What velocity increase per inch?

[Pond, James Pond]

Taking a .308 bolt loaded with typical rounds (155gr, or 168gr) charged with usual powders (N140, Varget or the like) achieving typical velocities of 2750 ish FPS from a 26", what kind of velocity drop or gain would you get per inch of barrel.

I ask this because I am mentally comparing the likes of my own CZ550 Varmint 26" (25.6") from which I can get performance like that above with a 155gr bullet, compared to carbine rifles such as Rem 700 Carbines or the Ruger Scout.

[tangolima]

Sort of rule of thumb I have been following for high power rifles; 25 fps per inch.

-TL

[Bart B.]

Page 22 in the following:

http://www.saami.org/specifications_and_information/publications/download/206.pdf

They're based on fixed barreled actions with the barrel cut back one inch at a time with the same lot of ammo. This is the only good and valid comparison. There can easily be 25 to 50 fps difference between two people shooting the same ammo in the same rifle.

Some barrels 26" long will shoot the same ammo 50 fps slower than others 24" long.

[green_MTman]

25 sounds right for a .308.there is still some debate as to whether the burning rate of the powder effects this.
some say all the powder is burned within inches of the chamber after ignition.some say not,.
and that slower burning powders can give more velocity


with a .308 either way 25 fps would be right.now if your using something like accurate 870 or retumbo or IMR 7828 in a magnum,some believe you could get 50 or even more extra fps with an inch of barrel lenths

[hdbiker]

I have too .308's,a 18 inch carbine and a 22 inch rifle.The same load will shoot just over 100FPS slower in the short barrel carbine .hdbiker

[jmr40]

Comparing velocities from 2 different guns is useless. I've seen 100 fps difference with guns of equal barrel length. That is extreme, but the 25-50 fps Bart noted is common and to be expected. Recording velocity from a known barrel length and using the same ammo, under exactly the same atmospheric conditions and recording it again as the barrel is cut shorter is the only valid way to determine. Even then it will vary by the load. I'd expect 10-20 fps/inch with a 308 with barrels longer than 20".

Somewhere between 20-24" is probably the best compromise for a 308. As you get longer than 24" gains in velocity come in smaller and smaller steps. Above 24" 5-10 fps/inch might be all you would see. When you get shorter than 20" the losses come in bigger and bigger steps. Here I wouldn't be surprised to see 25 fps/inch or even more.

Two inches either way means nothing. It would not be unusual at all for individual barrels to shoot faster than other barrels 2" longer. When you start comparing 20" guns to 26" or longer then those small steps start adding up enough to make a difference.

[Art Eatman]

Generalizing: Taking the .308 as a sort of "standard" case, cartridges like the '06 are considered "slightly" overbore. True maggies are definitely overbore.

The velocity loss per inch of cut-back is greater than for the .308. Maybe 50 to 75 ft/sec/inch for such as the '06. Maybe 75 to 100 ft/sec/inch for a .300 WinMag et al.

Back from the 1940s and into the 1960s, about every three to five years or so, somebody from the American Rifleman would get out a hacksaw and a chronograph and do some playing. My memory has it that the old '06 was the most common victim. So, I've always figured some 50 to 75 as "the number". :)

[Brian Pfleuger]

I used QuickLoad, set it for .308 Win with default case capacity. I used a 165gr Hornady BTSP and a 105% charge (46.0gr) of Varget.

QuickLoad predicts:

26" - 2820
25" - 2799
24" - 2777
23" - 2754
22" - 2729
21" - 2702
20" - 2674
19" - 2644
18" - 2611
17" - 2576
16" - 2538

[Bart B.]

Powder type and amount does make a difference. And as most rifles with all sorts of barrel lengths show flames coming out of the muzzle if observed in total darkness, that's usually considered proof that there's still powder burning outside the barrel. And with some loads of some powders, unburnt granules can be seen on the ground in front of the muzzle

[Pond, James Pond]

Those quickload figures are very interesting to read. I can see why some hunters prefer a carbine: light, but still able to produce usable velocities.

[pax]

Try www.ballisticsbytheinch.com

Fascinating stuff.

pax

[Pond, James Pond]

BBTI.

Should have thought of that: it is indeed a very good site.

I hope they start to add more calibres too, and perhaps more examples of the ones they have.

[mete]

If you used a real cartidge like my 45-70 the velocity loss would be a lot less !:rolleyes: I didn't bother to calculate the loss from a 28" cut to 22" .

[JohnKSa]

Some interesting information in this article.

http://www.chuckhawks.com/rifle_barrel.htm

There are a couple of lists of velocity changes based on the muzzle velocity range of the rifle. Here's a summary of the two lists.


