I know I saw it somewhere here but I’ll be darned if I can find it again.

My question is; if, for example, I load a 45 LC, 255 gr. LSAC bullet, with 8.5 gr. of Unique, and fire it through a 5.5” barrel revolver which will travel at 846 fps; how fast will that same load fire that bullet through a 20” barrel?

Please demonstrate that calculation so I can use it again.

TY

there's not one .. if you shot it in a sealed breach rifle, you will have entirely different performance than an open revolver ...

in an pistol (7.62 tokarov in my case) the 5" pistol gets 1300fps, in a 19.75" boltgun, it gets 2050 ... but then again, its still not apples to apples, as there's no energy lost in recoil.

i would GUESS 300-400fps faster in the 20" barrel

That makes sense; I guess I could compare a pistol to a rifle rather than a revolver. The chambers would be more alike then.

That is a lot of difference in inches (5.5 to 20). There is a good chance that 8.5 gr of unique will have stopped combusting well before the bullet reaches the 20" mark and it may actually be slower than that the revolver (due to bearing surface friction).

Otherwise, I would guestimate that each inch of barrel would equate to about 100-200 fps.

That’s interesting, but if I go to the rifle section of my loading book, it says for a 230 gr. lead bullet through a 16” barrel, 8.5 gr. Unique, it goes 1043 fps. I doubt 4 more inches will be detrimental. Incidentally that’s a Cowboy load I’m stating here.

Incidentally you comment fits my desire to experiment with Alliant 2400 for these loads which would answer your concern anyway.

I Googled and found a couple of points of interest like this post which appears to be a rule of thumb which is more like what I saw before.

Posted 28 March 2010 - 02:21 PM

Rentedmule, on 28 March 2010 - 04:59 PM, said:

Anyone know a quick rule of thumb or a resource to calculating muzzle velocity?

Here's my problem: My velocity readings on my chronograph are 100 fps above the listed load. The load data was for a 4" barrel where I am shooting a 6". My chronograph is 10' from the muzzle. I know I will gain some velocity from a longer barrel.

No pressure signs but I don't have much experience in looking for them. If my chronograph is accurate then my velocities are over the max listed in the manual. The load is actually for a 38 special but I am posting it here as there is more traffic here. If it helps my load of 5.6gr of unique for 125gr Hornady XTP HP is listed at 900fps and my chronograph is reading 1000fps.

So how do you correlate the chronograph reading to actual muzzle velocity? Is the drop in velocity from the muzzle to the chronograph enough to worry about?


10' is a little bit close to the chronograph and it can sometimes mess with your readings. It normally makes readings low because the blast and powder residue get to the first sensor early, but it could be possible to have it happen the other way around.

The rule of thumb is that you'll gain 25-50fps per inch of barrel (up to a point), so 100fps higher than the manual isn't totally crazy with a 2" longer barrel.

Alliant's online guide shows 5.7gr of Unique with a Speer 125gr GDHP at 980fps using (I think...not exactly clear) an OAL of 1.44". So, it's not like your numbers are too unusual...granted, this is a different bullet, but we're still talking about the same ballpark. R,

Then there's this one if there are any mathematical geniuses out there that can decipher this one:

Hopefully it posts OK with all the tabs in it and stuff.

Didn't work very well here's the URL:
http://www.loadammo.com/Topics/October05.htm


