Occasionally someone will ask me if the muzzle crown is all that important. In the past I’d probably say something like “only if you want the bullet to go where you’re aiming!”, but I’m trying to reduce my percentage of flippant answers. Today I’d put it more lawyer-like: “it depends…”
The crown is the edge of the bore at the muzzle. It’s important to point that out, because it’s not unlike the edge of a cliff. Once you’ve fallen over the edge, you have no chance to change your path (unless you’re Icarus, in which case I’d really like to talk to you.) The edge of the bore, where the rifling ends, is likewise the last chance for the barrel to properly direct the path of the bullet.
The edge needs to be perfectly perpendicular to the axis of the bore; if it’s not, as the bullet leaves the barrel one side might be clear of the barrel, but the opposite side will still be touching. This can introduce instability to the bullet, reducing the accuracy of the shot.
Even when correctly squared, a crown with a nicked edge can have the same effect. If the last thing that touches the bullet imparts any directional friction, like a nick or burr, the bullet path will be compromised.
It’s amazing now small an imperfection can affect the accuracy of a barrel. I recently had a battle of wills with a Mossberg M44US rifle. This was a target .22 that Mossberg sold on contract to the U.S. military back in the late 1940s. They have a reputation for being quite accurate, and every example I’ve ever shot held up that reputation – except this one.
I could not get a decent 5-shot group out of the gun to save myself. I tested 15 different loads in the gun, went over it with a fine-tooth comb, and still got flyers in every group. I looked at the crown, and it seemed perfectly fine, but still the gun wasn’t accurate. After exhausting every other possibility, I decided to recrown the barrel.
The edges of the bore seemed fine, but the first pass with the crowning reamer told the story: the crown was ever so slightly crooked. We’re talking perhaps a couple of thousandths of an inch, which isn’t a lot. I cut a perpendicular crown, and took the gun to the range.
Night and day.
The gun now shot like a 44US is supposed to! Beautiful groups from Wolf Match Target (aka SK Standard Plus, aka Lapua SC), which had shot no better than cheap Remington bulk prior to the recrowning. The crown had seemed to be a non-issue, even under magnification, but before and after targets proved that even tiny imperfections can make a huge difference.
-=[ Grant ]=-
- Posted by Grant Cunningham
- On July 20, 2009