Since we're not making decisions based on cost alone, I think you should consider a .300BO as a "pistol caliber carbine". Sub-sonic ammo for shooting steel up close and hi-vel ammo for hunting and defense. All the same ergos as your fighting rifle and cheap to build. I have a QC 10 9mm pcc...
I realize this thread has aged, but I thought I'd offer another option for the OP. I have two SilencerCo Omega 300's, a Hybrid .46 and two Octanes ( 9mm & .45). I have found the Hybrid to be the most versatile (can be used on a rifle or pistol) and an excellent first can. For those of you in...