Something to think about here, relating to velocity...

How many of you folks have read somewhere that short barrels aren't really any disadvantage velocity-wise because all of the powder in a center-fire rifle load is burned in the first 4", 6", or 8" of the barrel (pick any one of those you want)?

I'll say right up front that there's usually no huge chronographed difference in velocity between 20" and 24" barrels, but does that truly mean that it never pays to put on really long barrels...like 27, 28, 29. or 30" tubes for smokless powder rifles?

How about this thought? "It isn't the powder burn time that counts but how long that hot gas produced by the burn continues to expand in the tube faster than the bullet moves." Doesn't the gas then keep pushing the bullet faster and faster as it expands?

Question:
Having thought it over, is there ever a real life advantage to a really long barrel?

Choices:
Yes, sometimes (and here is where I think the advantage lies...)
Yes, but only for target rifles...
Never. Rifle handiness is always more important than bullet speed...
Very seldom. Here's the problem that quite often offsets the advantage of higher velocity
None of the above. Here's what I think...

 

I d have to agree only on some of what you said, specifically about the gases.
I ll keep it short for now.
Now I found for the slower burn rates (Example 30-06/308Win) that if you have a barrel shortened below 18"-20" you will start to lose your fps. But for the faster burning powders in the smaller calibers like the 223Rem you would have peeked your psi long before exit.

Just remember your major advantage is not in FPS but time in barrel gives your better accuracy. More distance on lands on groves gives your bullet better/more time to stableize before exit.
FPS are useless if you cannot hit anything.
Whats the real point of maxing out FPS? Shorten case life?

As long as there is "positive" pressure in the barrel, the bullet will continue to accelerate. And, the area under the curve dictates max velocity.

DMB,
Since my field is medicine not physics I may be wrong but don't you really mean that IF the pressure exceeds the frictional resistance of the projectile, the bullet will continue to accelarate?

I would think, but have no supporting evidence, that if the pressure is steady state and one increases the resistance,i.e., rough barrel or narrowing of bore, the speed, not only acceleration would diminish.

You know, I think I have just entered into the land of "how many angels can dance on the head of a pin".

Anyway, I like a longer barrel for certain circumstances but not in my carry rifle when hunting.

Gary

quote:

Originally posted by DMB:
As long as there is "positive" pressure in the barrel, the bullet will continue to accelerate.

Only true within the realm of "normal" barrel lengths.

If the total force exerted by the gas is LESS than the frictional forces, then the bullet will decelerate.

IIRC, they have done this before and this point is about 28-30 FEET from the breech.

As the powder in the cartridge burns, it produces hot gas which causes pressure to build in the cartridge. As the pressure and temperature inside the cartridge increase, the rate of combustion increases, causing higher and higher pressure to build. At a certain point, the pressure in the cartridge exceeds the friction that holds the bullet in place, and it begins to move. The powder continues to burn, increasing the pressure in the chamber, until it reaches a maximum when the bullet is a short distance down the bore. As the bullet moves down the barrel, it accelerates, and the pressure decreases slightly as a result of the expanding volume behind the bullet .

A full length or longer barrel produces higher velocity until powder burn is completed then it begins
to drop off and barrel friction works against velocity .

A happy medium is necessary for maximum efficiency . IMO

Slow powder VS Faster powder VS barrel length requirement . Don't most " Mags " come in around 22.5-26.5 " Sloppy large bore ( No offense meant ) use shorter length as they tend to shoot under 250 Yd. and brush swing advantage is a PLUS for a 2200FPS 500 grain stop knocker !.
'

quote:

Originally posted by MuskegMan:
[QUOTE]Originally posted by DMB:
If the total force exerted by the gas is LESS than the frictional forces, then the bullet will decelerate.

THIS IS FACT!!

At what distance this happens is dependant on a number of factors.
A light load of fast powder in a 22 hornet pushing a heavy,long bullet will decelerate at a shorter distance than a short bullet in a 30-378 using a heavy charge of very slow burning powder.

there are specialty applications where it matters. And, if you are REALLY a stickler for that last 100fps, its FAR easier to get with a longer barrel than shorter ...

however, within a range, faster powder can result in higher velocity in a shorter barrel than slower powder in the same shorter barrel.

the differences can be exaggerated ... say, 257 robets, 16.5" barrel, and compare h335 vs h1000 ... you will have realized MORE of the potential of the h335 load in 16.5" than the h1000 ...