Barrel Length vs. Muzzle Velocity
It is a usually accepted fact that gun barrels of different lengths will produce different muzzle velocities with the same load. Generally, the longer the gun barrel, the higher the velocity...at least up to a point. Only a few decades ago the common length of center fire rifle barrels ranged from 26" to 30". These rifles were heavy and unwieldy to carry around in the field all day. Today, the more common lengths for sporting rifles are in the range of 22" to 24", resulting in a lighter and more portable firearm. With this change many shooters want to know just how much velocity is lost in switching to the shorter barrels or how much velocity gain can be achieved in a longer barrel.
It seems that little useful information can be gained by comparing the muzzle velocities of two different rifles, of the same caliber, but with different barrel lengths. Varying dimensions in the chamber, throat and start of the rifling affect the velocity too much for a meaningful comparison. The more obvious solution is to use a single rifle, cut the barrel off, in given increments, and carefully measure the muzzle velocity at each barrel length. Many experiments were performed using this technique on both rifle and handgun. The problem was that the variation in velocity, from shot to shot, with the same ammunition, was often larger than any effect noted by shortening the barrel in 1" or 2" increments. However, if the data were plotted graphically to smooth out the irregularities due to variations in velocity measurements then a reasonable relationship between velocity and barrel length can be shown.
Various mathematical equations were developed to relate muzzle velocity to barrel length, but one of the simplest relationships was developed by Homer S. Powley. He defined the relationship between muzzle velocity and barrel length, as one giving muzzle velocity (v) as a function of charge weight C, bullet weight B and expansion ratio (R). Here the expansion ratio is defined as the ratio of the barrel volume plus cartridge volume (total volume of the gun) to the cartridge volume. The equation relating to these factors is represented by:
v = K[C(1- R^-.25)/ (B + C/3)]^.5
Where, v is in f.p.s., C and B are in grains and K is a constant that depends on chamber pressure and other factors in the gun. The expansion ratio is dimensionless. From this equation, it can be seen that for a given gun, with a given powder charge and bullet weight, the muzzle velocity is dependent only on the expansion ratio. By cutting off the barrel the barrel volume is reduced, thereby reducing the expansion ratio. The relationship can be represented by:
F = [(1-R2^-.25)/(1-R1^-.25)]^.5
Where, F is the correction factor to correct the muzzle velocity at expansion ratio R1 to that of the reduced expansion ratio R2 (shorter barrel). For example, a rifle chambered for the .223 Rem. cartridge, has a 24" barrel, and fires a 50 gr. bullet at 3,080 f.p.s. when loaded with 25.1 gr. of IMR 3031 powder. The expansion ratio is 8.5. What is the muzzle velocity if the barrel is shortened to 22"? The new expansion ratio is 7.8. Plugging these values in the equation we get:
F = [(1-7.8^-.25)/(1-8.5^-.25)]^.5
= 0.9846
v = 0.9846 X 3,080
= 3,032 f.p.s.
The new velocity for the 22" barrel is 3,032 f.ps. The velocity loss for removal of two inches of barrel is 48 f.ps. (3,080 - 3,032).
The table below lists the velocity change for different barrel lengths for several popular calibers.
Cartridge Bullet Wt., grs. Charge Wt., grs. IMR Powder Type M.V. in a 24” Barrel Difference in velocity for a 2” change in barrel length, f.p.s.
20”-22” 22“-24” 24”-26” 26”-28”
.223 Rem. 50 25.1 3031 3,080 46 48 42 38
.243 Win. 100 40.6 4350 2,920 56 50 45 32
7mm Rem. Mag. 154 66.8 7828 2,885 89 54 48 44
30-30 Win. 150 32.4 3031 2,228 32 29 25 23
.308 Win. 150 40.5 4895 2,531 40 36 32 28
30-06 Rem. 150 52.7 4320 2,770 50 44 39 35
.338 Win. Mag. 200 66.9 4831 2,684 52 38 41 37
.375 H&H Mag. 270 70.8 4320 2,460 39 41 35 28
.460 Weath. Mag. 500 96.0 4831 2,164 34 36 27 24
Note that the greater velocity increase, per inch of barrel length, generally occurs in the shortest barrel length. As the barrel is lengthened the velocity increase, per inch of barrel length, decreases. For instance, the .223 Remington shows a 46 f.p.s. velocity increase by lengthening the barrel from 20" to 22" This drops to 38 f.p.s. when the barrel is increased from 26" to 28". Some of the magnum calibers show more variation in velocity from barrel segment to barrel segment, but the velocity change, per segment, is generally less as the barrel is lengthened.
The powder charges and powder types shown in the table are for illustration purposes only and are not to be used as load data. For details on loading for a particular cartridge the reader should consult a good loading manual or internal ballistics software program. The software program offered at this web site will quickly calculate muzzle velocity for any barrel length from 18" to 50".
Watch our web site for the next topic of interest. Until then, shoot safely and know where your bullets are going.
Sincerely,
The Ballistician

Either the Cast Boolit or the 24 Hour forum have a post to a web site where 4 guys did an extended test of velocity and barrel length for several calibers and something like 22 inches long to pistol length barrels.

Thanks, that site looks like it could be valuable to my situation here.

I don't have a formula, but I do have real world experience with what you are talking about. Using a 260 grain cast RNFP over 7.0 of W-231, I get 850 fps from my 7 1/2" Colt SAAs and 1025 fps from 19" & 20" lever action carbines. Depending more on the particular revolver than the length of the barrel that same load goes anywhere from 775 fps to 825 fps in 4 3/4", 5 1/2" and 10" Colt, Ruger and USFA SAs. These are all chronograph velocities. Hope this helps.

I think it's time to buy a chronograph.

quote:

Originally posted by wboggs:
I think it's time to buy a chronograph.

Oehler is taking orders for a limited run of their 35P model. You may want to check them out.
IMHO there isn't a better chrono available to the shooting public.

muck

I'll check em out thanks.

Then I get this; actual results:

http://www.ballisticsbytheinch.com/results.html

wow!. i am going to be doing about the same thing in the next month or so, comparing 375s

Saeed did one of these experiments with a rifle barrel and cut it off 1" at a time a few years back. You might want to search for that also.

The results are going to vary by caliber and specific barrel characteristics.

quote:

No pressure signs but I don't have much experience in looking for them. If my chronograph is accurate then my velocities are over the max listed in the manual. The load is actually for a 38 special but I am posting it here as there is more traffic here. If it helps my load of 5.6gr of unique for 125gr Hornady XTP HP is listed at 900fps and my chronograph is reading 1000fps.

You've learned something, there are no absolutes when it comes to vel. with any given load. I have 4 diff 4" 357mags. They all shoot the same load to diff vel, one 100fps less than the other 3. Chamber size, bore/groove size & smoothness, cyl throat size, it all plays into vel/pressure readings. You can not go 100% by the data results in reloading manuals. They are tested in ONE firearm, either an actual gun or pressure bbl. Your results will almost always vary, sometimes quite a lot.

Each rifle will have its own velocity.

This was demonstrated to us when we had 5 Mannlicher rifles, all identical, in 270 Winchester.

We had Norma 150 grain SP ammo.

The differences between the slowest rifle and fastest one was 150 fps!!?

OK got the chronometer and went to the range today. My load was 200 gr. LSWC, CCI 300 primer, 5.2 gr. Unique. The weapons; Kimber Tactical Ultra (3”); Kimber Pro Carry II (4”); Kimber Custom Target II (5”).

3” average 856 FPS
4” average 900 FPS
5” average 943 FPS

So we’re looking at about 50 FPS per inch of barrel. SWAG