Ruger Mini-14 and Mini-30 Ruger Mini-14 and Mini-30 family of rifles

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Old 01-19-2012, 11:39   #1
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Question How does crowning a barrel improve accuracy?

When looking at target crowned barrels, all I see is where they have machined a bowl or a cone shaped recess into the top of the barrel....

So, what does that accomplish?
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Old 01-19-2012, 12:08   #2
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The recess is just to protect the crown from damage, i.e., bumping into something. It also helps protect everything else from the sharp edge of the crown.

The crown itself allows the powder gases to exit the muzzle evenly. If the gases on one side of the projectile exit sooner than the other side, the projectile will be "pushed" by the later-exiting gases and directed slightly to the other side. Wow, I hope that makes sense...because I know I know what I mean, but I'm not too sure in my ability to explain it.
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Old 01-19-2012, 17:29   #3
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Originally Posted by COBrien View Post
The recess is just to protect the crown from damage, i.e., bumping into something. It also helps protect everything else from the sharp edge of the crown.

The crown itself allows the powder gases to exit the muzzle evenly. If the gases on one side of the projectile exit sooner than the other side, the projectile will be "pushed" by the later-exiting gases and directed slightly to the other side. Wow, I hope that makes sense...because I know I know what I mean, but I'm not too sure in my ability to explain it.
That makes sense, I just couldnt figure out why cutting a recess into the tip of the barrel would improve accuracy, but I guess every little bit counts.......
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Old 01-19-2012, 18:11   #4
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An uneven crown..or damage to the crown causes the bullet to yaw(spin like a poorly thrown football) upon exit from the muzzle.
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Old 01-19-2012, 18:14   #5
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Originally Posted by COBrien View Post
The crown itself allows the powder gases to exit the muzzle evenly. If the gases on one side of the projectile exit sooner than the other side, the projectile will be "pushed" by the later-exiting gases and directed slightly to the other side.
That makes me wonder if muzzle brakes and flash hiders are not the antithesis of a shallow crown with respect to accuracy? Especially with something like the AK slant comp...

At least muzzle devices stop you from dinging the edge of the muzzle proper.
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Old 01-19-2012, 18:24   #6
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Originally Posted by COBrien View Post
The recess is just to protect the crown from damage, i.e., bumping into something. It also helps protect everything else from the sharp edge of the crown.

The crown itself allows the powder gases to exit the muzzle evenly. If the gases on one side of the projectile exit sooner than the other side, the projectile will be "pushed" by the later-exiting gases and directed slightly to the other side. Wow, I hope that makes sense...because I know I know what I mean, but I'm not too sure in my ability to explain it.
Thank you soo much for posting that. Ive always known that a f'd up crown decreases accuracy but always been afraid to ask why. Thats awesome
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Old 01-19-2012, 23:29   #7
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Likewise makes sense now I know, cheers for asking the question, and the great answer
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Old 01-20-2012, 01:19   #8
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I do not think there is enough pressure between a muzzle break, or compensator to effect the bullet's accuracy, but at the point where it exits the barrel, the bullet is in direct contact with the barrel, and pressure there is much higher, and does effect bullet "release" from the barrel.

I could actually see where a well designed compensator would enhance bullet speed, and accuracy. (Sort of the effect a well made/installed suppressor can result in increased FPS out of the same weapon/ammo, and improved accuracy.)

The "turbulent" gas is stripped away from, or removed from where it can effect the bullet as it goes down range after being acclerated, and spun up in the bore.

