The small arms sounds are hit-and-miss. In some cases (such as the German MGs, the BAR, the M2 .50 cal and the M1 carbine), BFC has made sure they sound distinct. But pretty much all the rest of the sounds are far too generic, as all of the rifles and the SMGs respectively share the same audio file. The Bren, M1917, M1919, and the Vickers all share the same sound as well.
Do they sound different in reality? Yes. I own a good number of the rifles simulated in the various WWII titles, and yes, they all have their distinct sound. The various Mosin carbines, for instance, have a significantly different sound than that of a standard M91/30, even though they share the same action. About the only rifles of mine that I can say "share" the same sound are the various models of Mausers I have (K98k, VZ24, and M24/47), but that's for fairly obvious reasons.
I don't really buy that. BFC could add more varied weapon sounds to the game, and it wouldn't hurt things at all. We're talking about very small audio files here, and even the most basic onboard audio sound card can handle them just fine.