Actually, looking at the RGDs, the grease guns look like nerfed MP40s. I didn't look at everything, but from what I did look at.... they have the same damage, accuracy, and moving accuracy, but the MP40 fires longer bursts at a higher rate of fire and reloads faster and doesn't have to reload as often.
Hence why RGDs say so little some times. The grease guns work about twice as good as the mp40's atm, and will tear them apart at medium range.