Naive question about counterboring

[Capybara]

I was cleaning my 91/30 (it is counterbored) last night, used my bore light and really took a good look at the bore for the first time in a while. This is a stupid question but am I correct in thinking that when they counterbored a Mosin, it was to put a clean exit point at the crown?

Mine is counterbored about an inch and a half but the rifling stops where they began the counterbore? So obviously the projectile is spinning nicely by the time it arrives at the non-rifled end of the barrel where it is counterbored? Does counterboring negatively affect accuracy? I have shot this gun just a few times and it seems fairly accurate but it just seems strange that the last inch or two of the barrel are a smoothbore musket.

Please educate me a bit about counterboring Mosins.

[paul0660]

The idea is the crown was worn, from getting stuck in the dirt or poor cleaning technique. Counterboring creates a new crown, within the barrel. Seems to work fine, and the last few inches don't matter, because the spin has been spun.

That is not what she said.

[Capybara]

That's what I figured. It is just weird to peer down the bore of a modern rifle and see some 1760s smoothbore musket. Especially in the 91/30, that barrel is so long, I would think that projectile is spinning pretty seriously by the time it encounters the smooth part.

[Anubis Laughed]

+1 for paul0660's comments. By the time the bullet gets to the counterbored portion, it already has more than enough spin to retain range and accuracy.

I've seen counterbored rifles sometimes passed off as 'special modification done for night fighting, to reduce flash", which is pure BS. It does nothing to reduce flash or recoil, and is just a way to repair an otherwise solid barrel with a damaged crown.

[paul0660]

That's what I figured. It is just weird to peer down the bore of a modern rifle and see some 1760s smoothbore musket. Especially in the 91/30, that barrel is so long, I would think that projectile is spinning pretty seriously by the time it encounters the smooth part.

It is, and it does not touch the smooth part. It is just surrounded by gas, theoretically evenly displaced by the reground crown.

[hermosabeach]

If you cut 1" or 2" off the barrell and then recrowned the rifle.... What would happen to velocity and accuracy?

We have access to amazing tools that were not available 30 years ago....
Manson reamers and the like....

If you had to repair the crown on thousands of rifles that got plugged with dirt, back boring would be an easy fix and would prevent future damage to the crown

I never really thought about the military flash hiders also serving as a crown protector...

[paul0660]

The rifle in my avatar is counterbored, and SUCKS as a flash protector.

[Justin Case]

The rifle in my avatar is counterbored, and SUCKS as a flash protector.

Ha! Yep, piss poor flash hiders

[Capybara]

I just bought an M44 specifically because I want a gun that shoots out a 3-4' flame, I need to roast some marshmallows.

[Flyin Brian]

Counter boring was the easiest solution to fix a bad crown... since they could not just chop 2" off the barrel like would be done on a modern hunting rifle. The front sight is usually within 2" of the muzzle, and they needed to keep the capability to mount a bayonet, so that is an easy fix.

I've got an old Finnish M24 that must have a 4" counter bore... that one is a trip to look down the muzzle and try to figure out where the rifling is

[C&Rtrader]

A counter bored rifle should actually have better accuracy "in theory" because it has a new crown. YMMV though.

If you are looking for a shooter a counter bore is a positive thing