Ambivalent Skeptic

December 12, 2010

The 308 and the short barrel.

Filed under: Weapons — Clint1911 @ 9:22 am

The more I look at it, the more I see 16” barrels on rifles chambered in 308 Winchester or 7.62 NATO rifle as usually a bad idea.

I understand CQB is all the rage right now (CQB is Close Quarters Battle, although I prefer the British term FISH, Fighting In Someone’s House), and I know a guy who thinks he can make a rifle both an ultimate CQB gun and a sniper rifle.

I know how Mr. T feels about him.

Now, short barrels can be accurate, and if you want one go right ahead; just understand that there are some serious trade-offs.

First you lose velocity. Most loads have a listed speed from a 24 inch test barrel which may, or may not, give higher velocity that real world rifles. Along with that comes trajectory. The slower the bullet the progressively worst the bullet arcs during flight. Bullets work like American footballs. When launched from a height of 5 feet, in order to hit a target 5 feet off the ground the ball must arc up to a max height and then fall down to the target while still going forward. A thrown ball goes up many yards to travel a short distance, a bullet goes only a few inches to go few hundred yards.

However the slower the bullet, the more arc. This means at extreme range a mistake in hold-over is much easier and will cause a miss due to the bullet flying over the target (if one over compensates) or the bullet hitting the ground before reaching the target (in the case of under-compensating). You can correct for trajectory if you know the distance but the trick is knowing the exact distance. And the more arc to the trajectory the more important exact distance is.

Second, that loss in velocity reduces terminal performance. Let’s face it. When you shoot something, or someone, you want them to get hit pretty good. And the more effective the bullet, the better your shot placement translates to a downed target. And the less you get shot at.

A 16” barrel is going loss close to 200f/s up to 300f/s compared to a 24” and 20” barrels. (Depending a mitigating factors; each gun is an event unto itself .) Oh, and the effect is NOT linear. It is possible for a load that goes 2800 f/s out of a 24” barrel to do 2700 f/s from a 20” tube and only 2500 f/s for a 16 inch barrel.

And you lose velocity with distance traveled. Basically a bullet that is most effective at 2700-2800 f/s may not be very good at 2500 f/s.

Now, these reasons may not be much concern for a CQB sitch were you are defending yourself in an “across the street” or “across the living room” distances. But there is one aspect where a short barrel is very relevant to short range defense:

Muzzle blast.

Now, if I have to choose between deaf and dead, that’s a no brainer. (although some might choose dead over blind) But the point a self-defense is to ultimately go back to one’s “normal” life, no worst for wear.

So having my ears blown out does no good. And if you believe that hearing damage only hurts after the fight. Sorry, but no. Your sense of balance is dependent on the tiny organs of the ear. If those go kaput you can be severely disorientated and lose you balance. It’s somewhat like the early scene in Saving Private Ryan where Tom Hank’s character is disorientated after a near miss with explosives. You can reduce muzzle blast by using silencer. Unfortunately the government taxes and regulates what should be simply safety device pretty seriously even though, is a partial solution. So few own or use silencers, with is a shame.

So how much worse can muzzle blast become?

First, energy does not just disappear. It is either used one way or it is used another. According to the 2010 edition of the Hodgdon reloading magazine modern cartridges are about 17% to 47% efficient. In other words, the gunpowder (smokeless propellant) releases its stored chemical energy as kinetic energy to the bullet thus propelling the bullet forward. About 1/6th to 1/2 of the available energy is used for the intended purpose of pushing a bullet. The rest is “waste” energy with the 53% to 83% going into the gun or the air as heat and vibration (and a little bit of velocity). More on this in a moment.

The velocity/energy relationship is expressed by the equation E(k)=.5*m*v^2. Roughly speaking, an increase in velocity of 10% needs a 21% increase in energy. Or from a different point of view, one needs 21% more energy to gain 10% more speed. It also means if you drop 10% of your “nominal” velocity you have 21% less kinetic energy. So where does it go?

The short answer is -surprise- muzzle blast. The long answer is that the gun soaks a good portion of the the waste energy. The gun get warmer, the barrel will vibrate as the bullet travels along the bore, and the rifle with be pushed back against the shooter. A very small portion of the energy is transferred to the shooter for reasons best left for another post. For now, let’s leave it at a bullet with a kinetic energy of 3000ft-lbs going forward may have the rifle pushing back with only 20 ft-lbs.

When a rifle has a shorter barrel the gun won’t absorb as much heat due to the bullet and the burning power having less time in the barrel. The barrel also will not spend considerably more energy on vibrating. And due to less bullet dwell time, there may be less energy for vibration (however with a smaller barrel, there could be more vibration even with less available energy).

This means that all the increased waste energy goes into muzzle blast. Muzzle Blast has three primary components: Muzzle Flash, the light; Muzzle Report, the noise; and (for lack of a better word) Concussion which is the pressure wave coming from the muzzle.

All three of these things are disorienting when thrown in your face. And in-your-face they are because with that shorter barrel, the muzzle and hence muzzle blast are that much closer to your eyes and ears. And being human your instincts really want to protect your eyes and ears. In fact, muzzle blast is by far the leading factor in causing the dreaded “flinch.” A flinch is when your body protectively spasms as you use the trigger in a vain attempt to minimize any damage. Since keeping the sights on target until the shot is made is the most important factor in accuracy, you can see how a flinch is a very counter-productive. Especially if your life is on the line.

OK, let’s wrap it up.

With a shorter barrel you lose velocity with an increase in blast that will adversely affect your ability to shoot. So with a bullet weight of 150 grains going from a velocity of 2800f/s to 2600 f/s you lose 360 ft-lb of energy. From 2800 to 2500 f/s you lose 530 ft-lbs of kinetic energy. And we know where that energy is going.

That’s assuming the powder has burned completely, which maybe it didn’t. Once the bullet leaves the barrel the propellant has a fresh supply of oxygen so, yeah, even more muzzle blast.

Pushing a 150 gr bullet at 2500 f/s is the performance one would expect from the good, old, 300 Savage. Which was a shortened version of the 30-06. The 308 is a basically a lengthened 300 Savage at higher pressure. So by losing that speed we are losing a main advantage using the 308.

There is nothing wrong with 300 Savage cartridge. It is a good hunting round and its performance is half way between the 308 and 7.62 NATO (7.62×51) and the AK round, the 7.62×39. So it would make a very good combat/defense round.

Now if the shorter barrel made a 308 into a 300 Savage in terms of both velocity AND muzzle blast, hell, I’d prefer the shorter tube. But sadly that is not how it works. You pay for 308 Win, you have 300 Savage performance, and you get 300 Win Mag muzzle blast.

That’s a lose-lose situation.

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