The magnum .55 is the most powerful, but some Desert Eagle has the highest calibre bullet.
Other than a toy gun or water gun, the next most powerful one a citizen can have without a permit is a "B.B. gun".
577 T Rex
star cannon
10 gauge shotgun
Probably the siege gun.
It is the dragon destroyer.
Yes the ray gun is lethal, the ray gun is one of the most powerful guns in nazi zombies
600 Nitro Express
If you are referring to the present, today's most powerful sub-machine gun is the H&K UMP .45 cal sub-machine gun, or the Kriss Vector SMG. if you are referring to the past however then its undeniably the Tommy Gun. also a .45 cal sub-machine gun.
My guess is the M1917 Browning machine gun in 30/06
A BB gun would be more powerful
Tactical 50 cal