The crown is the very end of the barrel where the rifling ends. It's the last place where the barrel affects the bullet, and if damaged accuracy tends to go to hell. Even tiny burrs or dents where the rifling ends can either damage the bullet slightly or let gas leak out at that spot, throwing the bullet off-balance by just a fraction.
Yes, this is one of my patented, "you can do it yourself but don't yell at me if it don't work" posts. Cleaning up the crown is actually fairly simple, and can be done without a lathe or other fancy(read 'expensive') tooling.
The most basic is you find that, after being dropped, the crown has a little ding in the corner of the bore. Or you find that someone cleaning from the muzzle(and some rifles and pistols have to be) let a steel rod ding the crown. Assuming that's all you have to repair, you've got two ways.
There are cutters made just for this, like this one from Brownell's:
It's a tool steel body with a handle that screws on. Pilots are made for different bore diameters and held in place with a set screw. They make one that's about 3/4" wide and square, for making sure a newly-cut muzzle is true, and a couple of smaller diameter ones with the cutting flutes at different angles, depending on what crown you want. This one is 79 degrees, and that's a .311 pilot for a .303 British rifle. The first step with either of the methods I'm using is make sure the damn gun is unloaded; it'll really, REALLY ruin your day/week/life if it's not and it fires while you're doing this. Then put a tight patch on a cleaning jag and run it up to about 2" below the bore with this cutter and leave it there. With a clean-from-the-muzzle arm, I run the jag in the appropriate amount, then unscrew the rod from it; in either case it'll keep shavings, cutting oil, whatever from falling down the bore and into the action.
With this cutter/guide setup, use a small paintbrush to coat the cutting flutes and crown with oil, actual cutting oil being the best but just plain oil will do, it helps the cutter turn and cut smoothly. Ease the pilot in until the flutes contact the crown and then, with light, even pressure turn it. ALWAYS CLOCKWISE. If you turn it in reverse it can cause a burr to be rolled up on the cutting edge which pretty much ruins it until it can be honed to remove it. Turn it two turns, then take it out. Wipe off the shavings from both pieces and inspect. If you need to cut a bit deeper, oil and repeat.
When this is done, you may be able to use it as is, but you'll generally need to polish the new crown to get rid of the tiny burrs the cutter leaves. Which brings me to the next method, which can be used all by itself to clean up a crown. This high-tech method is
Yep, that's a round-head screw in a drill. Brownell's makes a brass piece just for this, or you can use a small carriage bolt(make sure the head is smooth) or round-head machine screw of steel or brass. I read of one gunsmith who takes steel ball bearings, anneals them, drills a hole in and solders in a piece of drill rod for a shank; he thinks the hard ball works better than a softer metal. Some swear by one with a screwdriver slot, as it'll hold some of the abrasive. They all work, as long as the curve of the head fits into the muzzle properly; small enough diameter & curve to contact the crown, large enough to not be able to slip into the bore. Power can be a cordless drill like this- as long as it is variable speed- or a crank-type hand drill. And you'll need some abrasive compound. I'm using some valve-grinding paste from a tool or auto parts store, this kind comes in a two-sided can with one side coarse and one fine. I've never needed any but the fine stuff.
If you're polishing after using a cutter, pull/push the patch out to clean oil & shavings out of the bore, clean it out then put in a fresh patch. If just using this method, put your patch in. Secure the gun or barrel so it's vertical. Dip the head of the screw or whatever into the compound. Gently set it into the crown and start it turning slowly. I repeat, SLOWLY. You'll be able to feel the compound grinding at first; when you stop feeling that, remove and dip the head again.
No, it's not vertical here; I'm holding with one hand and shooting with the other, so shut up.
Don't just hold the drill vertically, after the first bit start moving the drill in a small circle. After the second time I coat the head I put it in reverse and continue for a minute. The idea is to make sure the cutting occurs evenly around the crown and polishes both sides of the lands and grooves to get rid of any burrs or roughness.
Take your time. After running it both ways, set the rig aside and clean the crown off and inspect it. I use some cotton swabs, put the end in the bore, preferably without touching the compound, and use it to swab the stuff out of the bore. When it's done you'll have a clean, bright ring where the rifling ends, nice and smooth. Use a bright light and a magnifier of some kind to check it out. Remember, if you just had some small dings or a touch of wear it may only take a minute or two of this to completely remove the damaged section, so check often.
When it's done to your satisfaction, use more swabs and patches to wipe off all the compound you can, then pull the patch out to remove anything it caught. Clean the bore thoroughly to make sure you don't leave any trace of the abrasive, and oil. The mechanical part is done.
At this point you can use a bit of cold blue to get rid of the shine, or leave it as is and the next shooting you do will take care of most of that.
That's pretty much it for the basic crowning. The Serious Recrowning would be with, for instance, an old military rifle that was often cleaned without a guide, and the rifling for the last inch or two of the bore may be ruined. The only fixes for that are to either counterbore- basically cutting a cone out of the bore from the muzzle back for a couple of inches, lathe required- or cut an inch or two off the barrel and recrown. Which would ruin it as far as collectibility and authenticity and require putting the front sight back on. If you've got one like that you'll have to decide what you want to do.
Years ago I bought an old Turkish Mauser to turn into a hunting rifle. When I first shot it the best groups it would get were about 6-8" at 100 yards. I cut the barrel off right in front of the front sight, which only removed about 1/2", squared and crowned it and with the same ammo groups shrunk by half. In the end I took five inches off that 29" barrel, which removed all the damaged bore and that rifle will shoot 1" groups or a touch better, depending on ammo and if I'm doing my part that day.
This is one of those "It's simple, but it has to be done right" things. Most gunsmiths don't charge too much to cut & crown a barrel, so if you're worried about keeping things square that may be the way to go. If all you need to do is clean up or recut the crown itself, it is something you can do.
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