Why do SXS barrel bore diamiters vary?
 

I have been looking at several American SXS guns and notice that bore diameter can very by 10K from each other? If gun is factory original why would that happen? or is this not normal practice and avoid guns with wide variations. Say a 16ga....left is .655 and right is .668 Would this be indication of hone job? Should it be a deal braker if all else is in spec? I would think that a proper bore job would be the same for both tubes or are the barrels bored prior to jointing them together and luck of the draw? thanks all SXS ohio My question is...did the barrels leave the factory bored the same in each tube? Like .662 and .662 ???/ Thanks all SXS ohio....(-:

Kenny,
When I see a gun with one barrel more than a couple of thou larger than the other I assume the larger barrel was back bored to increase the choke in that tube. This may not always be the case but I have seen barrels that lettered Cylinder or IC that are now bored .010 over nominal and have .010 or more of choke.

SxS:
Much depends on the type and age of the gun in question. But, in general, yes. Both tubes should be within a couple of thousandths of the same spec. But remember... you a talking about a shotgun. It is intended to throw a cloud of pellets. and the charge expands to fill the bore on its way out - which is the principle of back boring. A little "tolerance" is not going to matter.

In the final analysis, the more metal you have in a SxS shotgun tube, the better. General rule. So... if one tube is thicker than the other, be grateful. And yes, it could well be the result of responsible honing that took out only as much metal as needed.

Bottom line: Don't let any of this worry you.

But always be certain of the barrel wall thickness before you buy a gun with questionable bore diameters.

I agree that generally each bore should be within say .003" diameter of the other. But keep in mind that most shotgun makers bought rough tubes to their individual specs from suppliers, and an overrun for one maker could be a bargain buy for another maker. And thus, tubes with different bore diameters could find their way to the storage racks of some of the makers. Generally the barrel assembler would choose two tubes of like bore diameter for a barrel set but that may not have happened all the time. For example: an Ithaca 16 gauge on my bench now that has right tube at .685" bore diameter and choke .671 = .014" constriction. Left tube .674 bore diameter and choke .646 = .028" constriction. Gun is stamped 2 & 4 indicating Mod & Full and the choke diameters are spot on. This gun is absolutely "factory".

For those not familiar with barrel honing (not backboring) it can only be done UNIFORMLY using a Sunnen barrel hone or equivalent machine tool, and they are very expensive and not usually found in the shops of gunsmiths. Most gunsmiths used/use simplistic tools to "hone barrels" - such as emery cloth on the end of a mandrel spun by a drill motor or maybe a lathe. This does not produce UNIFORM bores. Therefore if you're reading larger-smaller-larger (variable bore) with your barrel mike, it's probably been "honed" by some gunsmith using emery cloth or similar. But if your mike is showing one bore is larger than the other, but it's UNIFORM to within .001" diameter along its entire run, that barrel is likely factory. The other possibility is it's been backbored using a lathe and reamer(s) but in that case a close visual inspection will show a difference in surface finish/appearance from one bore to the other.

Just my take on it fwiw.

I have always believed that Parker Bros. would rebore a barrel to achieve the desired choke/pattern/pellet count. And that would account at least partially for the varience we often see in Parker bore diameters.

I wonder if bore variation on a SXS is normal and we don't see it because we don't look or is it something that we should be concerned about?
When does a variation get "out of limits"....001, 002, 003" in or whatever?
Thanks for your consideration

Linn

I'm not sure of what you mean by " out of limits ". As stated previously, there are reasons for a difference and as long as the barrels are thick enough there's really not a reason to worry about it.

What I meant by "out of limits" comes from my belief that when newly made a shotgun barrel has specifications--- like .650 plus or minus .002. This means it could be .648 to .650 and still could be within specification. This is not so??

Parker bores from side to side, in my experience, vary little, a thousandth or two at the most. If I see a .003 variance, I suspect that the bores have been cleaned up.