Backboring:

Pete Wass

Pete Wass

Well-known member
Backboring: ( Counterboring)

I had a good conversation a while back with an older friend who has been doing his own "Smithing" since high school days. He told me he had learned it all from books, had made his own one flute reamers, etc, etc.. I brought up a rifle I had purchased that had a huge rust crater about an inch or so inside the muzzle. I have seen a number of older "Hunting Rifles" with blued steel barrels with similar craters in them. He said he had once bought a rifle with the same crater in it and the rifle did not shoot well. He thought and thought and decided to try simply boring the crater out. I don't remember the size of the bore but he decided to use a twist drill slowly fed into the barrel. He bought a drill and honed the cutting surface it until it was shiny. He then used it to bore the muzzle out to just past the crater. My friend said the rifle shot well after this bit of work. He said you could not really notice the barrel had been backbored by casually looking at it. I have a couple of old barrels that have these craters up front and I am going to try the same process to one or more of them and see if I can be successful.
 
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If I understad you correctly

A quibble: There is already an established use of the term "backboring" in gunsmithing, for shotguns, and it has a different meaning, it's used with reference to fixing/improving chokes.

All you're really doing is cutting a recessed crown... The hot setup for RF benchrest sporter rifles requires the same technique.
 
Many of the russian 91/30's imported into the country recently have this done to them, called counterboring. I have several, and they are accurate. Are they benchrest quality, no, will they hold the 9 ring on the NRA 100yd highpower target, yes.
 
PT&G makes reamers for making integral muzzle brakes that are .020 over bullet diameter per caliber. They will ream several inches deep and leave an 11° crown at what ever depth from the muzzle you choose, limited only by the length of the tool. You can special order other than 11° crown if you prefer something else but usually 11° is in stock.
These are piloted reamers and would work well for what you are thinking about doing.

By integral muzzle brake I mean you can counter bore 1.5"( example length) deep from the muzzle .020 over bullet diameter and leave a nice little crown job at 1.5". Then put your barrel in your indexing head on your mill and drill/mill all your ports into the barrel so that you end up with a finished nonremovable brake on your favorite hunting rifle.
 
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Anybody ever watch a high speed video of the bullet being passed by combustion gases that are traveling 7000-9000 FPS when the bullet clears the bore?
Not good for accuracy.

Dave
 
If you decide you still need to do it you might try what a gunsmithing buddy recommends, below.


The coconuts in BR C are talking about running drill bits down the muzzle to counterbore and remove bulges. Providing the bulge is no more than an inch and a half down and it is 6.5mm to 338 caliber the excepted way to do this is to set up the barrel in the lathe to cut a chamber at the muzzle end not the chamber end and use pilot adapters "available from JGS" to put a larger than standard pilot on a smaller reamer. Use a 222 or 223 Rem reamer and drive it in beyond the bulge. The shoulder will cut a perfect crown and the pilot will keep it square to the bore. Using drill bits to do this never works worth a . If you look at a counterbored crown cut with a drill with a bore scope you will find the crown is ragged, has chatter marks and leaves a rolled burr on the edge of the crown. Small boring bars seldom will cut good crowns over that distance and diameter either. You can buy a 223 reamer for $100 and charge $100 for the first one you do to get your money back. The next 40 you do are all gravy.
 
If your only talking about 1" from the muzzle, why not just cut the barrel off that one inch and re-crown? Most factory crowns could use a nice sharp recrowning anyway. Lee
 
Dave Tooley said:
Anybody ever watch a high speed video of the bullet being passed by combustion gases that are traveling 7000-9000 FPS when the bullet clears the bore?
Not good for accuracy.

Dave
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Dave,

I can believe that.

But in the case of a muzzle brake, where there is the necessary clearance between the bullet and the brakes I.D. (I hear .020 clearance alot), does all of the combustion gases get diverted up and out of the brake, or does some of it sneak by the bullet via the bullet to brake clearance?

If it does sneak by, is there enough of it to be detrimental to accuracy?

