Palmetto State Armory has their popular PSA 16″ 5.56 NATO M4 Carbine Classic Rifle Kit in the FDE colorway for $429.99. You save $70.00.
If you’re looking for “most powerful” then you’d always choose a rifle caliber over a pistol carbine and a carbine over a pistol.
The PC Carbine M-LOK model is available in 9mm and uses Ruger SR9 magazines. The idea is that you can share not only ammo but magazines between the carbine and your Ruger handgun.