As soon as you modify the semi-automatic rifle to accept double stack magazines (technically, readily available high capacity), you must comply with 922(r) which restricts you to no more than 10 (out of the list of 20 key components) imported parts. Just expanding the magwell, you have Buttstock (which includes the trigger housing and rear trunion block all as one part), receiver, barrel, piston, operating rod, bolt carrier, bolt, trigger, hammer, sear, handguard, and then the magazine body, baseplate, and follower. A total of 13 parts.
With U.S. made AR mags, and a U.S. made bolt (which you'd need to change in order to feed from double stacked magazines) you would be at 10 imported parts, and compliant with 922(r). Usually what catches people up, is when they replace the SL8 buttstock with more import parts because G36 rear block, butt stock, and trigger housing are each seperate parts.