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Another Way To Recrown aka Counterboring

7K views 1 reply 2 participants last post by  FrankenMauser 
#1 ·
Okay re-crowning is a nasty word if you own a rifle that needs it. This is a set back crown or counter bore.

This will be the easiest way to fix a damaged crown on your rifle and not have to reset your front sight ramp.

Say you have a 30-30 that needs it. Groove diameter is .308 to clean up a bad spot you need a drill that will completely cut the old groove. Most common size would be 5/16 or .312 this only removes .002 per side not much.

Scary, actually not that bad. A drill will follow a hollow even and square. Even if you are using a hand drill.

So you have decided to give it a try and here you are. I would start by estimating the amount of damage in barrel depth, say 1/2 inch. Okay wrap a piece of tape around the drill 1/2 inch from the end. When you've got it in that far stop and your done.

.35 Rem is .358 groove you would use the most common drill near it .375 or 3/8 about .009 per side.
Or buy a specialty drill bit that is at least .005 larger.

.44 or .444 groove is .429 7/16 is .437

.45 LC is .452 and 45-70 is .458 using the larger number for both 15/32 or .468

Again specialty drill such as letter drills or metric sizes can many times get you close to the .005 needed to just clean up your damaged crown, without removing the sight ramp and setting it back or bobbing the bbl.

But then a lot of folks still want a shorter gun and this will also make a nice crown when bobbing a bbl back by recessing it in the bbl which makes it square and even if the bbl is cut out of square by a little.

Hope this helps all you home do it your self folks. PS I have done this a few times and currently have one Marlin with a recessed crown and it shoots less than a minute at 100yds.
 
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#2 ·
Some notes on counter-boring, that should be considered before you attempt this at home:

If anything resembling accuracy is desired.... Once the counterboring is done, the barrel needs to be crowned ... way down at the bottom of the counterbore.
Since obtaining the proper tools to cut a new crown inside a counterbore is rather difficult for the average DIY owner, that can be a show stopper.


Counterboring is for last-ditch, gotta-save-the-rifle-without-making-it-appear-altered, or gotta-fix-the-crown-without-removing-something-attached-to-the-barrel. ...And the necessity to counterbore is almost always indicative of terrible cleaning methods with steel cleaning rods, and/or massive muzzle damage from abuse (throwing the rifle on top of tools behind a truck seat, slamming it into rocks, storing it with a metal hook or rod inside the muzzle, etc.). Counterboring really shouldn't be done, outside of circumstances where you have to avoid cutting a barrel back (such as to keep the barrel legal length, to avoid having to relocate a front sight, or to avoid moving a barrel band).


To see the benefits, pitfalls, and reasons for counterboring, look no further than surplus military rifles - especially Mosin Nagants. It saved some badly abused barrels, but ruined most of them, due to the low quality crown left by the counterboring job. There are metric tons of information available on what counterboring their barrels did for the Russians.


Most notably - counterboring introduces its own potential problems for accuracy. Once a barrel is counterbored, the gases propelling the bullet no longer stop acting on the bullet as soon as it leaves the crown. With a counterbore, the gases vent around the sides of the bullet, after the bullet leaves the crown, and can act unevenly upon the bullet until it passes through the actual muzzle, into 'clean' air. This effect is magnified with barrels that have some erosion, pitting, wear, or uneven-depth grooves. Any point that allows the gases to vent unevenly (before the rest of the crown) will push the bullet in the opposite direction, until the bullet leaves the actual muzzle. This problem has been shown to "throw" bullets as much as 8 feet off target, at just 100 yards; and with bullets that are marginally stabilized by the rifling twist rate (say a 300 gr bullet in a micro-groove .444 Marlin), the uneven gas pressure can almost instantaneously cause the bullet to tumble.


A 1958 test done by Vernon Speer verified that a bad crown caused by bore erosion at the end of a counterbore had otherwise stable bullets completely sideways at just 7 yards. Accuracy at 50 yards was a 4.5 foot "group".


It is best to exhaust all other options, before turning to counterboring.
 
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