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Anatomy of a Counterbore

4K views 38 replies 14 participants last post by  Like_Old_Guns 
#1 ·
Somewhere here recently I was discussing counterbores either on this forum or with someone and I cannot for the life of me remember now. Either way, got in a batch of barrels, this one was in "good" condition according to the seller. Garbage to me...

 
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#6 ·
Counterboring is definitely an attempt to improve a damaged rifle's accuracy without going to the trouble and expense of replacing the barrel. Much more often than not, it does the job.

You are correct about counterboring for muzzle damage.

Counterboring is also done if the rifling at the muzzle has been worn down, most often due to careless cleaning practices. The British have a word for this: "cordworn".
 
#7 ·
Ive done lots of counterbores over the years,generally on corroded barrels,not always.At one time some new guns event had small counters,about 1/2" deep to protect the muzzle.One of the recent ones was to correct a cockeyed crown,bevelled at 45 dg but offset.Restored original accuracy,no more keyholes.Only needed to be about 3/8" deep.The first ones i did were with drillbits,but later with a lathe.I know the K98s were all modified with a 30mm deep counterbore late in the war.I recently did one on a gun that had a squashed in muzzle to demil it.Had to cut 1/4" off and counter bore another 3/4" deep.Its now shooting good.They now horrify collectors,some of these same guys had me counterbore their guns years ago.
 
#13 ·
I agree with John.K. Why go so deep with the cutter if you were only trying to restore the muzzle? Perhaps there was another reason (see attached). This is from "Backbone of the Wehrmacht" by Richard Law.
SteveK
Sometimes there are fairly long sections of the bore at the muzzle messed up by careless cleaning. When counterboring one of these barrels, it just seems to make sense to me to go in further to be sure you get past all the damage.

One of my favorite rifles to clean after I have had it at the range is a dot44 I have that has a counterbore much like that shown in your picture. I use a muzzle guide, go in from the muzzle, and the patch enters the bore itself very smoothly after the counterbore has 'prepared' it. It doesn't get cleaning solvent all over my shoes.
 
#12 ·
Counterboring muzzles was an accepted practice by ordnance shops when rebuilding U.S. M1 Carbines (often worn because they could not be cleaned from the breech). This was not feasible on M1 Rifles because the gas port was located so close to the muzzle.

The difficulty with using an ordinary drill bit is that it's hard to keep an unpiloted drill exactly concentric with the bore; also, unless the counterbore is reamed thereafter, the shoulder of the counterbore is unlikely to be precisely perpendicular to the boreline all the way around its circumference.

The best and most precise way to counterbore is on a lathe, but on military rifles this often is not a simple job.

M
 
#14 ·
Jon Speed w/ two photographs

Paul, I saw on site talks on Counter bore of Muzzles and saw a person showed from Law book a drawing image of this technique form a Mauser drawing. Just to confirm and show the real thing here are 2 picks of the actual Mauser firm Blueprint of this aspect which is dated 27.12.44. This was done as precaution for 98k that from field armoury reports on how poor cleaning procedures or muzzle damaged occured during late war heavy combat conditions. Please put these up on site. Thank you. Regards, Jon
 

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#18 ·
One final word on counterbores....If you face the muzzle,or bullet exit point flat,ie at 90deg to the bore,exact centreing is not necessary for a correct result,if the exit is faced at an angle/cone ie drillbit point,it must be exactly centred on the bore. Mr Speed s drawing shows this detail.
 
#19 ·
How to Counterbore a Rifle

I've never had a problem counterboring, even with my little Chinese tabletop drill press. Just use a 3/8" top quality 45 deg. (standard) bit at the proper speed, set the barreled action up clamped vertical and parallel to the drill and so the bit will swing out right over the bore center. Make sure the drill is fairly tight, minimal wobble. Use plenty of cutting fluid or motor oil to keep it cool, make short cuts under enough pressure to cut the bore continuously. if you're too cautious and let it rub instead of cutting it'll just get hot and the drill bit will wear out. Pull the bit out and inspect for good rifling around the cut. When you hit decent rifling you're done. Knock any chips off with a piece of brass or plastic.

I've never had a problem centering either because the 45 deg. drill bit really wants to go right down the center of the bore or maybe because I'm so anal retentive I get it perfectly centered and straight. I've always gotten back within the milspec of a 3 in. group at 100 yds, benchrest, sandbags. But I'd be reluctant to try more than a 2 inch counterbore with this setup.

You do not need to use a lathe or cut a flat "crown". During WWII Springfield was crowning some M1 Garands with a 45 degree drill bit instead of a dedicated crowning cutter with no effect on accuracy. Those flat crowns are usually for target rifles, and lathe cutters wont track the bore like a drill bit will.

BTW if you "just cut an inch off the barrel", congratulations, Bubba. You've killed the rifle's collectibility. Counterboring doesn't since the military, especially with Mosins, did it regularly as a way of getting the accuracy back without interfering with the bayonet mounting. In fact the most accurate milsurp I ever owned was a Finn capture 91 with a counterbore. One hole at 50 yards, using milsurp ammo. The counterbore was short, about 1/2 inch, but a strange shape, like a funnel instead of a drill cut.
 
#22 ·
Great picture!

Obviously a staged photograph, but none the less! MGMike, I would agree, however, looking at the construction of the drill, it looks like the whole head floats. Drills will always follow a bore, and very precisely, unless your grind is off, the tool is worn, etc. My *guess* from looking at the construction of the motor assembly, allows for the head to float, correcting for any misalignment. I could be totally wrong as well...
 
#24 ·
I suspect my wonderful piece of Chinese engineering - the tabletop drill press - is wobbly enough to do likewise. It's "adjustable", but if you tighten it up so there's zero movement you can't move the bit up and down!
 
#30 ·
Military rifles are always long throated so that chamber pressure does not become excessive when the barrel gets hot enough to smoke the woodwork...................The drill is a big old alloy cased portable of the kind that would wind your arms up like noodles if the drillbit jammed.A very makeshift setup,and the way he is holding the gunbarrel would wind up his fingers if the bit caught .If you are ever silly enought to place your hand on a rotating shaft,never point the fingers in the direction of rotation.The hand will catch,winding the arm on.
 
#37 ·
Military rifles are always long throated so that chamber pressure does not become excessive when the barrel gets hot enough to smoke the woodwork...................
I've read that long throats were used to accommodate all potential types of cartridges, and to make an allowance for dirt and debris build up. As the barrel heats up it gets slightly longer and the chamber and bore diameters very slightly bigger, which would tend to reduce pressures.

In general, for civilian shooting, a bullet that just touches the lands makes for better accuracy.
 
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