Justin
 
Butch Lambert said:
...Providing the bulge is no more than an inch and a half down and it is 6.5mm to 338 caliber the excepted way to do this is to set up the barrel in the lathe to cut a chamber at the muzzle end not the chamber end and use pilot adapters "available from JGS" to put a larger than standard pilot on a smaller reamer ...
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Dunno what those adapters cost, but anyone who has bought an oversized bushing for a chamber reamer so as to fit a seating die blank probably winced at the price. The larger bushing is not in most sets, the reamed hole in a Wilson seater is quite a bit larger than bore size. If you could buy one adapter & then use bushings you have on hand for other bore sizes, it might worth the cost.

Simplest, since you're losing gas pressure with this "fix" anyway, is to just cut the barrel off. Add a weigh if it's now too muzzle light.
 
Dave Tooley said:
Anybody ever watch a high speed video of the bullet being passed by combustion gases that are traveling 7000-9000 FPS when the bullet clears the bore?
Not good for accuracy.

Dave
Click to expand...

So, how does one explain the accuracy of the Rimfire Sporters with a counterbore?
 
skeetlee said:
If your only talking about 1" from the muzzle, why not just cut the barrel off that one inch and re-crown? Most factory crowns could use a nice sharp recrowning anyway. Lee
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It would ruin the appearance of a lot of old rifles but doing so.
 
Charles E said:
If I understad you correctly

A quibble: There is already an established use of the term "backboring" in gunsmithing, for shotguns, and it has a different meaning, it's used with reference to fixing/improving chokes.

All you're really doing is cutting a recessed crown... The hot setup for RF benchrest sporter rifles requires the same technique.
Click to expand...



I stand corrected and did use the wrong term, My apologies.
 
Pete Wass said:
So, how does one explain the accuracy of the Rimfire Sporters with a counterbore?
Click to expand...
Because with a .22 Long Rifle, the gas pressure in the barrel has dropped to less than bore friction in about 16 inches of barrel. With a .22 RF, you get the highest MV with a 16-17-inch barrel. In short, ain't no high-speed gas to rush by.

"Ruin the look"? Well, you could loose weight so people will stop diverting their eyes to your rifle ;-)

Or, you could cut the barrel off, put on a good crown, then turn down & thread a small section at the muzzle, and screw on a real short bloop tube (much larger diameter hole). That'd help. You'd get a lot of questions, too, and if the rifle shot well, all sorts of people would rush to their gunsmiths to have the same "improvement" made.
 
how would an inch shorter barrel and a nice properly cut crown ruin the apperance of a rifle, compared to a bored out barrel that sounds like not a good idea anyway. Besides that, what good is a rifle that doesnt shoot well? New or old. Just sayin. Lee
 
skeetlee said:
how would an inch shorter barrel and a nice properly cut crown ruin the apperance of a rifle, compared to a bored out barrel that sounds like not a good idea anyway. Besides that, what good is a rifle that doesnt shoot well? New or old. Just sayin. Lee
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Well, lets say it was a rifle that had a soldered on front ramp sight, which was the case with my friend had or had a long magazine that would also need to be cut and re-fitted. Frankly, I'm sorry I brought this up and will be more careful in the future as to what I report here. Even though his rifle shot "wonderfully" after he did what he did, apparently that has no bearing on this forum, so be it.

Pete
 
Pete Wass said:
Well, lets say it was a rifle that had a soldered on front ramp sight, which was the case with my friend had or had a long magazine that would also need to be cut and re-fitted. Frankly, I'm sorry I brought this up and will be more careful in the future as to what I report here. Even though his rifle shot "wonderfully" after he did what he did, apparently that has no bearing on this forum, so be it.

Pete
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Pete, I don't see that it makes any difference how he solved the problem as long as it shot better after he did it. I don't blame him at all for not wanting to shorten the barrel and reattach a soldered on front ramp sight. All I can say is "Good for him that it worked."

Mike
 
Ya, that was my thought too. I just think the piloted tool would be worth buying if the rifle was worth the expense.

This thread has me thinking about running one of those reamers in like I was talkin about and drilling some ports in there at an angle so's the gas'd vent up and out each side of the front sight.
 
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