muzzle crowns

[moooose102]

ok, i am trying to learn something here, so, please no wise cracks, i have read that a muzzle crown affects accuracy. i am curious how the shape of the end of the barrel can do this. i understand if it was not square, or if it was deformed and doing something (like a burr catching the bullet as it exits) to mess up the projectile as it leaves. but how does the shape effect accuracy? it seems like as long as it was square, and there were no burs, that would be that. i have noticed that most barrels have a nice radius, and my handi-rifle has a recesed 2 step pretty much flat end, i think they call that a target style. the bullet is going so fast at that point, how does the shape make any difference? why are there differnt shapes, if one was more accurate, then why not just use that shape? what about pistols? theirs seem to be pretty much flat, of course the barrel end is quite thin, so not much is probably possible.

 

[Chipperman]

The part that affects accuracy is just the edge where the rifling meets the crown. A radius just helps to protect the crown, it does not affect accuracy.

 

[Conqueror]

Recessing the crown, either via a "step down" or an 11 degree angle, or whatever, is simply a way of protecting the crown. If it's recessed, it's harder for stuff to damage it.

 

[Sniper X]

I concur. But, it DOES look very very cool! So, there are actually two reasons!

 

[siglite]

I saw the worst crown I've ever seen just recently on a romanian AK74 (WASR-2 I think). This guy was watching me shoot and I guess was impressed, so he came over and asked for help with his AK. I looked at his rifle and didn't see anything wrong. Then he showed me a target. Yup, the rounds were going through the paper nearly sideways. I looked at the crown, and sure enough, the entire thing was bulbous and misshapen. The powder patterns were all to one side, and the bluing was completely worn off on the other. He asked me how much a crown job costs. I had no idea what to tell him. I've never needed to get anything recrowned.

 

[Funderb]

answer to that ak problem- hacksaw, file, and a hammer.
that's how ak's were meant to be repaired.

 

[Chipperman]

answer to that ak problem- hacksaw, file, and a hammer.
that's how ak's were meant to be repaired.

Careful you don't end up with an unregistered SBR. Most AKs are already at the minimum barrel length.

 

[Funderb]

oh, well, you probably better off just grinding the end down a couple mm.
still remember the hammer. primary ak service tool.

 

[GhostlyKarliion]

the muzzle crown is the last part of contact with the bullet.

if you look at fired bullets, you will plainly see the rifling marks


so if the crown is waped or damaged then the bulet is going to go all screwey when it tries to leave the barrel.

now, about different shapes? I have never seen anything except squared off, sure some are recessed and some arent, but the basic shape should all be the same.

maybe a picture of what you are talking about?

 

[siglite]

Careful you don't end up with an unregistered SBR. Most AKs are already at the minimum barrel length.

Yeah, I wouldn't have even considered advising him to chop that barrel. I suggested a gunsmith, as a crown job shouldn't take a LOT of labor, and therefore might be pretty cheap to get done.

 

[redneck2]

One reason the crown has a big effect is gas venting. If there is some type of ding or groove in the crown that goes to the bore, gases squirt past the base of the bullet as it leaves the rifling. These can kick the base of the bullet off to the side and disrupt the flight. The 11 degree thing puts the bore deeper inside the end of the barrel to minimize the chance of a ding at the bore.

 

[mekender]

i would imagine that the shape of the crown could impact how the gas expands as it exits the barrel... im not fluid dynamics expert, but to me a smoother gas escaping process would lead to less turbulent air for to bullet to pass through as it exits the barrel...

 

[moooose102]

the family is sleeping now, i will try to post pics (if i have any luck) tomorrow.

 

[JohnBT]

Sometimes a visual helps in understanding the pressure waves involved and the need for perfect symmetry.

www.phschool.com/science/science_news/articles/revealing_covert_actions.html



"Scientists today can observe the shapes and speeds of shock waves on a grander scale than ever before. This image, taken as a .22-caliber marksman's pistol fired, reveals the spherical shock wave that emanates from the muzzle. - Settles/ Pennsylvania State Univ."

Aw, what the heck...

 

[NASCAR_MAN]

In the book "Rifle Accuracy Facts" by Harold R. Vaughn, the author indicates that no one really knows how the shape of the crown (i.e., flat, conical, recessed) affects accuracy. Consequently, the author's research has not drawn him to a conclusion as to what is the best shape - and neither has anyone else's research.

The author maintains that as long as the crown is symetrical and centered on the bore axis, it's shape should make little or no difference.

http://www.amazon.com/Rifle-Accurac...bs_sr_1?ie=UTF8&s=books&qid=1208434794&sr=8-1


In the book, khe author gave an example (photos) of a bullet exiting a high-powered rifle (a 270). At one bullet length (0.86 inches), the pressure acting on the bullet base was still ~4000 PSI (obviously, the bullet is still being accelerated) and the entire bullet well within the muzzle-blast jet. At four bullet lengths (3.44 inches), the pressure acting on the bullet base was still in the 100's of PSI (still, the bullet is being accelerated) and the bullet had emerged such that it's front end was outside of the blast jet. Thus, a asymetric muzzle jet (like one caused from an asymetric muzzle crown) would most surely accelerate the bullet off bore axis.

Hope this helps.

NASCAR

 

[siglite]

Yeah, that guy with the AK had a VERY asymmetrical crown. Visibly so. Plus, the telltale "powder residue all on one side" kind of confirmed it. Along with his sideways-flying bullets.