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243 rechamber to 6mm Remington

Compare the 6mm Remington drawings with the .243 Winchester drawings. You will have to go in at least an inch before the .243 shoulder cleans up which means you will have to move the barrel shoulder forward and remove about an inch of the tenon. Since the tenon is likely not an inch long, you might as well start with a new tenon and thread it before doing the new chamber.

An inch is just a SWAG (actually semi-wild ass estimate) so you should do the math and add a little just in case the old job wasn't perfect.
 
You could use the old barrel but it would be an inch or two shorter which would cost you some velocity. Rebarrel is better, especially if the old one has seen a lot of use. And it gives you the opportunity to do it the way you want it.
 
Autos are subject to heat buildup which changes the parts in the re-seating process. With all the moving parts of a simi-auto I never shoot more than 2 without a cool down.
Owners come in and declare "this rifle will not group" after asking how many shots they fired in a few minutes they say "ooh 10 or 15". By that time the light weight barrel is corkscrewing with heat.

Shoot two and let cool!
 
Depending on the action you use, you might not be able to feed from the mag with the 6mm Rem.
I have built two 6mm RemAI on Sako L579 and AII actions and have to single shot both of them. On the second one, I opened up the action and made a new mag box for it, but still only able to mag feed the 70grainers. Any longer and I have to remove the bolt to eject live round.
I started shooting 88gr, 87 and 70 grainers from it. With 70 grain it rocks, anything heavier and you are just burning more powder without any gain over something like 6XC. Now shooting 105's from it, but will move to 6xc or SLR at the very next opportunity.

If you do want to go 6mmRem and not cut your barrel shorter, go 6mm RemAI - and do stick to 55-75gr range.
 
I realize this treat may be stale but I am a huge fan of the 6mm Rem and have had a stainless M77 MII 'converted' from a 243 win to 6mm.

The gunsmith took off about .5 inch from the barrel, retreaded it and reamed the chamber to fit. This is the most accurate of all my 6's as the rate of twist (1 in 10 inches) stabilizes the 100gr bullets better than the stock 1 in 11 inches.

This reworked rifle was made for my wife and was for hunting, meaning the look wasn't as important as the function. It also had to be safe, with mild recoil, and small to fit her frame. It was made in 1996 and now has a Hogue stock and is very reliable. So far it has been an outstanding rifle.

Ken in Wy
 








 
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