Earlier today I was hiding from the kids and the noise and realized on paper something most of us already know: the velocity gains aren't in line with all the hype associated with overbore magnums. Specifically long action ones.

7mm-08 compared to 7mm Rem Mag compared to 7mm STW

140 gr bullets fastest load in the book (hodgdon manual #27) 24" barrels across the board

7mm-08 H4350 48.0grs 2868fps

7mm Rem Mag H1000 70.0grs 3036fps
(45.8%) (5.9%) increases

7mm STW H1000 85.5grs 3284fps
(78.1%) (14.5%) increases

almost one and a half times as much powder in the 7mm Rem vs tne 7mm-08 and only a 6% gain in velocity?!! I thought there must be something wrong with the numbers so i checked several other books (speer #13, nosler 5th) and the same kind of results. i even looked at the same powder for each cartridge and different bullet weights...same results?!!

i'll do the math on the difference in drops at different ranges and post the results sometime.

might not be all that great out to practical maximum hunting distances (maybe 500yds)

Congratulations.....you have discovered the law of diminishing returns......

If you follow this far enough you'll see the fallacy of long barrels too.

knew of diminishing returns, the stw was actually slower than the weatherby with more powder on several occasions. don't know the longer barrel deal, please explain more...

i have heard of 22lr losing velocity because there isn't enough powder to continue the acceleration. is this what you mean?

IMO, the point of the longer overbore magnums was not to push light 140gr bullet 200-300fps faster than say a 280, but to push a heavy 160gr-175gr bullet 200-300fpsfps+ faster. My 280 will run a 160grNP @ just shy of 2800fps. My 7mm Dakota will run the same bullet to 3200fps. QUite a diff.

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don't know the longer barrel deal, please explain more...

same principle.....longer barrels don't add a uniform amount of velocity per inch.....the longer they get the less gain they add....until eventually the gains are negative.

We have a lot of folks that absilutely have to have 26" barrels and some demand 28".....

The faster the bullet travels through the barrel, the less time the pressure has to accelerate it.....a longer barrel increases velocity....so the next unit of length has less contribution.....sooner or later we run out of gains.....

Unless of course we go to very large cartridges like the .50 BMG....or artillery rounds!

Vapo is right, as usual.

With longer barrels, gas pressure drops in every inch (or fraction thereof) of bullet travel after the first ouple (or less). The following figures are drawn out of thin air, and are just for examples to make the logic clear:

With powder #R1-ZZ, gas pressure at 24" is 7,450 psi. At 26" it is 7,111 psi, at 28" 6,640 psi. Obviously during the time the bullet is pushed through each of those sections of barrel it has less pressure shoving it. So, it gains less speed during each 2" of further travel than it did during the previous 2" barrel segment. Eventually it won't gain any more velocity at all. Beyond that"no gain" length, it will lose velocity due to barrel friction.

If the loader uses a slow enough burning powder, he will lose less pressure as he goes, and that is why we commonly use slower burning powders for longer barrels.

Or perhaps if a person had a slow enough (truly progressive) powder, he could even even gain pressure.

But among those powders available to current loading factories or handloaders for small arms cartridges, that progressive powder has yet to be invented, so it just ain't gonna happen in barrels over a very short length.

Interesting post. I got my Kimber 7mm08 with 22 inch barrel 3 years ago and have not taken my 280 Ackley hunting since then. The Montana is 6 lbs with a Leupold VX3 2-8X while the Sako L61R 280 Ackley 24 inch barrel with a Swarowski 2.5-10X is 9 lbs. I have been dropping game out to 330 meters (360 yds) with the Kimber and Barnes TSX loads. It is so much easier to carry & I cannot shoot accurately any further than that. I know I should sell the 280 Ackley but I just love that rifle!

One of my favorite posters here, Ramrod340, has pointed out a rough rule of thumb that for a 4% increase in boiler room there can be a 1% increase in velocity. This is a fairly good observation for the size of current boiler rooms but take a look at the gains in the .22 WMR compared to the .22 LR

SAAMI pressures are the same, as I understand, for both yet the gains in the .22 WMR are quite a bit larger than the 1:4 ratio seen in cartrigde cases that hold much larger charges.

One can therefore estimate that the 1:4 ratio is not uniform but has a slope to the line favoring the smaller case capacities.

