As the title says. I'm interested in the one that mounts to the receiver. Is there any kind of drilling and tapping required?
Thanks,
-Steve

Same here.  A buddy of mine has an M3 that he says is adhesively attached (only) to the receiver.  I'll follow up with him on what it is, who makes and sells, etc.

I've always wondered if some enterprising devil would come out w/ one that mounts onto the picatinney rail, maybe even via ARMS throw lever? At any rate, those fixed rigs can be done, but I'm still thinking that the 3gungear velcro-fastened shell holders are the way to go. Velcro, if employed correctly, should provide sufficient mounting strength to maintain on a receiver w/o having to tap & drill mount one, which I'm not too keen on doing.

I was wondering about a receiver mounted side saddle that hooked to the rail system.

I don't see myself using JB weld to fix a side saddle to a 1300 dollar gun.
The Velcro setups may be the only choice.
-Steve