243 Winchester to 6mm Remington

[Clemson]

If any of you have made this conversion, how much barrel did you have to cut off to get the 6mm chamber to clean up the 243 shoulders?  The barrel is a bull-barrel from a Winchester model 70, and it is going on a Yugo M48 Mauser action, so I have to lose all the tenon anyway.  I still have roughly 1 1/4 inch of cylinder to cut a new tenon and threads and the boss for the safety breech.

Bill Jacobs

[Clemson]

Lest anyone think I am too lazy to try to determine this, my calipers, set on the shoulder of a fired .243 case, show me that I need to lose about 7/8 inches of that 243 chamber.  That will remove all the threads from the Winchester barrel plus about 1/8 inch of cylinder.  When I cut the new tenon, I'll have about 1/2 inch of cylinder left.  I would still be interested in your experience, however.  I would hate to go to the trouble if cutting, threading, and chambering and find that I'm leaving a line on the case where the old shoulder used to be.

[Dr.Hess]

What about looking at the SAAMI drawings, then doing the maffs?  Plus, the M70 and M48 may have a different amount of "stick out," for lack of a better term, of the casing from the back of the barrel chamber.  So, it might take more maffs.  Or, stick a 6mm in the chamber, measure how much it sticks out, figger out how much the stick out should be for a M70, back out the difference in casing lengths to the shoulder and that might be pretty close. 

[rdm1962]

I've never gotten that deep into a conversion. The 6mm Rem is a very underrated round and IMO beats the 243. The only plus to the 243 is ammo and reloading components are easier to get. 

Based on the work you've done for me, I would say if anyone can do it you can. 

[Clemson]

Thanks!  I am just trying to avoid cutting a chamber twice.  I'll over-correct on the initial cut and hope that it is enough to clean things up.  Basically, a miss would mean double work for me.

[Dr.Hess]

What if you had it chucked up and turned the face off until a 243 head space gauge stuck out the correct amount?  Then do the shoulder to match that?

 

[Clemson]

Well, the answer is..................

Zek hit it pretty close.

I took off the Winchester tenon and started reaming.  Had to take an additional 1/8 inch to clear up the old chamber visually.  I cut the new tenon, including the safety breech boss, and will thread tomorrow.  I think I will be around .010 short with the chamber based on what I have measured off the GO gauge.  I can deepen that amount by hand.

Interesting exercise.  I would not recommend this operation unless you have a really good bull barrel that you want to use and must have a 6mm

Bill Jacobs

[AzRednek]

On 6/29/2021 at 3:38 AM, rdm1962 said:

I've never gotten that deep into a conversion. The 6mm Rem is a very underrated round and IMO beats the 243. The only plus to the 243 is ammo and reloading components are easier to get. 

Based on the work you've done for me, I would say if anyone can do it you can. 

I agree the 6 MM Remington is under rated. From what I’ve read, I believe it was an article by John Amber author of Cartridges of the World. The main difference between the two is in the barrel rifling. Best I recall muzzle velocity of both were very close. What gave the 243 the advantage was their rifling twist stabilized heavier bullets where the Remmy did a better job with lighter weight varmint bullets. Remington promoted the 6MM as a varmint cartridge bridging the gap between 22 Centerfire and 25/06 for varmints. I can recall seeing a Remington 6MM rifle with their factory heavy varmint barrel. Winchester with its rifling twist doing a better job of stabilizing heavier bullets promoted the 243 as a combination varmint and deer rifle. I can’t remember the numbers in Amer’s article but I think he wrote the Winchester 243 rifle, not the ammo out sold the Remington 6MM rifle something like 10-1. I can recall hearing or reading the barrel life of the 6MM is considerably longer than the 243. From what I recall the forcing cone of the 243 wears prematurely.  

A bit off subject, a lady janitor I worked with at the Post Office. Her son was an avid tournament shooter. He had participated in Olympic rim fire tournaments but didn’t make the final cut. Her son was in the process of building a custom rifle chambered in 6MM Remington. I can’t recall the brand name but he was awaiting a high price barrel from Europe. I met him briefly, he was selling a Ruger 44 magnum Super Blackhawk with a 10 inch barrel. In our conversations he praised th 6MM Remington. He claimed it was the best choice for long range tournaments. He claimed the 6MM caliber was the best choice for dealing with wind. Claiming a 6MM bullet weight I forgot drifted less than a 7MM or 30 caliber in cross winds. I don’t know if he was really knowledgeable or blowing smoke. He showed me a heavy barreled custom rifle his deceased father passed down chambered in 222 Remington. He claimed the 222 was superior to the 223. Giving me a song and dance about the military adopting the 556 instead of the 222 preventing Remington for taking credit for developing the military round. Far as I know, the 222 and 223 are nearly identical.
 

We didn’t cut the deal for the Blackhawk. I was offering  Colt 38 revolver, Hong Kong police surplus and cash but he wasn’t interested. He did make me aware to my surprise the revolver was originally chambered in the 38 Smith and Wesson or 38/200 the British designation of the cartridge. Later the cylinder chambers were reamed to accept the longer 38 Special cartridge. The Brit round is a slightly larger caliber .361 I believe and he pointed out the accuracy would be terrible shooting .357 bullets. I read up on it a few days later and he was correct. 

[Downwindtracker2]

I think I remember reading the 222 Magnum is only slightly shorter than the 556. Someone who had more interest and a better memory can go into the relationship.

[Clemson]

222 Magnum is longer than 223 Remington (5.56 mm).

[AzRednek]

A chicken or egg first question. Was the 5.56 or 223 Remington first?? I know they are different cartridges but I can recall Bill Ruger claiming his tests with his then new Mini-14 claimed the difference was insignificant. 

[Dr.Hess]

223 Rem was first.  I agree with Mr. Ruger.  "The Internet" says you can't shoot 556 in a 223 barrel.  I have yet to see anyone damage a gun or themselves doing it.

[AzRednek]

On 7/21/2021 at 7:40 AM, Zek said:

The difference between .223 and 5.56mm is lawyers.

The military 556 has slightly thicker brass. I assume it’s the entire cartridge as the 556 necks eyeball a bit thicker. Internet baloney about 10,000 PSI difference is just something in one’s mind as far as I’m concerned.