With Ruger's proprietary gas piston system, swapping barrels may be unobtanium on the SR762.

Well, I can always build DI .308 AR10 with a 20" barrel.

Two of the 'big names' in gas-guns are John Holinger at White Oak Arms and Compass Lake Engineering (don't have a name at CLE). You could call either one and ask, as I'm sure they would know. Likely much easier and straight-forward then trying to reach thru the bureaucracy at Ruger.