90% of my customers start off with a 22 can to get started. Its the cheaper of the cans and never goes out of style. Doesn't matter what you get, Your screwed now.
If and when you do buy a 5.56 suppressor, I'd highly recommend going with a 7.62 suppressor that will also fit your 5.56 flash hider. Two birds with one stone.
I've got the AAC 51T Blackout flash hider just for that reason.
I looked at this a while back and the only way to save money is to do it all yourself. So what are you doing? A form 1 will run you $200.
SBS. You can pick up a H&R for $100 or mosberg 500 / Remington 870 for about $200. Engraving will run about $30 and a replacement fiberoptic sight $20.
Total on the build $450.
In an SBR an 11" ar15 lower will run you $450 and up. You could SBR a TC encore or contender frame and use pistol barrels but they cost as much as a "real" gun. Entry level rifles at Academy run $300-350 for a remington ADL. Then you have to send it to a gunsmith as cutting the barrel and recrowning is much more difficult than a shotty.
The numbers on the AOW just don't add up. While a 870 can be found for $200 I could never find a pistol grip shotgun for less than $500. And most come with full length mag tubes which means you have to cut and move the barel lug. Not a problem if you can weld. I can't. It gets real expensive really fast.
The sawed off shotty is the king of the NFA build. Brileys here in Houston will install chokes in SBS shotguns but I never found one was needed. At 30 yds a shotgun with a 12" barrel seems to shoot a 4" pattern. That's plenty for my needs.
Another vote for a .22 can. I didn't go cheap for my first can (Silencerco .22Sparrow) but you'll never use any NFA item as much as you'll use a .22 suppressor. Best bang (or lack thereof) for your buck. I've probably got more rounds through my can in the last 6 months since I got it than the rest of my guns combined. And I shoot at least 3 USPSA matches a month; sometimes as many as 7.