I was reading another post and replies where the subject of non-concentric chambers came up. This triggered a memory of a conversation I had some years back with a well known 'smith, now a Guild member. I mentioned a problem rifle, an aniversity marked (1976) Ruger 77 in .243. Nothing I tried could get me better than 3" groups and often worse. I assumed one of the bad barrel jobs. This gent told me that he and his partner had checked many of these poor performing 77's and discovered that non-concentric chambers were common to them. They set the barrels back and re-chambered, and the results were often dramatic improvements in accuracy. Incidentally my Ruger .243 had a 1 - 11.25 rate of twist. It became a Douglas barreled .257 AI. Thought some of you might find this of interest.
Larkin

larkin

The title of your post suggests that non-concentric and out-of-round chambers are the same thing, but they're not. An out-of-round chamber can be just as accurate as a round one if the neck and throat are in line with the bore. A non-concentric chamber means that it does not share the same center or axis as the bore and the bullet cannot get a straight start on it's journey and will most likely result in lousy accuracy.

Either one is unacceptable in a rifle but not uncommon in factory offerings and even some Benchrest barrels made by less than qualified gunsmiths.

Ray

I'm gonna second butchloc's question of few days ago: exactly how does one create an egg-shaped chamber? I never saw a reamer shaped like an egg and I can't envision a rotating reaming operation that would cause such a condition. I am assuming the egg shape represents a cross section perpendicular to bore axis. Maybe that's my misperception?

tigger

Good point. My post is edited accordingly.

An out-of-round chamber is the result of a heavy handed barrel butcher reaming a chamber by hand.

Ray

I threads I refered to were about both non-concentric AND out of round chambers. I did not mean to imply they are the same.
Tiggertate: not an explanation but an experience (and embarrassing admission): Prior to having a lathe I had only hand reamed short chambered barrels. Once I started to re-chamber a 6mm Rem to AI version. I got impatient with the slow progress and chucked the reamer to a power drill. BIG MISTAKE! It opened up the chamber quick but produced an egg-shape chamber that I did not see until examining the first fired case. I think the high turning speed caused an out of balance situation. Anyway it was just a cheap Ebay barrel and I know a lot more now.
Larkin

Cheechako
An out of round chamber is not only caused by hand chambering. There are several other ways to accomplish this. One is when Bubba makes a mistake by chambering in two seperate setups. Second is by applying too much pressure with the barrel vise. The second reason is the most common and most likly in production situation.

Too much pressure with a barrel vise??
Butch

quote:

Originally posted by butchlambert:
Too much pressure with a barrel vise??
Butch

Me too!

Wouldn't an out-of-round chamber show up the first time you tried to insert a reloaded cartridge that was fired in that ssame rifle?

Unless you put it back in the axact orientation it was in when fired the first time? Perhaps FL sizing MIGHT make it small enough, but probably not?

Butch/Jim
I have seen a few barrels that were actually crushed by a barrel vice. In the hands of Bubba a barrel vise can be a dangerous thing. There are still people out there who think that a barrel needs to be installed with a four foot cheater bar.. I am not saying that this is always the reason, but it is one possible cause that is sometimes overlooked.