3 actually. Pistol, Bolter and Heavy Bolter (minigun version).Also, no wonder Dan Abnett books are good, he is one of the founding fathers.
The storm bolter feels offended by being left out
Mea maxima culpa.Also, marines use different(=bigger) bolters than "regular" (SoB, IG officers, Inquisitors) infantry.
And then there are the different "patterns".