hey sharpshooterklj
Being a reenactor, I can tell you, that both the Mosin-Nagant and the K98 really dont smoke from the barrel much, if at all (I have never noticed it). My own K98, when I have fired live rounds through it, DOES smoke when the bolt is open, the receiver pores out some smoke. Aside from LMGs like the 34 and 42 you really wouldnt 'see' alot of heat coming off of the barrel. (although, no modern owner of a 34 or 42 in their right mind would fire either to the point, you can damage the whole weapon - and if you ruin the receiver, youre SOL, as the ATF hasnt papered new ones since 1986)
Although I dont own one, I have never noticed the MP40 smoke either, perhaps because it fires only 9mm. Being a German unit, we have no access to a PPSH41 (or any of the related SMGs - Soumi31 (Finnish), PPD40 and PPS43) so I dont know about that.