The original "firing port weapon":
The purpose of the mysterious "fin" on the bottom of the MP38's barrel (just behind the foresight) is to prevent recoil from forcing the muzzle backwards and into the vehicle from which it was being fired. The results of such a mishap are better imagined than experienced...
maximus otter
PS: I know it's not an MP38, but I couldn't find a good picture of a '38 on Google Images!
m.o.
thats the original one for sure, the M16 port firing weapon was pretty much a dedicated port firing weapon though ie it wasnt intended for gen'l use.