Most PCCs are overpriced range toys or aimed toward police departments that don't trust their pigs with real submachineguns or AR-15s. The exceptions are the Hi-Point carbines and the Kel-Tech Sub2000 which are worth consideration. You can get the former for like $270 and the latter for around $500. They occupy a niche in that are an economical substitute for an AR-15 for rifle problems and home defense/ fighting inside buildings like a pistol or shotgun. The higher end ones are pointless.
So 9mm is the best caliber for the PCC and the best kinda ammo is 9mm NATO, 124gr ~1250 fps. Hi-Point makes .380, 9mm, .40, .45 and 10mm carbines. Only the 9mm and 10mm are worth consideration because everything else has awful ballistics and fails to do the niche thing PCCs can do like quick accurate shots out to 100-150 yards. 10mm that's actually loaded beyond the energy of .40 S&W is like $1.40/rd and always out of stock. There is an ideal of a 10mm Carbine with like 1500 fps with a 180gr round and doing 1000+ ft/lbs energy from the barrel deadly 20-30-50 stendo round assault clips. You can modify any .45 hi-point mag to feed 10mm by pinching it's feed lips closer then it will hold capacity +10%. 44 Round 10mm drum mags.That's all cool but let's consider 9mm carbines to be the only real choice because the main draw of a Hi-Point carbine is being cheap and 9mm is the cheapest.
So for 9mm options, Kel-Tech takes 32rd Glock magazines is extremely lightweight and folds in half. Hi-point takes single-stack magazines of dubious quality ranging from 10 to 20. It's perfectly adequate. It's not a combat weapon unless you have to. Then donkey-dick 9mm stendo assault-clips, almost do it.>>383
Most pistol calibers reach max velocity at about 12". There's no real reason to get a barrel longer than 16".