About Barrel Length vs Velocity

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3006loader

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It seems to be common knowledge that a longer barrel gives more velocity than a shorter barrel. But is there a point where a barrel can be so ridiculously long that it would actually reduce velocity because of the friction on the bullet's bearing surface? Does anyone know what this barrel length would be for a given caliber? I searched the interwebs but I couldn't find anything that didn't have to do with "the great barrel length debate," which in regards to this question I couldn't care less about. Any help is much appreciated. Thanks.
 
I'd think and I'm by no means an expert . It would really depend on the caliber , bullet , powder burn rate , charge and barrel length . Example : I've seen lately guys shooting/loading reduced 223 loads using what would be considered pistol powders . Now I truly have no specific knowledge on this but using the fastest burning powder you can in a rifle caliber with a very long barrel . You may see the bullet actually decelerate while still in the barrel .

Or maybe a pistol caliber with the fastest powder being shot from a long rifle barrel could do the same .

Interesting thought though . Maybe one of the guys with quickloads can play around and see if that can actually happen .
 
I've seen data for 22 rimfire ammo indicating that barrels of about 24" or longer will often be slower than shorter barrels. There are a lot of factors to consider and this is not always the case depending on the individual rifle and ammo used.

With centerfire rifles somewhere around 28"-30" is about as long as you'll see a barrel and they are still gaining velocity at that length, albeit very small gains. But I'm certain that you'd reach a point where you'd start to lose speed. I just don't know where that length is and since no one makes a barrel long enough to test the theory it would be no more than a SWAG. It would be different for every cartridge; even different with the same cartridge with different powder/bullet weight combo's. There is probably someone, somewhere who has run numbers through a computer program and come up with an approximate theoretical number.

Manufacturers have pretty much standardized barrel length by cartridge for a reason. Most standard cartridges in the 30-06 family have 22" or 24" barrels. You can make them longer and get more speed, but the gains above 22" are quite small and not worth the negatives. All magnum cartridges are 24" or 26" for the same reasons and you'll find cartridges in the 30-30 and pistol caliber class work very well at 20" or less. The only reason to go longer than 24" or 26" is to squeeze that last few fps out for extreme long range target work.
 
Looking at ballistics by the inch it would seem that 16" is the max for .45 ACP. From 12" to 16" the gains are minimal and at 18" the velocity starts to drop.
 
I was in the belief that for every inch of barrel add or subtracted would yield an approx 20-25 fps change in real life velocities. As of late im confused by my chrono data. My r700 26" varmint 308 would yield about 2550 fps with 41.8 gr of w748 pushing a 168 match bullet. My 22" M1A with 41.5 gr of H4895 same 168 match bullet is chronoing at 2630 fps ? Now i understand im comparing two different powders but still my data seems to be backwards as far as velocities?
 
I would say any length after your powder is completely burned is entirely useless. How far after that until it starts to slow down? An inch? Two? Not sure

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I would say any length after your powder is completely burned is entirely useless.
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Those uses can include:

1. reducing noise,
2. increasing sight radius,
3. decreasing deviations in muzzle velocity,
4. decreasing velocity to a speed that produces greater accuracy,
5. changing weight and balance to suit a shooter's preference.
 
nhyrum said:
I would say any length after your powder is completely burned is entirely useless.
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No. As long as the gas pressure in the barrel is high enough to apply more force to the bullet base than is needed to overcome friction, the bullet is still accelerating and more barrel will give you more velocity. The gas pressure is still there when the powder burns out. Only when the pressure has dropped so low, due to the expanding volume it occupies behind the moving bullet, that the force it applies to the bullet base merely equals bore friction, will acceleration cease.

For the 22 LR that occurs at between 16 and 20 inches down barrel for most loads, which range from mild pistol target to hyper velocity loads. Once you pass that limit, velocity starts to drop, but it drops slowly. This is why, for calibrating a chronograph, you can generally count on match ammunition to be within 50 fps of the box stated velocity in about any rife chambered for it.

For the 22 LR the expansion ratio is huge. At 20 inches it is almost 50:1. The volume the powder is burning in grows that much before the bullet starts to slow.

For the much larger case capacities, an expansion ratio that large requires a very long barrel, indeed. A 30-06 with a 20" barrel has an expansion ratio of around 6:1. So the barrel would have to be about about 8 times longer (160") to match the 22 expansion ratio at 20". The acceleration behavior should roughly match at that point, assuming lubricated lead bullets and identical bore condition in each gun. That said, in a large chambering there are other factors involved that mess with the number, like having to push a large powder mass down the bore in the form of gas. QuickLOAD's simulator says a 170 grain lead bullet driven by H4895 at about the same peak pressure as a 22 LR would start to slow at about 120" of barrel. Due to its higher friction, it says a 165 grain jacketed bullet driven by a full load of 4350 would start to slow much earlier, at about 50" of barrel.

That seems like a reasonable estimate. We know from shooting .308 Win, that you lose about 25 fps per inch of barrel when you are near the 24" SAAMI test barrel length. But in a 30" Palma barrel it's down to 15 fps/inch due to the greater expansion causing an inch of travel to be a less significant percentage of the total volume.

Here's an approximation I worked out awhile back that comes pretty close. Note the difference in medium power and overbore due to the latter having smaller expansion ratios at any given barrel length.

Barrel%20Length%20Velocity%20Approximation_zpsvmsocp0k.gif
 
UN , am I to take from that slower powder burn rate is more likely to cause this effect or at least a slower powder would need less barrel length .

My thinking ( and really just that "thinking") a very slow pistol powder like longshot or AA 2400 or faster if doable in 308 would burn up fast and as the bullet traveled further down the barrel after powder stopped creating gas you would need less barrel length create this effect . Or maybe 45/70 with similar fast powder .

Then let's say there is a working firearm , load that actually can do what is being talked about here . I'd think you'd have a good chance of losing chamber pressure at the same time allowing the case to constrict back to extractable size resulting in gas leaking at the breach
 
I was in the belief that for every inch of barrel add or subtracted would yield an approx 20-25 fps change in real life velocities. As of late im confused by my chrono data. My r700 26" varmint 308 would yield about 2550 fps with 41.8 gr of w748 pushing a 168 match bullet. My 22" M1A with 41.5 gr of H4895 same 168 match bullet is chronoing at 2630 fps ? Now i understand im comparing two different powders but still my data seems to be backwards as far as velocities?
Click to expand...

There are 2 things going on here. First, the number of 20-25 fps/inch of barrel length is somewhat accurate with barrels between 20"-24" with 30-06 class cartridges. On barrels longer than 24" the number is much less, closer to 10-15fps/inch. With barrels less than about 20" you see numbers greater than 20-25fps/inch.

Magnum rounds will see greater differences per inch and 308 class cartridges will see less.

The 2nd variable is trying to compare data from 2 different rifles. The above data is only relevant if I take a long barrel with a known velocity and cut it shorter and measure speed at each length.

When you are comparing 2 different guns, with equal barrel length and shooting ammo from the same box 25-50 fps difference is VERY common. I've seen as much as 130 fps. With 4" of barrel difference it would be rare to see the 22" gun beat the 26" barrel with the same ammo. But coming close wouldn't. I have seen 20" guns shoot FASTER than 22" guns and match 24" guns with the same ammo before.
 
"...point where a barrel can be so ridiculously long that..." Yep. Think Physics.
".. "the great barrel length debate,"..." Has been going on since before any of us were born.
 
I would think it all depend on powder volume and the size of the expanding gas it produces.

Like comparing the 22 to the 45, the 45 will accelerate from a longer barrel than a 22.
 
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