Regarding the "less than 16" rifle" issue, Stormrider addressed that with his "$200 for the Short Barreled Rifle" (SBR) statement. You simply file a federal Form 1, complete with jumped hoops, pay your money and build it when your *approved form comes back* with "tax" stamp affixed.
I'd second the 7-08 suggestion. I shot a 7-30 Waters in a 16" T/C Contender quite a bit and it worked well. A little more power would have been welcome and the 7-08 will provide that.
If you're eventually going suppressed, and sub-sonic, bullet weight is your friend, and bigger calibers rule in the energy dept. FWIW, any (barely) sub-sonic round is going to have similar drop/drift figures Suppression w/o sub-sonic ammo still yields a pretty good "crack", though still not nearly as load as the report of an unsuppressed CF rifle.
The beatings will continue until morale improves.