How Barrel Length Affects Velocity

From GunCounter

By Combat Controller

So, in my never ending quest to find something to entertain you all with, I set forth to answer the question of how much does velocity change due to barrel length? This is by no means an exhaustive test, and one I will repeat, when I fix my chrony which took a bullet for you guys. Thank Vulcan Fast Rope had his with him.

In short, I took handloaded .357 rounds and shot 5 shot strings out of a 2,3,4,6,8,16 and 20 inch barrels to see how velocity is effected by barrel length in a real world application. As noted, I will try this one again when I load up some more ammo, and with another powder perhaps.

It was a fairly middle of the road piddly loading, 10.8 gr of AA#7 with a 158 gr JSP in new brass with CCI magnum primers. I was going to shoot more guns with shorter and in the middle length barrels, but the clouds hate me and readings got squirrelly. All readings are in feet per second.

2” Barrel AV:1027
3” Barrel AV:1015
4” Barrel AV:1110
6” Barrel AV:1178
8” Barrel AV:1214
16” Barrel AV: 1405
20” Barrel AV: 1449

I was not real happy with this load, I think with some 2400 it might smooth out a bit and bring the standard deviation down under 50 fps.

As you can see the 2 and 3 inch barrels did not have much difference, and therefor need to be retested with a more statistically significant number of rounds. say 10 each instead of the 5 I used for this test.

Adding two inches over the baseline gives us almost 100 fps better velocity and another two almost 70. the gap got smaller with the jump to 8 inches, but like I said, I was not happy with these loads and the extreme spread was 164 fps on the 8” which brought down the average. ditto the 2” it had almost the same extreme spread and has skewed the numbers a bit. notice the doubling of barrel length brought an instant 200 fps increase. 14 inches gives us almost 400 fps so we are looking at an average of 28 fps per inch of barrel in this test. I will rerun it and see if that changes. I will then also militec the barrels of these guns (none of them have it yet) and we can watch the velocity climb again in a reprise of last years successful (and yet lamentably eaten by the forum software) test. We have a larger baseline to work from now, and all in the same caliber. We can also do it with .308 rifles and show how cool this stuff is.

I hope this was educational and helped to illustrate why longer barrels are good, but shorter can be fine.

For those of you new to the whole thing, the reason we get more velocity is the gas has longer to push the bullet before being expelled, therefore making it go faster. How much faster has just been demonstrated on a small scale.

The guns used were
Ruger SP101 2 inch
Ruger Speed Six 3 inch
Colt Python 4 inch
Colt Python 6 inch
Colt Python 8 inch
Puma Levergun 16 inch
Winchester Levergun 20 inch

Follow Up discussion:

Question 1: At what length do you get maximum velocity, and is that barrel length practical? Viz, would it be 68” & therefore silly?

Answer from Chris Byrne: For rifle barrels in conventional chamberings, somewhere between 28” and 32” seems to be the maximum gain. Anything more than that and the lack of rigidity in the barrels (even in 2” benchrest barrels) more than offsets the very small increases in velocity.

Pistols dont seem to see much improvement beyond 8”, again because of rigidity; though some heavily constructed pistols fired from a rest may see improvements out to 16”.

Fundamentally, the more rigid the design and materials, the longer your barrel can be without inducing more inaccuracy.

For both pistols and rifles, it’s very much dependent on the chambering. The more powder, and the slower the powder, the longer a barrel it can take advantage of.

Question 2: Can you tune a load so that on that bell curve you get the maximum possible velocity with your barrel length? Or would you have such high pressures that it damages the firearm?

Answer from Chris Byrne: Absolutely. Every barrel, at every length and thickness has a “sweet spot”, where the pressure and velocity curves match the resonance curves of the barrel to produce both excellent velocity, and the smallest possible magnitude oscillation in the barrel; for best accurate velocity.

The problem is, it’s not easily predictable; and is effected by temperature, pressure, bullet design and weight, and the burning profile of the powder.

What you CAN do however, is control your experiments, altering one variable at a time. First, load for as complete a powder burn as you can get with a given bullet weight and barrel length, and a given powder. Now move up and down the scale in 2/10th gr. increments until you hit minimum and maximum loads, firing 10rds at each load level from a stable rest, and noting the results.

Typically speaking, most barrels and most calibers produce their best accuracy between 10% and 20% below max load; though with extremely rigid weapons, that margin moves upwards.

Follow up by CC: You can optimize a load (like I did) for leverguns, and one for pistols. You can also come up with a compromise load for both. I have a nice 300 gr load that works much better out of my .44 mag rifle than any of my pistols. It is powder dependent on where the sweet spot is, but slower powder will generally give better velocity in a rifle.

As for that test, it was a .44 mag with an 18 inch barrel. It came out at about the same as my test IIRC.

Question 3: What velocities do you need with that bullet to deliver effective energy to a goblin? I guess what I’m really asking is, “What length gives you acceptable effect on the recipient of your high speed little gift?”

Answer by CC: Those velocities are not so bad out of a pistol, out of a levergun they are fine too, but you can get them really cooking. A 1300 fps 124 gr .357 load will cause a lot of havoc.

Question 4: What’s militec?

Answer by CC: Militec is a lubricant that works via the sacrificial method, but will increase velocity in a barrel, in several documented cases, 100 fps better after treatment. I ran an extensive experiment with several guns, and documented it all, over a chrony, and it proved itself in rifles and pistols. IT is a popular lube in Iraq. I like it a lot, but actually found a better lube (and a rust protectent as well) that is not sacrificial, so I like it better. It is an industrial product but you can buy it as a combined product as Plate + and Machine Gunners Lube. It is great stuff.

Question 5: How do you know you’ve gotten a complete powder burn? Are you watching for flames out the barrel or something?

Answer by CC: Often that is a great way to observe unburnt powder. That is why shorter barrels have more muzzle flash, and certain ammo.