Is it possible for the bore and the chamber to be non-concentric ?

[cajun]

First clue was my bullets were engaging the lands unevenly. BHW said it was probably just a sharp edge on one side of the lands or a burr that would even out once it was broken in. But I was also getting pressure signs with moderate loads. That may have been caused by the short throat forcing me to seat bullets super deep and most loads were compressed. Accuracy was also terrible. BHW diagnosed the non-concentric bores and they eventually figured out that PT&G had incorrectly re-ground their reamer which they used on both of my barrels resulting in the short throat.

That just goes to show you anyone can have issues. I have a BHW 6.5 grendel thats a hammer.

 

[VinceMule]

Barrels are most likely the root cause. A lot (most?) of bar stock is NOT straight when new. IMO great barrel manufacturers check or straighten EVERY barrel BEFORE any machining process. (NOT just one per batch) If it's not straight before machining ain't nuth'n going to be straight or concentric after machining. Both V35BTC & Hummer70 allude to this & they're right-on. We must remember that the vast majority of barrels are mass-produced on machines designed to crank 'em out. (We like to picture a dude operating his lathe in his shop taking his time to eventually make one perfect barrel at a higher price. That MAY happen in some cases but not likely for most mass production) I worked for a major corporation for years in Quality Control & I can tell you that EVERYTHING affects a true bore, a straight/square steel stock, concentricity & bore diameters. Things like ambient temperature of the facility verses barrel stock temperature. Hydraulic fluid temperatures. (We never turned our pumps off!) Speeds & feeds. Tooling sharpness/wear. Even coolant temperature. (Most folks never stop to think: How can I get enough coolant on the cutter when it's halfway down the barrel?) Factors like this affect a uniform minimal rifling ID down the length of a barrel. As a drill or cutter heats up, ir expands, & makes a bigger hole. Sure, it can be fixed by the next operation but everything passes on down the line you see?) JMO
Everything matters....

There is a world of difference in gunsmiths in how they chamber.

As bore scopes become a standard fixture at the cleaning stations of serious shooters, many flaws will be found in barrels and qualtity of the chamber/throat reaming. There should be a separate category for Bore Scope pictures, which would up the game on what leaves the shop. Gunsmiths and barrel makers would scream to high Heaven over a category for "Bore Scope Pictures". Novices would learn volumes from bore scope pictures. It is probably best to:

See No Evil
Speak no Evil
Hear no Evil

It would be shocking to know how many gunsmiths actually bore scope a barrel prior to chambering, and how many inspect the chamber they just finished cutting prior to removing it from the lathe. The very best gunsmith I ever had poured a lead lap and ran it down the barrel prior to chambering and marked the outside of the barrel with the bore dia to the 0.0001 place. He was worth every -penny he charged.

 

[MagnumManiac]

Magnum, are you sure that that the decimal place is not off on this .015, .0015 would be a country mile!

This is correct, in other words, the centre of the chamber is .0075" off the centre of the bore. The rifle in question still shoots very, very well.

Cheers.

 

[MagnumManiac]

If the lands and the bore are off by .015", shouldn't the barrel maker exchange the barrel?

No, I chambered the barrel, and no, it was not caused by an indexing mistake, the reamer did what it wanted. Have seen this many times by other 'smiths too. It just happens sometimes.

Cheers.

 

[1861WHITWORTH]

I would think that if the barrel is centered in a lathe when the chamber is cut and reamed,
then the chamber would always have to be concentric with the bore, because that is how
the rifle bore itself is cut. Same with the barrel threads.
The outside surface contour of the barrel also must be concentric for the same reason,
although this is of somewhat less concern.

I'm trying to eliminate things to worry about. If the brass or ammo is out-of-round, that can be
dealt with separately and solved. I'd really like to believe that the barrel is never part of some
out-of-round problem.

I have a muzzle that was threaded non-concentric by a well known, well respected custom rifle builder here from CO (peyton area). You can physically see it just by looking at it.

He swears it can't be -- I wouldn't ever use him again

I have seen Remington 700 Sendero bores off center so much that after a gunsmith threaded the muzzle and installed one of his brakes the first shot fired from the rifle with the brake on, the bullet hit the brake and punched a half moon hole in the brake. My discussion with the gunsmith, I said you threaded the barrel and never checked or noticed how far out it was. His solution was to open up the center hole in the brake so the bullet would clear.

