What is counter boring?

[Irving]

Exactly what is counter boring? Why is it not desirable?

I tried to look it up before I asked, but the entry on wikipedia didn't cover relation to firearms.

[Backinblackrifles]

Counter boring isn't really undesirable it is simply where the inside of the muzzle end of the rifle is bored to a larger diameter to prevent damage to the rifling. Serves the same function as crowning really. It was popular in europe exspecialy with military firearms during WWII.

[Circuits]

It's actually to protect the crown - doesn't serve as the crown.

The crown is a uniform relief cut/angle at the very end of the rifling, to let gases behind the bullet escape evenly as it exits the bore, so they don't upset the flight of the bullet.

Counterboring is a depression cut at the end of the barrel, to make it harder for anything to hit and possibly ding the crown.

[Irving]

Okay thanks guys. I'll have to look up info on the crown next, as I've never been 100% clear on what exactly that is either.

[Backinblackrifles]

The barrel on the left is counter bored and crowned right side is just crowned.

[Irving]

Thanks very much. When I see people ask about damage to the crown of the barrel, they are just making sure that there is no change in the rifling, or any obstruction on the round right? For example, if the front of either of those barrels was totally rusted, as long as there was nothing obstructing the path of the bullet itself everything is a-okay right?

[Backinblackrifles]

When I look at a crown I look for damaged rifling dents burs and the sharpness of the lands the crown is the most important part as far as flight of the bullet is concerned should be nice and shiny because it is the last thing the bullet sees as it leaves the bore of the gun. Some crowns as cercuits said are used to try and make the exit of the round less violent such as an 11 degree target crown, But a hunters crown does nothing but protect the rifling it is all really interesting IDK if you can but you probably can find high speed camera footage on the internet of how diff. crown cuts can effect the bullet as it exits the bore.

[Irving]

If that footage doesn't exist, it should. I was mostly worried because the .22 rifle I inherited has some surface rust on the crown, but it isn't close to the rifling at all. I've been meaning to get a picture of it up on here. One day.....

[Backinblackrifles]

Sounds like it would just be a cosmetic problem.

[Troublco]

The thing is, counterboring effectively moves the crown back as far as the counterbore is deep. The muzzle crown ceases to have much effect on the accuracy of the gun. The new crown, as far as the bullet's concerned, is down in the barrel at the depth of the counterbore.

[Irving]

The only reason that matters is in relation to barrel length though right? I can't imagine only 1/4 makes that much difference, does it?

[Colorado Osprey]

In muzzleloaders it helps by setting miniballs and maxi-hunters in straight.
You don't really need a bullet starter with a counter bore.

Personally I have shot plenty of both, and love the counter bore.

[Circuits]

Whether the crown is angled or straight, what matters is that it's perfectly uniform and concentric.

Counterbored barrels do not have to have a straight crown - that only happens in the limited case where the counterbore diameter is exactly the bore diameter (as shown in the thumbnails above), but a counterbore doesn't have to come anywhere near the crown.

A lot of AR-15's have a very shallow counterbore, about half the width of the barrel, and only a tenth of an inch or so deep, in addition to an angled "target crown".