Muzzle Velocity Range/Change in Muzzle velocity per 1" barrel length change.
1000-2000 fps/5 fps.
2000-2500 fps/10 fps.
2500-3000 fps/20 fps.
3000-3500 fps/30 fps.
3500-4000 fps/40 fps.

There's nothing exclusively related to the .308, but there is a comment about the .30-06 which should be in the same general neighborhood. It suggested that the change in muzzle velocity based on an inch change in barrel length was 15fps.

I suspect that this goes back to a day when rifle barrels were typically much longer than they are today and bullet weights tended more toward the heavy side. I think they should be taken with a grain of salt when the barrel lengths get very short and/or bullet weights get very light.

Here's a much more involved treatment of the topic.

http://www.thetruthaboutguns.com/2013/10/daniel-zimmerman/the-truth-about-barrel-length-muzzle-velocity-and-accuracy/

The data from his article indicates the following.

Barrel length Range/Change in Muzzle velocity per 1" barrel length change
18" to 26"/18fps
16" to 18"/42fps
13.6" to 16"/53fps

It's pretty clear that there's a big difference in going from 14" to 15" as opposed to going from 25" to 26". Unfortunately, his data is not very finely resolved. It would be nice to see things broken down into a few more length ranges so we could get a better feel for where we really start to see diminishing returns.

Here's some data from a post from a thread on another forum.

http://www.ar15.com/forums/t_3_121/587662_Barrel_Length_vs_Muzzle_Velocity_in__308.html


26" = 2918 FPS
24" = 2873 FPS
22" = 2823 FPS
20" = 2766 FPS
18" = 2700 FPS
16" = 2623 FPS
14" = 2531 FPS
12" = 2417 FPS
10" = 2273 FPS
8.0"= 2080 FPS
6.0"= 1800 FPS

[44 AMP]

For many years, before modern studies and computers, the "standard" rule of thumb was, for most calibers, 35-70fps per inch.

Of course, this was a really rough guess, and many guns & loads fell outside this fairly broad range, as well. And much more noticeable in some calibers and loads than in others.

Most loads appear to have a "sweet spot" where a barrel length a couple inches longer, or shorter has a smaller effect on the velocity than the rest of the range. For example, many rounds show a much smaller velocity change per inch between 22-26 than between 18-22.

It is still 4 inchs of barrel difference, but which 4 inches (in this case) makes a difference in the rate of change per inch. Different powder burn rates will play a part in this too.

Velocity change per inch of barrel is linear, but not consistently identical per inch with everything.

[MDM]

I think it all depends on the barrel. I've went from 28 to 26 and didn't see any loss in a 6.5 creed. Went from 27 to 25.5 in a 6.5 Lapua and lost 40fps.

[natman]

There can easily be 25 to 50 fps difference between two people shooting the same ammo in the same rifle.

I assume you mean that there can be 25-50 fps difference between individual rounds from the same lot.

Otherwise you are going to have to explain how one person firing the rifle can cause velocity changes compared to another. Perhaps the first guy squeezes the trigger harder? ;)

[Brian Pfleuger]

No, one of the lets them gun recoil harder. A rifle locked in an immovable machine rest will shoot ammo faster than any human will shoot the same ammo and two humans will also shoot it at different speed. Of course, it's not a one shot thing. It's the average of several/many.

[tangolima]

I think it all depends on the barrel. I've went from 28 to 26 and didn't see any loss in a 6.5 creed. Went from 27 to 25.5 in a 6.5 Lapua and lost 40fps.

The barrel and the load. Long barrel with fast powder, little delta fps per inch, and vice versa. If you have a barrel that is way too long, shortening it can actually increase the speed.

-TL

[Metal god]

tag

[jimbob86]

Powder type and amount does make a difference. And as most rifles with all sorts of barrel lengths show flames coming out of the muzzle if observed in total darkness, that's usually considered proof that there's still powder burning outside the barrel. And with some loads of some powders, unburnt granules can be seen on the ground in front of the muzzle


+1

By choosing a faster powder than the ones at the top of the list for my 16" barrelled Ruger Frontier, I get a whole lot less flash and blast out of the muzzle, less recoil, and don't lose nearly as much velocity as one would think out a barrel 8" shorter than what the load data was developed in ....

[surveyor]

Same load, almost same jump to lands(within 0.006)
20" barrel semi auto
2683,2689,2688,2668,2706

30" barrel bolt action
2835,2828,2840,2835,2803

This is a 0.100 jump to lands and mag length

Velocities were higher at .020 jump, but I don't have a direct comparison.

I mainly went looking for a load for the 20" first, as close as the chambers were I thought it would give a little insight working a load for the 30"..

Not sure what the overall takeaway as far as barrel length vs velocity would be, as
They are diffrent rifles, actions and other factors involved.

These were 223..