What is needed is not progressive burning powders but accelerated burning powders. Powders that start burning slow but accelerate the burn rate over time such that as the bullet speeds up the burn does as well. Theoretically the best performance we can get is if we have a powder that applies a 65,000 PSI pressure to the bullet and burns such that the 65,000 is maintained all the way down the barrel.

If I get real wound up later, I'll calculate the velocity potential using such a powder.....

This ought to get the pressure pot stirred up a little!

What fun!

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SAAMI pressures are the same, as I understand, for both yet the gains in the .22 WMR are quite a bit larger than the 1:4 ratio seen in cartrigde cases that hold much larger charges.

The 22 WRM has a larger bore diameter then the LR. (.223 vs .224") It`s not quite a apple to apple comparision although close.

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Originally posted by vapodog:

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don't know the longer barrel deal, please explain more...

same principle.....longer barrels don't add a uniform amount of velocity per inch.....the longer they get the less gain they add....until eventually the gains are negative.

We have a lot of folks that absilutely have to have 26" barrels and some demand 28".....

The faster the bullet travels through the barrel, the less time the pressure has to accelerate it.....a longer barrel increases velocity....so the next unit of length has less contribution.....sooner or later we run out of gains.....

Unless of course we go to very large cartridges like the .50 BMG....or artillery rounds!

I absolutely agree with this, but I'm with Fred on this one....

Finding the point of maximum benefit per caliber per bullet weight, per barrel length, is fun.

This said, would you agree that by using a slower burning powder one can get more velocity from a 26" barrel than from a 24" barrel for a specific overbore cartridge?

Also would you agree that when the end user of a specifically chambered rifle wants to use a heavy for caliber bullet, that the longer barrel will be more efficient in getting the velocity up using said slower powders, to a certain point?

As in the above listed comparisons, sans barrel lengths, these use a 140gr bullet, which for the 7/08 is about the best overall weight for case capacity and efficiency for this particular cartridge. But if you toss in a comparison of the 160gr bullets you can see real quick where the bigger cases and slower powders and even a longer barrel will all benefit the user correct?

Or better yet, jump up to a .308 caliber, and compare the .308 to the .300 Win, .300 WM, and .300 RUM, use the 150gr loads compared to 180 or 200gr loads, and there again use a short 22" barrel compared to a longer 26 or 28" barrel. The Palma folks are using a .308 with generally a 30" barrel to get every last bit of squeak from their rounds as well, I am sure that most would love to use something around 22" but the fact remains that they simply cannot get the needed velocity from the shorter barrels to keep their loads going at the longer distances.

I concur that in some cases, with some calibers, the barrel lenght is not so much a relative thing once you get past about 24", but in some cases it does help to have a little added length to get a bit more velocity using slower burning powders.

If it wasn't an issue we would all have 20" barrels on our 30-30's and all drive Fords because these are the best of the best correct?

The question is: Why would someone want to shoot a 140gr bullet in a 7mag? Except of course someone who wants to impress non-shooters with his boyhowdylookwhatIcando velocity?

But, you put a 160gr bullet in a 7mag and it will motor along at 2900fps which is an excellent performance window of trajectory and sustained energy.

In their proper context, both cartridges do what they do very well. Your comparison is very skewed.

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This said, would you agree that by using a slower burning powder one can get more velocity from a 26" barrel than from a 24" barrel for a specific overbore cartridge?

Absolutely I'd agree.....but that wasn't the context of the statement nor was that what I said.

It was all about diminishing returns. It wasn't about NO returns!

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One of my favorite posters here, Ramrod340, has pointed out a rough rule of thumb that for a 4% increase in boiler room there can be a 1% increase in velocity.

Yep I use that "Rule of Thumb" maybe a touch too much.

As to Joe's observation. When I was working on my 280 wildcat I did a lot of plotting of velocity information for the various 7mm rounds. I later did it for 243s. If you plot it velocity vs capacity you will get a curve that starts out looking like one leg of the St Louis Arch. Once it reachs the top the other leg will show only a slight increase and finally become flat or even drop.

In my plotting what I found was you could fit a line on the steeper leg as well as a line on the flatter side. Thsoe two line would intersect. When I designed my wildcats I shot for that point. Any capacity above that point you would get smaller and smaller increase. Right or wrong I viewed that a the most efficent(I know not the best discription)capacity for a given caliber. Just so happens that a 7x61S&H hits that point on a 7mm and I designed my 280PDK to hit it as well.

