How would one do it? The only way I can think of is to put the magblock in, and weld the spots on the floorplate to the mag body.... Is that how it's done? Is that considered "up to snuff" with NJ's crazy laws?

Sure, if you can weld aluminum.

-.- I knew they were aluminum... I was just having this discussion with a friend, and he was telling me they were steel.

 

So how would one do it?

They sell a blocked 20 round version of the USGI mags. http://www.midwayusa.com/product/2228111195/ar-stoner-magazine-ar-15-223-remington-with-anti-tilt-follower-stainless-steel?cm_vc=ProductFinding

-.- I knew they were aluminum... I was just having this discussion with a friend, and he was telling me they were steel.

 

So how would one do it?

You could source some C-Products or HK steel mags and weld them. But those aren't cheap. Personally, for USGI mags, I really like Paul's solution.

If they are steel use a mig welder to tac the floor plate to the body. If they are aluminum you can either braze the floor plate on or use a tig welder. Of course, I'm sure there are other ways of going about this but I personally would use the ways I mentioned.

IIRC you can't braze aluminum. Besides, brazing is going to heat the hell out of a mag body, spring, and follower. I think the spring would fail.

 

Steel 30 rounders can be found cheaply. Under $10 online, usually from places like PSA. Not USGI, but still quality. Steel is also much easier to weld.