Just saw a website offering a small jig to position and drill the third hole in an AR lower for an M-16 auto-sear. I would never buy one of those-- mail-order or however you'd get one. I'm thinking the ATF would want to know exactly who has purchased such an item, and will be snooping through your on-line activity to see what you're doing after the purchase. I remember when the ATF believed that owning a short-barreled upper was "intent" to mate it to a stocked lower and by doing so, creating an SBR without a license. We have AR pistols now, so that argument is moot. But to have the jig that exactly positions the auto-sear hole, to me, is about 99% "intent" to drill that hole.