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.
Backboring
- Thread starter 25ring
- Start date
Military rifles were "Counterbored" to restore crowns.
WSnyder
Silver $$ Contributor
I'm not up on and never really was but didn't RFBR have a sporter class where no tuner was allowed but could have an oversized muzzle that was backbored and acted as a tuner of sorts? Calfee Sporter Taper perhaps???
Ah yes! IR 50-50 Sporter class. Calfee reverse taper dog knot barrels. Not exactly what you were asking but......
benchrest.com
Ah yes! IR 50-50 Sporter class. Calfee reverse taper dog knot barrels. Not exactly what you were asking but......
Rimfire Benchrest rifles (Calfee SPEC rifles)
Who will be the last one to speak. I have an idea.
benchrest.com
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Yeah I get that,I'm just wondering if that would eliminate the problem of the bore opening up where it's been threaded.
Yes
Just more stupid rules that turned their equipment into bizarre looking stuff. Although I will say I have a 10-22 barrel that has a muzzle weight built into the barrel that has won many matches.I'm not up on and never really was but didn't RFBR have a sporter class where no tuner was allowed but could have an oversized muzzle that was backbored and acted as a tuner of sorts? Calfee Sporter Taper perhaps???
Ah yes! IR 50-50 Sporter class. Calfee reverse taper dog knot barrels. Not exactly what you were asking but......
View attachment 1627043Rimfire Benchrest rifles (Calfee SPEC rifles)
Who will be the last one to speak. I have an idea.
I don't see any ARA or very few NRL or PRS competitors using threaded muzzles for tuners or suppressors and assumed that's the reason. That's why I'm curious about it.
For readers in general, are there any studies showing accuracy remains unaffected after threading the muzzle?
10XSHOOTER
Gold $$ Contributor
I have a Lothar-Walther 10/22 barrel that is back bored under the threads
zeke06
Gold $$ Contributor
I did some testing on this a few years ago. I threaded a barrel with VARY conservative speeds and feeds along with sharp tooling, one with my go to tooling for SS at the recommended speeds and feeds, another one that was back bored, and the last one I made the threads about 1.5'' long and re-lapped the bore before cutting to the proper length and crowning. I know I should run a number of each just to see if it had consistent results but here is what was determined. I slugged the bores before and after machining and none of them had a measurable difference after threading but the one cut with sharp tooling and conservative F&S the slug would push easier at the threaded section. The others all felt about the same as before threading. I did not test fire before threading but just afterwards. The one that was threaded long and lapped shot the best, the back bored and standard tooling with recommended F&S shot about the same and the one cut with sharp tooling and conservative F&S shot the worst.
My assumptions are the sharp tooling let the stresses free more easily and the standard tooling applied enough tool pressure to counter act it. The back bored crown was cut with a smaller carbide boring bar with 1/2'' of stick out so you're losing some rigidity. Lapping after treading might have allowed to have a constant bore diameter and round it back out.
Like I said before this is just what I found with the limited testing that I did, and my shop isn't setup to accurately measure under .0001''
My assumptions are the sharp tooling let the stresses free more easily and the standard tooling applied enough tool pressure to counter act it. The back bored crown was cut with a smaller carbide boring bar with 1/2'' of stick out so you're losing some rigidity. Lapping after treading might have allowed to have a constant bore diameter and round it back out.
Like I said before this is just what I found with the limited testing that I did, and my shop isn't setup to accurately measure under .0001''
TRSR8
Silver $$ Contributor
It’s been quite common, as stated, with IR 50/50 sporters, however they start with a fairly big OD at the muzzle and even then, not an easy job.
That said the issue seems mostly involving buttoned barrels, but with light cuts could be OK.
Thanks for the striker answer, by the way. Never had a Ballard but trying to by a Rigby Ballard, have a few HW’s and Stevens 44’s
The few CZ threaded barrels Tad has slugged were to tight in the threaded area. Constricted more than he felt was desired. They were back bored and new crown placed at desired location. We did shoot these barrels before and after during this and found gain. Working with factory class and rules is not what Tad enjoys doing so the number of rifles this was done to is low. Maybe 5 total so examples of results is low.
Todd
Todd
Maybe it was just the new crown that worked. Too many variables.
Maybe. Might have been barometric pressure. Just relaying the small number of test we have.
Todd
TRSR8
Silver $$ Contributor
From a straight accuracy standpoint, why wouldn't you simply modestly shorten and re crown?
With military rifles using open sights it eliminates the problem of recutting a dovetail and reinstalling the front sight.
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.
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.
Hi-NV Shooter
Gold $$ Contributor
I heard the same thing about hammer forged barrels that cutting the muzzle will tighten the bore.
On back boring removing weight and having an effect on how it shoots. it is because of the removal of weight at the muzzle. it is the same thing done for iR50/50 sporter barrels any time you alter weight at the end of the muzzle you will have an effect on bullet exiting timing. want to try and do an impromptu experiment get some stick-on wheel weights for balancing car tires and place them on a bare barrel at the muzzle. place them at various positions starting at 12'o-clock you will see a difference on how the barrel will shoot.
Lee
TRSR8
Silver $$ Contributor
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.
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