View Full Version : What does crowning a barrel mean?
May 9, 2002, 11:45 PM
Just curious, I heard that one cure for keyholing is to have the barrel of a gun "crowned". What does that mean? What is crowning?
May 10, 2002, 12:08 AM
Crowning generally refers to the contouring of the muzzle end of the barrel.
You can see it best in a rifle, where the crown is normally a roundish depression, with the exterior of the barrel extending up past the actual muzzle.
The idea is to both protect the rifling and also square the rifling so that the bullet is released uniformly from the rifling. If the crowning is off or damaged, it can have a seriously detrimental effect on accuracy.
Recrowning the barrel can be tried as an accuracy fix.
May 10, 2002, 12:11 AM
(as usual I'm sure that someone much more knowledgeable than me will be answering while I'm typing this in .....)
Essentially it is having the end of the barrel reground so that the grooves in the barrel end at the same point. If one is longer than another it will cause the bullet to tumble when fired, thus the keyholing. By having them all the same length it will (in theory) give a more stable spin to the bullet.
May 10, 2002, 12:13 AM
Yep, upon review of the thread I was right. Thanks Mike.
May 10, 2002, 12:15 AM
Mike nailed it. Greg too.
Accuracy and protection of end of the bore.
May 10, 2002, 08:33 AM
Mike has mentioned a target crown, but lets step back for a second. Manufacturers have learned that a particular ramp angle at the muzzle will deliver best accuracy. It's a counterbore from the muzzle into the rifling, and even if they are even, some angles have proven more accurate than others.
Rifles and high power pistols sometimes have to be re-crowned. That's because the hot, high pressure gases that push the bullet down the barrel escape irregularly as the bullet leaves the barrel, and may erode the barrel further.
Another cause of erosion that requires recrowning, if not rebarreling, is the erosion caused by a cleaning rod inserted from the muzzle. This is a common problem with the M1 Garand, for example, and sometimes a 1/4" has to be cut off, if the barrel can be saved at all. In the worst cases you may see keyholing, but more likely a rifle that was shooting 1/5" groups will start shooting 3" group over time. Throat erosion is usually less of a problem.
Most high power or service rifles will show a rounded shape at the muzzle, with the ramp not really visible. A target crown is recessed in a heavy barrel rifle such as a target rifle, to prevent the crown from being nicked.
On the Garand and similar rifles, the "bullet test" is used. A bullet from the standard M2 service ammo is inserted in the muzzle, and the distance measured from the muzzle to the case. If it goes all the way in, it's shot. A reading of 1/8" or more means it usually good for another 1,000 rounds or so.
When the previous owner of the CZ 85 Combat I bought complained that he could not get the pisto lto group well, one of the things I did was to have the barrel recrowned - it's usually $25-35.
May 11, 2002, 12:29 PM
what would one expect to pay for a rifle to be recrowned?
May 11, 2002, 10:01 PM
Hi, VVG and guys,
Cutting 1/4 inch of an M1 rifle barrel will put it right on the edge of failure to function. Gus Fisher recommends 1/10 as the max, and I agree with him. Trouble is 1/10 (or sometimes even 1/4) might not be enough to clean up badly worn muzzle. With an M1, wear almost always is from the cleaning rod. I have never heard cleaning rod wear called "erosion" and that is not a common use of the term. True erosion is seldom a problem at the rifle muzzle since the gas has cooled enough that what we usually call erosion rarely happens.
All in all, for an M1, the best solution is barrel replacement. With a manual action, or a gas gun with the gas port further back, backboring or severe trimming may well be an option.
A bullet test is a "quick and dirty" way to determine bore diameter at the muzzle, but a gauge is better since bullet ogives are not standard.
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