Barrel Thread Change

[Recce21]

I am working on a project that I originally believed was going to be a straight barrel swap but so far it seems that the barrel is a M21x1.5" and the receiver trunnion is a M26x1.5". Now, I cant confirm that since the original barrel is still attached to the receiver but I really don't think that barrel is going to work so off it will be coming.

Is there anyway to either convert my larger M26 trunnion to a smaller M21 without compromising the barrel or the receiver?

Does anyone have any ideas?

 

[tiriaq]

It should be possible to recut a 26mm - 1.5 thread barrel shank to 21mm - 1.5.
I don't do metric, my lathe is imperial, but I have recut 1 1/16-16 Remington shanks down to 1-16. If the major diameter of the 21mm shank is greater than the minor diameter of the threads on the 26mm barrel, it would be necessary to pick up the 26mm threads, and keep cutting until the shank is 21mm - 1.5.
There is also the issue of timing the barrel in the extension. This is where things could get interesting, if you are dealing with finished barrels with fittings.

 

[Recce21]

The donor barrel is a M21x1.5" threading and the receiver I have been told was M26x1.5". I am not 100% sure because that was for the 7.62x39mm chambering and not a .223 chambering. Not sure if the 223 would be smaller.