.223/5.56x45 and .308 or
12ga with stock barrel and rifled slug barrel and .223/5.56x45
Or, you could check out the Thompson Center Dimension rifles - they have interchangable barrels etc, for up to 8 or so different cartriges. and a .223/"
!.1. H&K USP in 9mm or .40 or .45.(Probably 9mm.)
1.2. Bushmaster M4 Patrol Carbine.
2.1. Remington 870 or Mossberg 500.
2.2. Any Existing Clone of the MP5, Whether it be a HK94, a Arms-5, or even a SP10.