Re: is it possible to build a bolt action 9MM rifle
If you load anything to subsonic speeds you get very quiet results. Keeping in mind that you have a 40 cal suppressor, I'd suggest you build a 9mm or a 10mm round. If you can get a 45 slug through that 10mm can then I'd strongly suggest you go with a 45 ACP
I've built a 9mm and a 357 mag into a Turk Mauser action as well as a 45acp. They don't feed at all. The 9mm needed to have the extractor modified and the 357 didn't, so the 9mm chamber was cut back and the 357 installed instead. I fired the 9mm a few times with 90gr bullets and super compressed loads of Blue Dot with results over 2600fps, clearly not subsonic, but nothing else in 9mm was going to get you velocities like that.
The limiting factor you're going to find on the 9mm is bullet weight. With 147gr bullets that suppressed 9's are good. My dad and I did a bunch of work with cast 158's last year to shoot in a 9mm and found that in order to get them to feed from a magazine the bullet had to be shoved so far into the case that it bulged out the body taper and the rounds wouldn't chamber. Using a 357 Mag with heavy cast bullets will get you the same thing with stronger brass and a long case to shoot heavier bullets from.
Subsonic rounds really are dependent upon bullet weight. At close ranges and with a maximum velocity that's common to all "subsonic" ammo you only get better knock down by a heavier bullet. Going with a 10mm bullet will get you 200-210gr commercial jacketed bullets and about 230gr cast bullets available easily. You can't get anything that heavy in a 9mm.
The 45 would allow a similar jump in bullet weight for an increase in knockdown. Casting your own bullets is a big deal when shooting subsonics, you can make much heavier cast bullets than you get from jacketed stuff. It's also incredibly cheap to do, just like reloading you'll see the financial recoup on equipment quite quickly.
An easy, turn key platform you might consider is the Ruger 77/44 in a bolt action 44mag. The bullet weights available for 44mag will get you some excellent clobbering power with the subsonic ammo, you can get off the shelf factory ammo and it's easy to load for, there's factory support, factory mags, etc. The rifle can be had for a little under $600. By the time you get it threaded and ready to go you're probably under $1000 for brass, dies, bullets, rifle, threading work, etc. My cast bullet reloads for my 44 mag revolver cost about $0.09/round
