It occurs with the Mk 18 CQB and the M249 as well. 5.56 weapons in general, it seems.
Recently, I've noticed that when swapping from a Sniper (like the Mosin, and SVD for instance) to a pistol, frames drop considerably for a second or two. Typically, I'm running around 100 FPS, and it dips into 40 FPS for those two brief seconds, before going back up. The odd thing about this, is that I don't experience this frame drop when swapping between a shotgun to a pistol, or a rifle to a pistol.
I have a GTX 1070 with an i7 5600k processor.