What I really should be saying is in a 06 based cartridges the gain per capcity was around 1 for 4. In smaller cases the gain was higher maybe 1 for 3%. Larger cases just like jumping from a 7mag to 7STW the gain is far less. But unless you are really overbore a 1 in 4 is a good

Part of it I "think" is being able to use the capacity. Smaller cases you are often capacity constrained so more space you can use maybe just the next slower powder. If you are only using 85% of the available capacity adding to it won't help as much.

So as said I often get lazy and lump it together and just call it all 1 for 4.

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Originally posted by vapodog:
What is needed is not progressive burning powders but accelerated burning powders. Powders that start burning slow but accelerate the burn rate over time such that as the bullet speeds up the burn does as well.

Isn't that what we used to have, Vapo?

Back when you and I were young, powder burning rates were in part determined by "deterrant coatings" being applied to the exposed surfaces of powders toward the end of their manufacture process. Those powders' burning rates (relative to the area of surface available to burn) sped up after the coating burned off.

(They did not necessarily speed up in total sum after the deterrant burned away, but did when compared to what they would have done without the deterrant. Put another way, they burned slower, but not as much slower as they would have if deterrant remained. The deterrant allowed us to use a lot more fuel, which helped keep the burn going strong farther down the barrel.)

Much of the rate was also controlled by kernel dimensions and perforations. As the outside surface burns it gets smaller and decreases burn rate. As the inside (perforation) surfaces burn, they get larger, thereby increasing burn rate.

Still pressures in total were lower toward the muzzle with those old powders, rather than rising or stable. They were not able to burn slowly enough at first, and quickly enough later, to keep pressures up or increasing.

The problem which offset the relatively "faster" burn after the deterrant was used up was that the powders then completed their burn too soon...while the bullet still had a good deal more barrel to travel through.

It was mainly to decrease that problem that we went to including the burning deterrants WITHIN the powders before they were shaped into "sticks" or "spheres". That way, so long as there is powder to burn, there is detterant present to help control the rate of burn, so we can use a great deal more of it within the pressure limits of our rifle barrels.

The only way I can see a powder working to maintain pressure is if different powders were available for specific internal barrel volumes (lengths & diameters).

At least in theory, one could slow the burning enough early in the burn to keep pressures down. Then, later, we would have enough powder remaining to be burned to keep pressures up for the last bit of the bullet travel and exit from the barrel.

If the thickness of the deterrant coating was engineered properly for the typical 24" .30 barrel volume, there would be no deterrant action beyond a certain point, allowing the remaining powder to burn a bit (lot?) faster than it would if deterred...and we could still use enough more powder at the start of the burn to increase the aount available to burn later without exceeding pressure maximums anywhere during the burn.


Of course, in artillery, Gerry Bull's "super gun" aimed to (pun intended), and would have solved the problem by fresh infusions of burning propellant at different distances down the bore..



Theoretically the best performance we can get is if we have a powder that applies a 65,000 PSI pressure to the bullet and burns such that the 65,000 is maintained all the way down the barrel.

Very true, but no such powder has yet appeared.

Best wishes. You've certainly posted some fascinating head-scratchers since you returned. Glad to have you posting regularly again.

P.S.: Maybe we need to solve the problem the way it has been more or less solved for space launch vehicles...multiple stage charges. Would probably require some separator(s) between the charges which would just burn through at the right moment to properly time the ignition of each following stage?

AC

i thought about the bullet weight issue and so did the research on 175 gr bullets as well, i just haven't done the math and i need to cook supper for the kids. Just a quess from the figures, but they seemed to follow the same pattern...

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The 22 WRM has a larger bore diameter then the LR. (.223 vs .224") It`s not quite a apple to apple comparision although close.

Is .001 really that important? and with lead bullets or bullets if very thin jackets?

Frankly, I don't know but it seems a very small consideration.

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Originally posted by vapodog:

What is needed is not progressive burning powders but accelerated burning powders. Powders that start burning slow but accelerate the burn rate over time such that as the bullet speeds up the burn does as well. Theoretically the best performance we can get is if we have a powder that applies a 65,000 PSI pressure to the bullet and burns such that the 65,000 is maintained all the way down the barrel.

What fun!