BUT

h1000 in a 30" barrel is not going to be all that much faster than the h335 load that was faster in the shorter barrel...

velocity is area under the time/pressure curve, and has little to do with PEAK pressure

In specialized cases, under a huge amount of powder, operating within a range of acceptable pressure, it CAN have an advantage to reach a certain velociy.

these parameters can be tweaked to favor either side, or to perversely, make a smaller cart LOOK better...

It is my understanding that maximum pressure is achieved within the first few inches of the barrel length. A balanced powder for any given bore / projectile will continue to burn the full length of the barrel providing increased acceleration until the bullet exits the muzzle, with a 95% burnt rate being about perfect.

There are other physical forces at work, more than simply measurable pressure. And I don’t pretend to understand those forces, like the effects of the kinetic energy force of burning powder on the back of a bullet.

If pressure alone was the Only factor to bullet velocity, this graph wouldn’t make any since.

With a long barrel the fast powder gives up the push too soon and velocity will be less. As you shorten the barrel you can use faster powder to maintain speed.
Remember also that a heavy charge of slow powder also adds to projectile weight and can reduce speed.
Take a 30" barrel on a BP rifle and keep adding powder. At some point you will find a max velocity but from then on as you add more powder the velocity will decrease with each addition.
The question is too general to answer.
The caliber itself is important in that a magnum rifle that uses a lot of powder needs a longer barrel. To chamber a 20" barrel in .300 RUM or a Weatherby just puts the rifle into a 30-06 class. Not only from the powder weight that does not burn but from the energy loss of the unburned powder.
I don't know of any rifle that burns all the powder in 8", it takes about 16" for a .22.

Let me say that the answer I come closest to agreeing completely with so far is Jeffe's. But, of course the comments are not all in and I could end up convinced of who knows what....

So long as the gas is expanding against the bullet's base with more force than required to:

1) surpass the friction and inertia the bullet has to overcome at that specific instant and,

2, to accelerate the column of air in front of the bullet,

the gas will be accelerating the bullet.

As soon as that expanding gas is pushing against the bullet-base and air column in front of the bullet with less force than is needed to continue accelerating them both, velocity will begin to drop off.

There is essentially no "stasis" length of travel where the bullet and air column are neither accelerating or decelerating.

It seems not to matter if the powder burn is completed, IF the gas is still expanding with more force than needed for the bullet to overcome both the bullet inertia and friction, and the resistance to movement by the air column in the bore ahead of the bullet.

Of course, that is a power situation which is difficult to maintain IF the powder has quit burning within about three inches or less down the bore, which it almost certainly appears to do in .22 rimfires. Even .22 rimfires do generate enough hot gas to continue pushing the bullet faster & faster for 16"-18" of barrel or so, but after that their bullets start to slow, whether in the barrel or in the air.

But it is not at all unusual for large charges of smokeless powder to generate enough rapidly expanding hot gas for those gases to continue to shove the bullet briskly clear to the muzzle and even a little tiny bit more.

Why, otherwise, would the time-honoured rule of losing 25 to 35 f.p.s. for each inch of barrel length reduction below 28" or so ever have come to be?

Of course, we can forever debate when the powder is done burning, too. Is the burn complete when all the "solids" are burnt? I dunno...is a gas a diffuse "solid"? Certainly some of the gases released by the decomposing "solids" of the powder burn in their turn all the way down most bores, raising the temp and burn rate of the gases still remaining and increasing the force exerted against the base of the bullet.

Anyway, I love it when some guys say a question can't be answered and then proceed to answer it.

It will be interesting to see what those who do vote in the poll end up deciding about the usefulness of longer tubes...

In the meantime, think I'll put on my flame suit and go here

Have fun, y'all.

jeffeosso has stated what I have been speculating.

A longer barrel can utilize the energy that can be stuffed into a case with a slower powder that would otherwise cause muzzle flash/blast. Another major advantage of a longer barrel is the intensity of the muzzle blast and proximity to ones ears of that muzzle blast - further away!

quote:

A longer barrel can utilize the energy that can be stuffed into a case with a slower powder that would otherwise cause muzzle flash/blast. Another major advantage of a longer barrel is the intensity of the muzzle blast and proximity to ones ears of that muzzle blast - further away!

