Barrel Length vs. Velocity

msgriff · Sep 5, 2022

Many of us have seen this compared before, usually starting with a 30"-34" barrel. MDT came out with a fun, semi-scientific test a few days ago where they started with a 72" .308 barrel. I won't give any spoilers, but thought it was interesting enough to share the video.

Joe13 · Sep 5, 2022

I would think the bullet weight and powder etc would change the 'ideal' length but I didn't watch the video.

I mean bullet weight changes the ideal twist as well so it seems like there would be a point of diminishing returns or we would see a lot of competition guns with 72" barrels.

American123 · Sep 5, 2022

Barrel length does have an inherent effect on velocity, but it has diminishing returns and there is clearly a "long enough" length for each caliber (with respect to how it is loaded because the specific loading makes a giant difference).

What is often overlooked regarding barrel length is the effect on muzzle flash/blast. A shorter barrel typically will produce a significant fireball compared to a longer barrel. For anyone considering shooting at night, this is a major consideration. Suppressors can mitigate this a lot, but there is far less to mitigate with longer barrels than shorter barrels.

Sound is another consideration, for example, firing the 18" AR in 5.56 vs firing the 10.5 AR in 5.56, there is a big difference in sound perceived at the shooters ear.

The Heretic · Sep 6, 2022

I would not expect a "slow down" due to barrel length with ammo that is faster than mach 2-3

Member 3687 · Sep 7, 2022

I was rather shocked the the curve did not look more like a bell curve, slower to start at full length, then speed up as you cut down towards 45 inches then start to slow down as you loose barrel length until no barrel is left.

So I wounder at what length it would truly start to slow down, the so called diminishing return length, and what point would be the fastest at least for that round as it stayed pretty steady for the first 12 to 15 inches of cut off.

it was very interesting though.

Certaindeaf · Sep 7, 2022

This is why traffic barrel length is standardized at four bums.

wired · Sep 7, 2022

Im not surprised at all. I never did believe that longer barrels slowing the bullet down crap. That corked up gas is still high pressure and conce the bullet is deformed in the rifling it isnt that much of a push to get t to move. I do think they would have had significantly different results with a semi auto .

The Heretic · Sep 7, 2022

wired said:
Im not surprised at all. I never did believe that longer barrels slowing the bullet down crap. That corked up gas is still high pressure and conce the bullet is deformed in the rifling it isnt that much of a push to get t to move.

It is with centerfire rifle pressures - 40K at least, more like 50-60K PSI - that projectile will still be pushed enough to not slow down. Plus that peak PSI curve probably won't drop off enough that it won't be effective enough to keep pushing the projectile effectively vs. friction.

I recall an article in, IIRC, American Rifleman, about electronic ignition with (I think) liquid propellant (by Remington?). The idea was to reach a peak but not too high pressure, and via the electric ignition and the nature of the propellant (turned into a plasma?) the pressure was maintained for the length of the projectile travel to the end of the barrel, resulting in something like 5K fps?

Now if you had a low pressure cartridge, like .45 Colt (14K PSI), then you might see a drop in velocity with longer barrels (30"+?)

I seem to recall where .22 rimfire cartridges exhibit velocity drops beyond a certain barrel length.

TTSX · Sep 7, 2022

I would like to see this same test with a faster, higher capacity cartridge. Since this one was already .30 cal, I think doing a .300 WM would be "fair"