On a sbr you have to list a caliber . I have 2 sbrs and 2 cans form1's and form4's all of them you have to list a caliber and barrel length.
I have a 308 H&R im putting a can on. A intergal can would be the single stamp ticket as long as it ended up with a 16.5" length. You also CAN NOT have any "extra baffles or end caps. one extra will be considered another can and a unregistered nfa item. You list more than on caliber on a supressor it will throw up flags im sure.
Red Jacket fire arms can build you any caliber can/barrel you want. My can is all stainless and full auto rated. tested on a 300 win mag. He is good check out his website and watch the supresed 50 cal bolt acton.
If you call ask for Will and tell them Will from arkansas told you to call.