Regards
303Guy

True, when I bought my .300 Weatherby I bought 26", same with my .220 Swift. The long barrel gives performance.
It irks me when someone says they have trouble in the brush with a long barrel---what the hell are they doing in 30-30 country with a big magnum?
There are a million other calibers suited to shorter barrels but to shorten a barrel on a rifle caliber that needs a certain length to be efficient is plain silly just because an inch or two longer "is hard to handle".
Look at all the silly guys wanting a 2" barrel on a .500 S&W! Soon they will want the barrel removed so they can just shoot from the cylinder!
I spent years stalking the woods and killing deer with a muzzle loader and a 42" barrel, I must have been wrong, should have used 4" instead!
The trend to short barrels is getting worse and it burns my butt that I can no longer buy many decent rifles.
Archery has gone off the planet with bows so short they can't be shot with fingers anymore. I can't even buy decent arrows, most way too light to hunt with because of the speed freaks out there. 9 out of 10 guys can't even shoot through a deer, what do they do for an elephant or cape buf????
I hate to see this ongoing move to satisfy weaklings and city folks that get all experience from gun rags.
I love it when a guy says his .357 or .44 and even a .45 ACP just kicks too hard and hurts their tender hands! Then does a goofy thing like buying an aluminum, titanium .44 or larger because they hate a heavy gun. Then they send it off to have the barrel chopped down to nothing and want to know how to make the thing shoot. They take one shot and start to look for super light loads with no recoil and I have to ask why the hell they bought the stupid gun to start with.
Thanks for letting me rant a little, people get more and more funny as time goes by.

AC, you're having too much fun with this thread.
When we are talking the barrel length that causes acceleration to change to deceleration, does the air in the ""remaining barrel length"" at that point have any real contribution to deceleration? I think not. Friction force is in the hundreds of pounds. The remaining air in the barrel may account for an ounce of force or so. If you doubt the contrast between friction force and force to remove air try beating a bullet down a rifle barrel with a rod and hammer. Now put your mouth on an empty barrel and blow. Water Melon vs. poppy seedroger

AC, it is true it can't be fully answered to fit every gun, barrel length and caliber along with all the different powders used.
The questions and poll are just too general and many of the choices don't fit the questions.
I would have a large problem picking one to check.

Guys- I AM having fun with this, but it is also serious.

For those who don't think we can figure out the answer, consider this example:

You have a .30-06 casing. You put 47.5 grs. of IMR 4895 in it, and place a 150 gr. pointy bullet on top.

Then you fire it in an approximately 24" long barrel.

Okay, is there enough pressure at the end of that barrel to keep accelerating the bullet?

Before you say we can't know, read on, please.

Well, let me give you a little more info. The rifle you just fired it in is a Springfield Arsenal manufactured M-1 Garand. Just an inch or two from the muzzle end of the barrel is a tiny hole drilled into the barrel which lets SOME of the gas escape to where it can hit the face of a motionless several-ounce piece of metal over a foot long...the operating rod.

That remaining, tapped-off gas pressure then moves that operating rod briskly enough to compress a pretty strong spring, moving the whole operating rod in the process, and then in turn moving back the several more ounce action breech bolt enough to extract the empty cartridge case and throw it 6-8 feet out of the gun.

The empty cartridge case alone is heavier than the 150 grain bullet being pushed by the much larger volume of gas still behind it during the last inch or two of barrel travel.

In fact, there is so much gas pressure left at the point where any is tapped off to operate the action that we have to be careful not to use powders significantly slower burning than IMR 4895, or we will BEND the operating rod and/or batter the action to where it maybecome unreliable, BECAUSE OF TOO HIGH GAS PRESSURE.

For the same reason, we also have to carefully make certain the hole where the gas is tapped off the barrel is not TOO large....and it is already nothing at all like the size of the hole which is the bore where the bullet is being hit by the gas.

So, whadda ya think of what that tells you?

(What it tells me is that sometimes we get so deep into theory that we completely overlook answers which are staring us right in the face.)

It is a fun thread!

MickinColo's graph provides the answer. At muzzle exit, the pressure is still close to 20,000 psi. 1,000 psi will accelerate a bullet!

Putting things in perspective, a 22LR continues to accelate the bullet for 18" or so.

quote:

Friction force is in the hundreds of pounds.

Not sure. Maybe. Remember that the pressure behind the bullet is also acting on the bore and that stretches it thus releasing some of its grip. On the other hand, the pressure acting on the base of the bullet is also making the bullet 'obturate' and press against the bore, so ...... ? Anyway, once a bullet has been engraved into the rifling it pushes through with cleaning rod force or maybe a little more. But then, lubing the bullet and/or bore raises muzzle velocity and maybe even reduces peak chamber pressure.

