Backboring

[TRSR8]

Threading a barrel (especially a button or cut rifled barrel) can / will cause the bore to open up at the treaded section. Normally it only a few ten thousandths of an inch (0.000x), but this is enough to effect the accuracy potential of that barrel. This isn’t to say that you will not end up with a phenomenal barrel, it just isn’t optimal (same size bore to the muzzle or maybe a slight choke). I know this because I have measured it several times and have had sev other very reputable gunsmiths agree.

One thing I dont know about is a hammer forged barrel. Robert Heart told me many many years ago that stress from the rifling process is imparted in the metal. Button & cut barrels have stress that is going away f the bore center line, & hammer forged barrels have stress that is going toward the bore centerline. So possibly hammer forged barrels may have a tendency to tighten up when threaded. But I have never tested that.

For counterboring, I have seen counter bored barrels shoot better after the counter bore is cut off. Not significantly but noticeable. What I don’t know is if this is due to a harmonic change with weight removal (I kind of doubt it because it is such a small amount of weight, but maybe) or because the bullet is entering cleaner (less turbulent) air with out that fairly tight tunnel to fly through. Hard to say either way because I have seen excellent accuracy with rimfires running bloop tubes, but they are significantly longer than a back bored muzzle.

It is all interesting stuff, just hard to scientifically prove anything because barrels are so unique.

Likely, your point has zero to do with anything to do with air turbulence. All of the best IR sporters exhibit that format, usually a good inch plus, and often shoot as good as the heavy guns.
I’d guess it likely has more to do with re acquisition of new tune after change.

 

[VLNBYR]

I suspect you might remember Lee, several years back and still here on the gun review article page, Joe Fredrich shot some great targets with a Billy Myers gun with about a pound of wheel weights taped around his Harrels tuner. What to make of that deal…..I wish I knew.

It goes to show that accuracy, like gold, is where you find it.

 

[Hi-NV Shooter]

I suspect you might remember Lee, several years back and still here on the gun review article page, Joe Fredrich shot some great targets with a Billy Myers gun with about a pound of wheel weights taped around his Harrels tuner. What to make of that deal…..I wish I knew.

Tim, yes, I seen pictures of the Von Aherns tuner with a ton of wheel weights but that is not what he ended up with as the final setup. the 2015 record 4 target score was done with the highly modified Von Aherns tuner seen in the picture with the red rest. he described it as a Roger Von Aherns lite tuner.

Getting back about adding a ton of weight it can work, but IME it narrows the window of lots that will shoot. because the barrel will never reach peak rise consistently velocity range will be narrower on lots that shoot the best.

Lee

 

[seymour fish]

Conventional wisdom says that threading the muzzle on a rimfire can be detrimental to accuracy because the bore will open up slightly. Has anyone ever tried backboring the muzzle to eliminate this problem? I have a Ballard 22rf that has been backbored and it shoots great and have seen older military rifles that have had this done to restore accuracy. Always been curious about this, would appreciate any thoughts.

Sporter- class method, laborious, attempt to tune. Have experience of one. Lights out

 

[jrm850]

Re: counterboring... I ran into this issue a couple of weeks ago experimenting with accuracy issues of a lower cost bolt .22. I found that I could drop a .218 gauge pin into the muzzle about the same distance as the threads. Slugging it showed a tight spot just behind the threads. I cut it back to the tight spot and re-threaded using very sharp hss tooling. After I was finished, I could still drop a .218 pin in the muzzle about the same distance as the threads. No Change from factory. I ended up counterboring to .309" diameter for .545". I wasn't very scientific about which change made the most impact, Rechamber, lap, or recessed crown, but I will say it shooting the best that it ever has now.