If a gun scores a hit on Infantry Target Type and the subsequent DR on the IFT is a 12 does the Gun malfunction? Ammo shortage does not apply. I couldn't find the reference.
No. A Gun never Malfunctions on the Effects DR following a TH DR.
C2.28:
2.28 BREAKDOWN: All Guns have an inherent B# of 12 if not listed on the counter. In cases where malfunction was more frequent, a lower B# is printed on the counter. All normal malfunction and repair provisions apply (A9.7-.74). If a non-vehicular Gun permanently malfunctions it is immediately removed from play.
A9.7:
"...Whenever a SW participates in an attack in which the Original IFT resolution DR (or To Hit DR in the case of ordnance weapons) is ≥ that SW’s B#, that SW malfunctions and is inverted..."
The references are A9.7 and C2.28: "Whenever a SW participates in an attack in which the Original IFT resolution DR (or To Hit DR in the case of ordnance weapons) is > that SW's B#, that SW malfunctions." C2.28 says that Guns are treated like SW for malfunction.
In some cases I have seen it wrongly played that people think a 12DR on the IFT-effects would be a Dud.
This is not the case. Duds can only occur on a 12 TK DR (C7.35).
Basically, you can say if you roll a TH, then Malfunction is possible. When you roll for the effectsof a hit, then it is not. Note, that an IFE shot (like the dreaded 20FP shot with a FlaK-Vierling for example) does not require a TH roll, so IFE shots are subject to Malfunction.
I tend to remember it as, any weapon can malfunction on each shot, and this is determined by the first DR for the shot. I'm sure someone will come up with an exception to this rule, but it works pretty well.