To get some sort of idea of the pressure at the muzzle. just observe what the muzzle blast does to nearby objects. One nearby object in my case was a suppressor I had failed to glue down properly. The distance it was flung down-range was amazing! Then consider how a muzzle break works - up to 40% reduction in recoil! So, as AC says .... !

bfrshooter, I was thinking just what you said - long barrels work just fine in the bush. The problem is carrying it without poking the muzzle into the ground. I have solved that problem by making my barrel longer! By adding a myzzle device that protects the muzzle from the ingress of mud and stuff and also happens to reduce noise and recoil. The added muzzle weight also happens to improve hold steadyness by damping down my shaking and improve the guns balance - for me anyway. I do accept that the longer 'muzzle' is now going to hit brush and the ground more often but it doesn't matter!

Confused 1K lb pressure will accelerate a bullet ???.

How about a whole lot less than that !.

AC ; The pressure displacement which is and has escaped all around the bullet is leaving the bore prior too the bullet leaving the barrel . Now there is still plenty of pressure still behind the bullet but a far as peak pressure it subsides LONG BEFORE the bullet exits the bore .

Muzzle flash is a result of hot gaseous and burning powder exiting the bore after the bullet has

exited the bore . So the larger the muzzle flash means powder didn't complete it's burn cycle fully with in the length of the bore .

Slower burning powders means longer sustained pressure curve faster powders have shorter pressure

curves .

How far do you feel the bullet must leave the chamber in inches ( Approx) , prior to the

peak pressure curve subsiding ?. I know I want to see if anyone else does .

quote:

How about a whole lot less than that !.

Well, maybe not a lot.

There is a point usually missed (I think) and that is that although the gain in velocity seems small (50 to 100fps does not sound like much) but the gain in energy is a a function of the square of the gain in velocity! Another thing to consider is that the faster the boolit, the shorter the flight time and hense the shorter the time for the boolit to slow down in flight so the down-range gain is actually a little greater than the gain at the muzzle.

I'll buy the latter part of your post .

The mass IE bullet weight and design bore friction air pressure density are just a few factors of forces with respect too pressures needed to accelerate it.


AC ; Didn't see the last part about Opp rod thingy . Pressure is relative to venturi size and shapes

when referring to gas operated by products of Bore pressures .

Anyone that has ever seat beside the guy at the range with a 10” 45/70 TC shooting factory ammunition knows the theory of long vs short barrels all too painfully will.

quote:

Originally posted by Doc224/375:
AC ; The pressure displacement which is and has escaped all around the bullet is leaving the bore prior too the bullet leaving the barrel .

Am not sure what you are trying to say with this line, Doc, but the amount of gas which escapes before the bullet leaves the bore dpends mainly on when the bullet obturates the bore, doesn't it? The way the rifle is chambered and the way the cartridge fits that chamber has a lot to do with whether ANY gas gets past the bullet enroute.
------------

Now there is still plenty of pressure still behind the bullet but as far as peak pressure it subsides LONG BEFORE the bullet exits the bore.

Definitely true, buit it is not peak pressure which determines velocity. It is the length of time that more than enough pressure to accelerate the bullet is applied to the bullet.
---------------------------



How far do you feel the bullet must leave the chamber in inches ( Approx) , prior to the

peak pressure curve subsiding ?. I know I want to see if anyone else does .

My guesstimate is about 1/2 an inch, or maybe less, depending on chambering and powder used in the chamber.


Now, Doc, let me ask you a question...How far down the barrel will gas pressure still be high enough to continue to accelerate the bullet in a typical big-game hunting rifle using a standard factory cartridge appropriate to big game hunting?

How about in a naval rifle?

quote:

Originally posted by Alberta Canuck:
[QUOTE]Originally posted by Doc224/375:
How far do you feel the bullet must leave the chamber in inches ( Approx) , prior to the

peak pressure curve subsiding ?. I know I want to see if anyone else does .

There is no single answer to that question taking into account the many calibers and varied burning characteristics of the plethora of powders. I do not only refere to burning rate but stuff like adequate temperature and pressure to sustain burning,etc.Bullet characteristics ( shape, jacket material, bareing surface length )would also enter into it.
Interesting and thought provoking but how meaningful is it to the average loader? roger

quote:

Interesting and thought provoking but how meaningful is it to the average loader?

That's easy - the more confusing the more the fun!

quote:

How far do you feel the bullet must leave the chamber in inches ( Approx) , prior to the

peak pressure curve subsiding ?. I know I want to see if anyone else does .