Then again, I think all things being equal, the shooter has much more to do with accuracy than the rifle/bore/round in most cases.
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Old 01-20-2012, 18:31   #9
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Don't think shooter has that much to do with accuracy. Any person gun person on this sight knows how to hold a sight on target and squeeze a trigger. Why do you think high level comp guns hav so many dohickys? The crown is very important if you do it with a file just aim in the general direction and hope to hit your target, do it on a mill or lathe with precision equip and expect great improvement. It's one thing to shoot from a res and a whole different can o worms to shoot unsupported from standing, then it might be the shooter.
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Old 01-21-2012, 07:59   #10
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Crowns are the last part of the barrel to influence the bullet. Whether it's a 90 degree crown, an 11 degree crown, rounded crown, etc. is really irrelevant most of the time. The termination of the lands and grooves needs to be exactly square all the way around so that the bullet and the gas pressure behind it exit evenly. Anything past the termination point is extra material to help prevent impact damage to the termination point. That's fine and well, but the termination can still be damaged by cleaning rods, corrosion from environment or erosion from gas pressure. Razor sharp crowns are generally undesirable - think about a sharp blade and how quickly it can dull, so most of us cutting crowns will cut something like an 11 degree crown and lightly chamfer it with a narrow 82 degree bevel to ease cleaning rod entry while still maintaining the right terminus (think of a drilled hole with a slight countersink). Even if you clean from the chamber, if you pull the rod back the brush/jag has to enter the muzzle.

So then we add a compensator, brake, flash hider, etc. These generally have a large expansion port inside, and the exit point of the part is preferably 0.020 - 0.022" over bullet diameter to allow for a bit of back pressure and keep the bullet from striking the part. Look at some parts and you'll see they are well over that specification. Since the manufacturer has no idea if you'll do the job with a file and die, or set it up correctly in the lathe they have to leave excess room to prevent bullet strike. This is one of the major factors in determining why one brand works so well and another similar one doesn't. So a comp or brake uses that short period of restricted forward gas pressure to redirect it and reduce recoil, but it doesn't really affect the bullet as it isn't supposed to touch the bullet which is technically already in flight and has achieved all the velocity it's going to and the gun has no more influence on it's performance.

Your factory Mini barrel has a 90 degree crown with a shallow bevel in it. Look, you can see it. Why not use an 11 degree crown? Fact is very few factory guns wear 11 degree crowns. I'm not sure how it came about exactly, but the benchrest community decided the 11 degree crown was the perfect shape some years ago. Someone smarter than me might be able to explain why. I cut 11 degree crowns because people want them, because they're called "target" crowns. Depending on the application I cut 90 degree crowns for ease of maintenance or better fit with a muzzle part.

Just remember to inspect your crown every so often, under magnification if possible. Even if you never fire the gun it will eventually erode due to oxidation of the exposed edges, and if you shoot a lot it will wear down from the gas pressures and cleaning chores. If the crown starts looking rough take it to your smooth for a touch up or use one of the excellent home tools available these days, but don't do it so often you wear the barrel down like a pencil in a sharpener.
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Old 01-21-2012, 08:30   #11
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Originally Posted by gundoc View Post
.... but the termination can still be damaged by cleaning rods....
Doc, I hope you are doing well. I'm no metallurgist, but I've often wondered how much damage a cleaning rod can do then the metal it is made of is waayyy softer that the metal of the barrel.

I do recall cleaning the M16 in boot camp and that the DI's weren't so worried about not putting the brush in at the exit end of the barrel instead of the chamber end.
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Old 01-21-2012, 08:52   #12
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It's not necessarily the cleaning rod metallurgy, but the abrasive particles adhered to it as it runs down the bore. Cheap aluminum rods are the worst offenders with soft metal and rough joints. Just like a brass screw with valve lapping compound can erode metal (cut a crown), a rod full of powder and copper residue can do the same thing - over a longer time of course. I like carbon rods, but I have a bunch of coated Dewey rods as well. I just keep a paper towel handy and every out and back of the rod I wipe it off with the towel.
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Old 01-21-2012, 18:11   #13
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Thanks Gundoc for the write up!

