M1a threading question

[Ravage]

So I have one with a 22" barrel I want to get cut down to 18.6" now I already have a extra 30 cal muzzle device it has a 5/8x24 threading now looking around I see that M1a have different threading and I see adapters and stuff now if I get my barrel cut it needs to be threaded is there a reason it cannot be threaded to 5/8x24 ?

 

[CanuckR]

The barrel is not large enough in diameter to thread 5/8x24.

 

[Claven2]

The best solution, though expensive, is to buy a Criterion 18.5" non-restricted barrel and have it installed. They are factory threaded to GI spec.