JB wrote an article in which he stated that a bolt gun was quieter than an AR and gave a better chance for extra shots.
I wonder if a Contender/Encore might also work well.
Bluemonday suggested a .300 Blackout. I believe Ruger has a supressor-ready RAR Ranch that only needs a can added. 200+ grains at just over 1000fps are easy-peasy. Cheapest option without a doubt.
Just so happens I'm picking up a new .44 rifle today. I believe I'm going to leave this one "loud" though.