 

[L.Sherm]

Barrel manufacturers do not straighten barrels, it's true no barrel is perfectly straight from end to end. The Deep hole drilling machine is the major culprit on how straight a blank is from end to end.
This is why in my opinion a good smith who indicates at the throat is who I want chambering my barrels.

 

[ORTW1]

I would think that if the barrel is centered in a lathe when the chamber is cut and reamed,
then the chamber would always have to be concentric with the bore, because that is how
the rifle bore itself is cut. Same with the barrel threads.
The outside surface contour of the barrel also must be concentric for the same reason,
although this is of somewhat less concern.

I'm trying to eliminate things to worry about. If the brass or ammo is out-of-round, that can be
dealt with separately and solved. I'd really like to believe that the barrel is never part of some
out-of-round problem.

I have seen several Ruger with muzzle threads off center of the bore as much as .020. I found it when my customer wiped out all the baffles in his suppressor. Yes it can be.

 

[ORTW1]

Remingtons are anywhere from .006 to as much as .018 out side barrel to bore.

 

[BCC72]

There's a fantastic episode of the Hornady podcasts with Jeff Siewert (spelling could be wrong) that talks about this and many other items. Barrel specific.
He is one of the worlds foremost barrel testers and examiners for the military.
With your point in particular, I remember there being cases where non concentric barrels being more accurate than some that are due to 'disturbed vibration' (think harmonics) and a bunch of other idiosyncrasies.
Anyway, better to watch the podcast rather than listen to me.
Really informative.

 

[brian11]

It is absolutely possible. In fact, it's "probable", unless steps are taken to eliminate it. Most (and all custom) chambers are cut on different machines, at different times, by different people, than the boring and rifling operations. When the chamber is cut, it's a simple and routine matter to make sure the barrel is mounted in the lathe concentric to the bore. If this step is not taken, or taken in haste or done poorly, then your chamber is not going to be concentric with the bore.

 

[RuninL8]

Just to show the bore in relation to the OD of a barrel and then the difference between a lapped bore on factory rifle before and after pics. Also, the expectation should be less than .0001 runout on axial alignment which starts on the bolt face of the receiver all the way to the end of the cut with the chamber reamer.

 

[Taj]

I would think that if the barrel is centered in a lathe when the chamber is cut and reamed,
then the chamber would always have to be concentric with the bore, because that is how
the rifle bore itself is cut. Same with the barrel threads.
The outside surface contour of the barrel also must be concentric for the same reason,
although this is of somewhat less concern.

I'm trying to eliminate things to worry about. If the brass or ammo is out-of-round, that can be
dealt with separately and solved. I'd really like to believe that the barrel is never part of some
out-of-round problem.

I've been barreling rifles for over 50 years and have seen some very unusual barrels in that time. I used to cut factory barrels and install brakes and have seen plenty of barrels that were not concentric with the outside diameter of the barrel where the cut was made. Chambering set ups must be made on on the bore and not the outside of a barrel for that reason. Some gunsmiths rely on floating reamer holders to compensate for set up errors which is not a good thing. All you end up with is a tapered or oval chamber even when using a piloted reamer. A good gunsmith will set up his barrel blank in either a 4 jaw or 6 jaw chuck and indicate the bore with a close fitting pin inserted in the barrel that is at least as long as the intended chamber. The "stick out" portion of the pin will be indicated in two places. One close to the barrel end and one at least as long as the chamber out from the end of the barrel. A "spider" or some other sort of chambering fixture is used to zero out the pin in both location. This assures the chambering reamer enters the bore straight and cuts a chamber in line with the bore axis. After doing this it is not uncommon to see the out side of the barrel at the muzzle end runout several thousandths . It is really tough for barrel blank makers to drill and ream a perfectly straight hole the full length of a barrel blank. Some are better at it than others but they all have some runout.

 

[BillLarson]

I have a muzzle that was threaded non-concentric by a well known, well respected custom rifle builder here from CO (peyton area). You can physically see it just by looking at it.

He swears it can't be -- I wouldn't ever use him again

Again be careful realizing what your looking at.... most barrel bores are not concentric with o.d. at muzzle.... that said... if your looking for a smooth transition out at end of barrel with the brake or moderator... it requires 2 separate lathe set ups....

 

[Taj]

Again be careful realizing what your looking at.... most barrel bores are not concentric with o.d. at muzzle.... that said... if your looking for a smooth transition out at end of barrel with the brake or moderator... it requires 2 separate lathe set ups....

Bill is right on. This is very common.