Barrel Length & Velocity-Extreme?

[Kawabuggy]

I have heard & read on many sites that a longer barrel will generate higher velocities as compared to a shorter barrel with the same load being fired.

This begs the question-at what point does a longer barrel's advantages give way to frictional losses?

If I fired a round from a 22" sporter barrel and recorded it's velocity, and then fired the same round (bullet weight and powder charge) from a barrel 30" then 40" & then 50" (if barrels of this length were available), what would the velocity look like at the longer barrel lengths?

Where do the advantages of a longer barrel end?

[SL1]

The experiment that you suggest has already been done with a 22 Long Rifle cartridge. The answer is that a barrel longer than about 16" adds nothing to the velocity and may start detracting from it.

But, there is little powder in the .22LR, compared to what we reload. I think that the practical answer for our centerfire cartridges is that you can't carry around a barrel that is long-enough to slow the bullet before it exits.

The more typical question is "How much velocity do I lose (gain) by having a barrel that is x" shorter (longer) than the data in this manual?"

We can answer that question pretty well for barrels of practical length if we know the cartridge, bullet and powder charge.

SL1

[Kawabuggy]

I'd still like to see the actual muzzle velocity of a 30-06 round out of a 50" barrel, and then compare that to a 22" sporter barrel.

I realize that if you had a barrel of that length, that you would use different powders to better utilize the additional time you have for pressure to act on the bullet. I just wonder what type of velocities could be achieved using the right powder(s) and an extremely long barrel.

And yes, I do realize that it is not practical to carry a 50" barreled gun around while hog hunting.

[Archie]

Kawabuggy, this is one of those recurring questions that only occur to those who actually think about what's going on inside the firearm.

SL1 has a grasp of the problem and is on the right track, but it's still more complex.

Bullet velocity at the muzzle (in conventional firearms) is dependent on three major factors and a host of minor factors.

Most obvious factor is the ratio of powder weight to bullet weight. In other words, the more powder (everything else being equal) the faster the bullet goes. That's why 'magnum' rounds always have bigger cases. Second obvious factor is expansion ratio; the ratio of volume of chamber space of an unfired round to the total volume of the chamber space, throat and barrel to the muzzle. People think of this as 'barrel length', and it certainly is the controlling factor, but it is in fact the space involved. Third factor is the burning rate of the propellant powder.

Barrel to bullet friction is a faction, but rather minor.

If one looks at the interior space of a barrel, it is obvious a larger bore diameter gains volume with barrel length quicker than a smaller bore. To word differently, a .22 bore gains much less volume than a .45 bore in the same length of barrel. Therefore, with identical chamber volume, the .45 has a much greater expansion ratio per length of barrel.

The complication comes in with the practical application. To compare a .45 anything with a .22 anything is difficult in this fashion. The lightest .45 bullet of which I'm aware is 155 grains. A 155 grain .22 bullet would be rather impressive in length; whereas a 40 grain .45 bullet would be odd looking at the least. Seating and crimping a 40 grain .45 bullet would be a real treat as well.

Okay, back to more or less reality. A longer barrel gives more expansion ratio, but that is limited by practicality. A 36 in barrel is pretty long. However, at some point, the pressure level within the system must drop low enough to not be pushing the bullet anymore. It is the pressure difference between the pushing part - inside - and the atmospheric pressure - outside - resisting the movement of the bullet.

This whole science of interior ballistics - which is what you're asking - is far too deep to adequately discuss here. I can recommend two good sources for your study and information. One is the book Pressure Factors by Dr. Lloyd Brownell, published by Wolfe Press. The other is the "Powley Computer" at http://kwk.us/powley.html.

The original Powley Computer was a slide rule sort of thing, designed by Homer Powley, a gunsmith, ballastician and freaking genius. If you explore this devise and read all the notes, you will begin to get an idea of how this works.

In short, there is no single "one inch of barrel increases velocity by X amount" statement applying to all cartridges. But it is a fascinating field of study.

[SL1]

Kawabuggy,

In order to do the calculation that you want, I would need to turn-on a barrel friction factor in QuickLOAD and tell the program how much pressure is needed to ovecome friction and slide the bullet down the barrel.

I can put in any number, but it is a case of garbage-in means garbage-out if I don't have a decent estimate of barrel friction.

I think there are a few measurements out there, and I would not be surprised it they were for a common bore like .308". But, I don't have the time to look for them right now.

If you want to do some research on that and give me (1) the bullet weight and shape, (2) the bullet diameter, and (3) the pressure needed to move the bullet down the barrel smoothly without acceleration, then I can give you some calc results to satisfy your curiosity. But, for the moment, I am more curious about how my taxes are going to come out for 2008.

SL1