Crown and barrel porting vs. Accuracy

[Badger Arms]

Another thread caught my eye about muzzle crown. Accuracy is effected by the crown, for sure, but why? Conventional wisdom is that high-velocity gas escaping behind the base of the bullet impart instability to the bullet to a slight degree, having an effect on accuracy... so here's the thought.

You can 'port' a barrel or drill holes in the barrel to release gas before the bullet exits the muzzle. The most common type of port is a gas port for operating autoloading firearms. There's also porting for shotguns where holes are drilled in rows behind the muzzle to compensate for muzzle climb and recoil. Some submachineguns are ported to siphon gas into the suppressor for increased effectiveness. A few rifles are ported near the muzzle by outfits like "Mag-Na-Port". The key is that the projectile is still in the barrel when gas is vented behind it.

Since pressure is being reduced behind the bullet, does that reduced pressure increase accuracy any? Can a longer barrel or a ported barrel reduce the disruption to the bullets flightpath simply by means of reduced pressure at the point where the bullet separates from the muzzle?

 

[Art Eatman]

In porting, not enough gas escapes to materially affect muzzle velocity. Think of the location of the ports; they're where the gas pressure is already on the downside of the curve.

For instance, the pressure at the gas port of a Garand is down to around 2,000 psi, whereas the peak pressure was around 47,000 psi.

A hickey in the crown can mean an off-center pressure differential across the base of the bullet--and for whatever reason it might not be uniform from shot to shot.

 

[benEzra]

Muzzle crown issues affect accuracy because the asymmetric forces induce nutation in the bullet (remember that the gas pressure behind the bullet at the instant of muzzle exit can still be 3000 psi or more):

http://www.exteriorballistics.com/ebexplained/4th/46.cfm

Think of a badly thrown football that wobbles as it spins; a bad crown can cause a bullet to do the same thing, and the resulting aerodynamic forces open up the groups. The slant brake on an AK does the same thing (off-axis forces), which is why an AK with a target crown or a flash suppressor will generally shoot somewhat better than one with a slant brake.

Porting doesn't cause nutation because the bullet is still in the rifling as it passes the ports. UNLESS the porting is badly done and a sharp edge of the port shaves the bullet jacket, messing up its aerodynamics a little.

 

[Badger Arms]

... Agreed. I think I understand the forces well enough which is why I think that this would work. So let's assume 2,000psi at the muzzle. If we radically port the muzzle, say twenty 3/32" holes about 2" from the muzzle, then we would lower the pressure behind the bullet at the muzzle and get a 'cleaner' separation. Should mimimize the disruption from the gas escaping at the muzzle. That's my theory at least. All other things being equal, 200-500psi at the muzzle should disrupt the bullet less than 2,000psi.

Crown is important, yes, but all that gives you is concentric forces. If you bevel the base of the bullet to 10 degrees, you're going to screw up your bullet every time, methinks. Same thing if you cut your muzzle on a slant. But, if you cut your muzzle AND bullet on a slant and there were ZERO pressure (theory) behind the bullet, your bullet should be as accurate as if you had a concentric bullet and crown, correct?

 

[benEzra]

Theoretically, yes, reducing the pressure would reduce the potential for disturbance at the muzzle. But you can never reduce the pressure to zero; even after the bullet is an inch or two past the muzzle and the static pressure has dropped to ambient, there is significant dynamic pressure on the bullet base from the several-thousand-mph tailwind it is experiencing. For that reason, a good muzzle crown or flash suppressor entry and a smooth-base bullet will always be important for the best accuracy. I believe boattail bullets also help reduce off-axis aerodynamic loads as the bullet exits, but I may be wrong there.

Porting can also improve your accuracy by reducing muzzle jump and felt recoil.

The big downside of porting is NOISE and increased blast. A ported gun is far louder than an unported gun.

 

[marsche]

If it would work or if it would not work - this appears to be a solution to an easily corrected problem. Rather than drilling holes in the barrel to put a band-aid on a dinged crown, why not just square off the muzzle and recrown it. On the other hand, if it is simply a curiosity, ding up your muzzle and try it.

 

[Badger Arms]

Well, the point was to squeeze accuracy out of a gun, not to avoid recrowning a barrel. The operative phrase would be "all other things being equal". What I mean is that if two guns had identical crowns and were in all other ways equal, would a proper porting job decrease groups by nature of reducing the pressure behind the bullet at the point it exits the muzzle.

But you can never reduce the pressure to zero;

On the contrary, the pressure can be reduced to zero or even negative pressure. Captive-piston or "Silent Cartridge" systems have been used whereby the propellant gas is contained in the cartridge to either be slowly vented or just remain trapped. The Russians had one in production. I suspect that this actually created a vacuum of sorts as the bullet actually pushed air out the front of the barrel and sucked it in the rear as it travelled down the barrel.

That's academic, though, because pistol cartridges aren't intended to be 'bench-rest accurate.'

 

[benEzra]

On the contrary, the pressure can be reduced to zero or even negative pressure. Captive-piston or "Silent Cartridge" systems have been used whereby the propellant gas is contained in the cartridge to either be slowly vented or just remain trapped. The Russians had one in production. I suspect that this actually created a vacuum of sorts as the bullet actually pushed air out the front of the barrel and sucked it in the rear as it travelled down the barrel.

OK, the pressure in a firearm using non-encapsulated propellant gases cannot be reduced to zero. I suppose a railgun could have a vacuum behind the bullet as well, but we were discussing firearms powered by hot, high-pressure gases accelerating a bullet.

 

[Badger Arms]

OK, the pressure in a firearm using non-encapsulated propellant gases cannot be reduced to zero. I suppose a railgun could have a vacuum behind the bullet as well, but we were discussing firearms powered by hot, high-pressure gases accelerating a bullet.

Of course, the point is to squeeze fractions of MOA out of a bench-rest gun. Any advantage gained in any other type of a firearm are drowned out by the dozens of other factors that degrade accuracy. I hear alot of different things about sleeving the receiver, tensioned barrels, cryogenic treatments, muzzle crown variations... I'd never heard of porting as an accuracy tool though.