It depends on the load too but according to Quickload about 2.25 inches in my Swift, 2.50-2.70 inches in my 7mm Rem, and 1.50-1.70 inches in my 7mm Mauser.

The time it takes to get there is less then half a ms.

Mike*** Can you run the number for a 30-378 w/ a 150 gr. bullet, the .375 Ultra Mag with a 300 gr. bullet and the hornet with a 34gr. bullet.Thanks roger

quote:

Originally posted by bartsche:
Mike*** Can you run the number for a 30-378 w/ a 150 gr. bullet, the .375 Ultra Mag with a 300 gr. bullet and the hornet with a 34gr. bullet.Thanks roger

You have a PM.

The bullet dwell time is the time between cartridge ignition, and the time the bullet leaves the barrel.

Explosive gas propulsion continues to drive the projectile out the bore once the projectile is clear of the bore excessive gaseous along with unspent power burn off, as muzzle flash and residue deposits .

A certain amount of Gaseous always proceeds the projectile leaving a bore as any slow motion film of that easily shows , it's still a loss of peak or extended pressures driving the projectile .

AC; IMO in doesn't stop driving until the projectile actually leaves the bore muzzle flash still has

inertia Hence a longer bore produces + FPS to a point !. So a Big game round still accelerates until the muzzle flash .

P S, I also wrote the previous posting wacky as A SMALL Amount of gaseous escapes via the rifling which proceeds the projectile exiting the bore is what I should have said . Which slightly contributes to the loss of peak and extended pressures captured behind the bullet . Sorry .


Correct Roger generally believed to be with in half the length of the case as theory has it !.
there is no single answer to that question taking into account the many calibers and varied burning characteristics of the plethora of powders. I do not only refere to burning rate but stuff like adequate temperature and pressure to sustain burning,etc.Bullet characteristics ( shape, jacket material, bareing surface length )would also enter into it.

MickinColo ; I just read what you posted in relationship to pressure peaks , is that about half of the cases length ?. It's one of the " Old " things I remember from ballistics .

quote:

Originally posted by Doc224/375:

A certain amount of Gaseous always proceeds the projectile leaving a bore as any slow motion film of that easily shows , it's still a loss of peak or extended pressures driving the projectile .


This is true of fixed ammunition fired in standard rifle industry chambers. However, in what is called breech-seated-bullet ammunition, such as used by Schuetzen competitiors, the bullet oten isn't even in the cartridge case at the time of firing. It is already in the bore, fully obturating the entire hole, including the grooves. The cartridge case carries the propellant. the primer, and a wad of highly frangible foam to keep the powder in place.

On firing, the powder gas is trapped by the bullet and only a very insignificant amount of gas may get past the bullet, if any. In my cast bullet benchrest cartridges and rifles, I set them up to mimic that situation.


AC; IMO in doesn't stop driving until the projectile actually leaves the bore muzzle flash still has inertia. Hence a longer bore produces + FPS to a point !. So a Big game round still accelerates until the muzzle flash.

I agree with your conclusion. However it is certainly not just the inertia of muzzle flash which continues to accelerate the bullet. It is the continued high pressure expansion of powder gases.

As to "flash" having inertia...well now we are starting to border on pure physics "string" theory, as that gets us into whether energy has mass or not....which leads us to whether "light" has mass...after all, flash IS light, isn't it? That's only a semi-serious comment. I really don't want to go into a debate on whether light has mass.

Without even referring to theory, bullets do accelerate all the way to bore exit in both full-bore hunting rifles and naval rifles. And the last part of that acceleration is caused by part of the same forces which are used to operate gas-powered rifle actions.

Which is why I started this thread. I got tired of reading profundities about how the powder burn was all done in a very short part of the cartridge case or barrel, so further barrel length didn't help as the pushing was all done at that point. Purely and simply, NOT SO.

In many (most?) instances the pushing is not all done even when the bullet is about to exit the barrel. And though the target may not feel any difference, or be any more dead because of a longer barrel, there are times when a longer barrel is definitely a good thing ballistically no matter when the powder burn was completed.

Anyway, I am ready to go fry some other fish. How about you?

(BTW, I consider the "best" answer to the poll to be #1, which also happens to be the answer the majority of the respondents selected. That doesn't make it THE answer, just the one I feel most comfortable with personally.)

quote:

MickinColo ; I just read what you posted in relationship to pressure peaks , is that about half of the cases length ?. It's one of the " Old " things I remember from ballistics .

