AR Extension threads & chamber wall

[Bimbie]

I'm looking at a chamber chop job
to repurpose a Mauser 7.65arg barrel onto a AR 300 Blackout (303blackout) ...No-Gas Repeating Rifle SideCharger

By the time I remove everything south of the 7.65 Neck, there will just 14-18mm of full inch shank left to turn for AR 13/16 Barrel Extension threads. Past that, barrel profile has a immediate shoulder to 3/4".
Seems simple part-off at where the existing 7.65arg chamber shoulder starts,
face down to the start of the neck,
cut extension threads, ream & headspace new chamber like a regular barrel blank for AR.


Sanity Check please:
223 / 300Blk will be ok with a chamber just .75" od past the Extension (+ few mm tighten face),
since AR 13/16 threads minor dia leaves just .75" undisturbed as well?


Casual observation makes one believe the 3/4" (minor) under the extension implies a 3/4" od chamber would work. Casual can get you blownt up when gunsmithing, so I better check with others before proceeding

 

[tenmetre]

It will all work, but the barrel twist rate will be the deciding factor if you want to stabilise a subsonic round.
If you are doing this for yourself as a "can this be done" exercise go right ahead, I will be interested in the outcome. If this is a .303 barrel be aware it is a .313 bore so a soft full base bullet will help getting it into the lands and making a seal.

If you are doing this for a customer as a cheap way to get a .300 BLK I would caution you not to.

 

[Bimbie]

Oh, its definitely for personal use.

The 7.65 Argentine uses the same diameter slugs as 303Brit, and there are a few round & flat lead mold choices for 303Brit slugs in 160-180gr range. Those should be just short enough to stabilize in the clearly-not-fast twist rifling of the Mauser (1:11 I've seen online). I have no delusions of getting a typical Blackout 220gr Pencil Slug to stabilize in thet slow twist, I an ok with the moderate weight stubby slug subs

On the bright side, the slower rifling should cause less deceleration of the slug after the Blackout powder burns out.... I hope. It will be some fun experimenting to see what wins: longer barrels give higher velocity, vs long barrels slow slugs if your powder is 'too fast'.

 

[GGaskill]

Don't take this as gospel as you haven't posted a drawing of the barrel as intended so I can't be sure about the starting numbers, but with a .75" OD and a .313" bore, a 60Ksi cartridge will develop 85Ksi stress in the barrel wall and 93Ksi in the thinner chamber wall. The wall thickness is about 69% of the bore diameter but only 57% of the chamber diameter which is substantially below the recommended 67%.

Since you don't really KNOW the barrel material or strength, I think there is more than the usual amount of risk in this. Even the first generation pencil barrels were .98" over the whole chamber.