Learned a few things, confirmed some others, and gave me some ideas!
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Old 01-21-2012, 21:40   #14
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Great info. I had a gunsmith do a little work on my old 3" S&W and one thing he talked me into was the barrel crown. Had he explained it better beforehand, I probable wouldn't have bothered for such a short barrel...or does the barrel length not matter much?
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Old 01-22-2012, 04:15   #15
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Originally Posted by geigersd View Post
Doc, I hope you are doing well. I'm no metallurgist, but I've often wondered how much damage a cleaning rod can do then the metal it is made of is waayyy softer that the metal of the barrel.
As a follow on to Doc's answer, the rod may be an aluminum alloy, but the outer layer is Aluminum Oxide and that is quite hard (it's often used as the grit on sand paper and for "sand" blasting media).
Soft metals also hold grit/dirt quite well, which is why they use lead to hold the abrasive when lapping a barrel.

Personally I use a bare SS rod from the breach end, and remove the tool (brush, jag etc) when it exits the muzzle (IOW the tool only get's a 1 way ride through the barrel)
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Old 01-22-2012, 08:38   #16
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Originally Posted by Brian S View Post
...or does the barrel length not matter much?
Technically speaking every muzzle needs a crown. Even shotguns need the muzzle to be nice and square. As long as we continue to rely of gas pressure to launch bullets we'll need good crowns to make accurate barrels.

+1 Tailgunner - much better answer than I gave.
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Old 09-17-2014, 11:23   #17
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gundoc: As a Muzzle brake manufacture for Witt Machine, I could not have said it better myself. Thank you for passing along some great knowledge
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Old 09-18-2014, 02:00   #18
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Side note to the subject: In the same way that an even crown affects accuracy, so does the tail of the bullet. A bullet with a crooked or otherwise messed up tip will fly straighter than a bullet with a nick in the tail. This was proven in the "box o' truth".
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Old 09-18-2014, 02:13   #19
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Originally Posted by Brian S View Post
...Had he explained it better beforehand, I probable wouldn't have bothered for such a short barrel...or does the barrel length not matter much?
The shorter the barrel, the shorter the effective range of the given cartridge. This is because a shorter barrel doesn't allow all of the gunpowder to be burnt before the projectile leaves the barrel. This wastes the gunpowder by turning that energy into flame, instead of a propellant gas. This is why shorter barreled rifles have a larger muzzleflash, or fireball. Money down the tube. That said, I love my 20" barreled Lee Enfield Scout
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Old 09-18-2014, 02:24   #20
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Originally Posted by Capt. Killingfield View Post
Side note to the subject: In the same way that an even crown affects accuracy, so does the tail of the bullet. A bullet with a crooked or otherwise messed up tip will fly straighter than a bullet with a nick in the tail. This was proven in the "box o' truth".
Actually it was proven LONG before the BOT guys were even a itch in their daddy's pants.

See "The Bullets Flight from powder to target" by Dr Fredrick F Mann, published in 1909
The Bullet's Flight from Powder to Target: The Internal and External ... - Google Books

The above is something that ALL serious shooters should read, as it is the definitive study of ballistics
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Old 09-18-2014, 02:35   #21
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Originally Posted by Capt. Killingfield View Post
The shorter the barrel, the shorter the effective range of the given cartridge. This is because a shorter barrel doesn't allow all of the gunpowder to be burnt before the projectile leaves the barrel. This wastes the gunpowder by turning that energy into flame, instead of a propellant gas. This is why shorter barreled rifles have a larger muzzleflash, or fireball. Money down the tube. That said, I love my 20" barreled Lee Enfield Scout
Actually the powder that is going to burn will all be burned long before the bullet is near the muzzle, the fastest pistol powders will all be burned before the bullet leaves the case, with even the slowest rifle powder being completely burned within the first 12" of bullet travel.
What you loose with a short barrel is TIME. That is the time that the high pressure gas has to do it's work (IE: accelerate the bullet).
The reason for the increased muzzle flash? Hot high velocity gas. The longer barrel (gas expansion) allows the hot gasses to cool and slow. AKA "Expansion Ratio".
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