As noted, the belt has to be for a pre-'45 firearm. A 1919A4 semi in 7.62x51 is not be a problem, in and of itself, but the belts would have to be for a pre-'45, and as pointed out, that can be a problem; belts were not designed then for 7.62x51.
If a conversion system was available for the M1A or m305, it would be quite legal, BUT it would have to use pre-'45 design belts, if more than 5 rounds were to be loaded - and there are none intended for 7.62x51. IF a belt for another calibre would work, fine.
Having said that, a belt fed conversion for one of these rifles would be quite a piece of work. I would suggest that you look at a few belt fed guns' feeding systems, look at a M305's, and then ask yourself howinhell a belt feed conversion could be made to work.