It's barrel travel. The distance that the bullet left the case. Is it the half the case length rule? I have no idea. But your ideas are as good as mine. Maybe better.

From 20-24" there is still a sizable advantage. For long range guns that don't have to be packable, 30" barrel will give 200+ fps more than a 20" with large cases.

gents, some of you are mis-reading these graphs ..

the units of pressure are on the LEFT vertical.. 10-15K psi... boys, 15K is a HIGH PRESSURE shotgun peak pressure...

in fact, gents, "slow" is about burn AND expansion ...

Barrel length for hunting has a lot more to do the just vel.

I have long barrel rifles and short barreled rifles I find the short barrel ones easier to carry but the long barreled one easier to shoot.

I don't like the muzzle blast of the short ones. I like the way the long ones hang for off hand shots. I have more longer barreled ones then short barreled ones. I hunt deer in the woods with a 99 sav with a 24 inch barrel I have no trouble killing running deer with it.

I hunt pigs with my 18 inch scout rifle I have no trouble killing running pigs with it.

The key is to paractice with them all and they all well work,

I have a theory on the 'optimum' barrel length - it's to do with expansion ratio. Simply put, to maintain an acceptable muzzle pressure, a longer barrel for smaller bore and shorter for larger bore. To this end I wanted to build a bush carbine on a Lee Enfield using a 303-375 wildcat. (Then I got made redundant! But one day maybe I also wanted to build a long barreled 303-22/220Rimmed Swift so as to achieve Swift velocity at the lower peak pressure the No.4 Lee Enfield can handle. One day maybe! )

As to "flash" having inertia...well now we are starting to border on pure physics "string" theory, as that gets us into whether energy has mass or not....which leads us to whether "light" has mass...after all, flash IS light, isn't it? That's only a semi-serious comment. I really don't want to go into a debate on whether light has mass.

Flash is simply the by product of unspent powder under ignition from either burn or pressure induced friction ignition .

The gas represents the force driving the projectile as the gas is no longer contained

with in the bore and now has changed direction thus no longer contributing

to the forward motion of the projectile .

Yes I agree #1 was for all purposes the correct answer . No String theory's Quantum physics is a

difficult subject. I apologize to all concerned for not posting a clear and straight forward answer initially . My mind has been focused on another project ,which is another reason I've not been in attendance as of late . First clue should have been Gaseous ? How about gases !.

Plus my Deer and Elk hunts are coming up pressure to complete the project is weighing on me heavily .

So if I'm not around for awhile you'll understand . Doc ...

Good luck on the deer and elk hunt Doc. See you later.

what doc said....

there's a reason paul made his barrels 28.9( or what ever it was) inches long. mathamaticly he "needed"( as in engineering talk) that length to get the bullet to the end of the barrel just as friction over took drive with the current powders/bullets/manufacturing technology of the time.
different, (dare i say better?) powders/bullets/technology and in large proportion, consumer prferences of more modern times have shortened barrels to what we now accept as standard length.
guns were deigned by engineers as war tools and ballistic efficiency was held far above ease of handling.

quote:

Flash is simply the by product of unspent powder under ignition from either burn or pressure induced friction ignition .

I`ve always (well maybe only for "just" and not "always") some time believed the flash was the result of the hot high pressure gas re-ignighting when it comes in contact with the oxygen in the air.

The "powder" burns, or however much of it is going to, in the 1st couple of inches of barrel. Then the bullet is propelled further by the hot expanding gases left from the burn. The longer the barrel the more time the pressurized gas has to act on the bulet until a point is reached the gas no longer has enought pressure behind it to accelerate the bullet anymore. Most carridges will produce enough gas to keep the bullet accelerating for quite a bit more then a 28" BBL lgt. This is a large part of the reason for big flashes from shorter barreled rifles and pistols, there is more pressure/denser gas, and therefor higher temprature behind the gas at these shorter lenghts when the bullet exits the barrel.
The 22 LR though, is one I can think of off the top of my head that loses pressure quickly and is reported to stop gaining velocity after ~16-17" of BBL.

I think most folks generalize the theory that a given bbl length is always faster, IE I hear guys say X rifle with a 24" bbl will be faster than Y rifle(Same Cart) with a 22" bbl. It's a diff story when you are talking the same exact bbl that's been cut shorter and shorter to compare data.

Don't forget about bore dimensions.

I have seen quite a few shorter bbl rifles shoot faster than longer bbl rifles of the same cart with the same loads. Bore dia plays a role as well.

Reloader

Nice thread AC.