oh yea here is that extension for AR im using
Les Baer Custom Barrel Extension AR-15 Steel in the White - MidwayUSA
Green mountain has a sale on 7.62 blanks $33, ordered a couple Im putting an extension on 1 for AR chambering for it 7.62x25 an doing other one for AK chambering it for 7.62x25
Gunsmith Edition 20" AK Barrel Blank
7.62x25 should really have a .308-.309 bore IMOO. I have done a bunch of x25 barrels all of them were done in .308 bore.
I have a few of those blanks and they are what you pay for. IMOO you can do much better using a good take off barrel from a bolt action for a x25 build.
I may chamber one in 7.62x25 and try it just to see how it shoots. and then if its not great I can always rechamber to x39.
why not just make it a blow back??????????? no extension needed. Im not a AR guy so Im not sure whats involved on a blow back AR. I just know that 7.62x25 is super dirty and that the AR system has enough problems with out adding really dirty ammo to the problem.
as dirty as the surplus is (i have rommyand bulgy) Im suprised the gun dosent grind to a halt. seems llike you could get away with out having to buy a standard bold and all that with a blow back. I would not mind having a x25 on a AR would make a nice light fun gun.
I already have an AR that shoots 5.45x39 it would not take long to drop a magwell adapter in and change barrels, The gun shoots surplus 5.45.
The way I think is if a gun wont shoot ANY ammo you put in it, from surplus to the best match grade Im uninterested. you may not always have a choice!
They now have .22s in various twists, .311, .308, 9mm, .451 (.45 ACP), .40, and 6.8mm.
I don't know what you'd do with a 20" long .40 barrel, other than cut it up for three or four pistol barrels. .400 Whelen, maybe?