I've heard a rumor that Hornady may be developing a duplex load to do something similar. Instead of a fast powder under a slow powder, these will be blended and the slow powder will have some type of extreme deterent so that it burns much later in the ignition sequence. Supposedly top secret stuff with a major powder manufacture so they can outdo the Heavy Magnum loadings.

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Originally posted by vapodog:

If I get real wound up later, I'll calculate the velocity potential using such a powder.....

Well.....I did decide to break out the old numbers machine and apply some basic physics.....

As it turns out.....if one was to start a .308 diameter bullet of 150 grains down the barrel at 65,000 PSI and was able to keep the pressure up over an entire 24" of barrel (bullet travel inches....not actual length) the velocity would be approximately 5378 feet per second.

This gives us the potential in maximizing the use of such a powder.

Frankly, I have no idea what the pressure is upon exiting the muzzle of todays loads but clearly it's substantially less than the 65K PSI

For those that want to raise pressures.....have a ball....the performance is in finding the powder that best keeps the pressure up the entire barrel length.

[[/QUOTE]
Well.....I did decide to break out the old numbers machine and apply some basic physics.....

The interesting thing about physics is that theory and reality don't often meet.

I just broke out a dbl shot of jim and a coke...goodnight all.

Ted

quote:

Originally posted by vapodog:

quote:

Originally posted by vapodog:

If I get real wound up later, I'll calculate the velocity potential using such a powder.....

Well.....I did decide to break out the old numbers machine and apply some basic physics.....

As it turns out.....if one was to start a .308 diameter bullet of 150 grains down the barrel at 65,000 PSI and was able to keep the pressure up over an entire 24" of barrel (bullet travel inches....not actual length) the velocity would be approximately 5378 feet per second.

This gives us the potential in maximizing the use of such a powder.

Frankly, I have no idea what the pressure is upon exiting the muzzle of todays loads but clearly it's substantially less than the 65K PSI

For those that want to raise pressures.....have a ball....the performance is in finding the powder that best keeps the pressure up the entire barrel length.

You certainly started the stirring...

How about developing a new powder of tapered thickness & which is like cordite - (long stands) - Then we could load a tapered bottle neck case with a powder that is thick at the bottom & burns slower and thin at the top were it burns fast!

What would the effect of such sustained and prolonged pressure be on the brass case?

Going back to the original post, I wonder about book information. In a 7 mag, 3036 fps seems absurdly slow with a 140 grain bullet. I have gotten 3000 fps with 175 bullets in that cartridge and would expect to be somewhere near 3200 fps with 140s. In several different 270s I have gotten 3100 fps with 140 grain bullets and in 30-06s I have exceeded 3000 fps with 150s. I have been chronographing everything I shoot for more than 15 years and I no longer believe the book unless I can confirm the information with the chronograph.

what about using the method of front ignition (not sure of offical name but Elmer Keith experimented with it and wrote a short brocure on it) similar to a pin fire style with different burn rates of compressed powder tablets/donuts stacked in the casing (like a duplex load starting slow and getting faster). it would have to be a striaght wall case (for the donuts to fit in the neck and around the tube that the primer flash traveled up) and ideally ignight and burn in the case from the bullet back toward the head so as not to stir or disrupt the order of powders (hence the front ignition)??

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In a 7 mag, 3036 fps seems absurdly slow with a 140 grain bullet

Not sure of what book. Nosler for example shows over 3300 for the 140 and just under 3000 for the 175. Could be they are stopping at 61,000 the Saami max.

Rocky Gibbs did some experimentation with front ignition but IIRC decided that the loss of case capacity was too much to over come.

Maybe someone can chime in here but I believe some artillery casings have front ignition tubes installed.

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Rocky Gibbs did some experimentation with front ignition

Thanks vapodog I forgot to add that. IN Rocky's writings he claims some good gains. Or so I remember. Anyway I have a copy of the article is anyone wants them.

PM me an email and I'll send it.

yeah, i'd love a copy, been trying to find more info for a long time.

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Originally posted by vapodog:

Frankly, I have no idea what the pressure is upon exiting the muzzle of todays loads but clearly it's substantially less than the 65K PSI

In modern rifle cartriges and 22-24" barrels muzzle pressure runs a little north and south of 10,000 psi depending on the cartrige particulars. Roughly 8500 psi to 13,000 psi for the majority.

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Roughly 8500 psi to 13,000 psi for the majority.

I would have guessed something like that but thanks for the data.

It's appreciated.