Wayne I think you might have stumbled onto a similar issue as Blitz 2 's Maus.. it has some odd issue with it's muzzle flash similar to what you've described.
If you'd be so kind as to tell me the EXACT name of the material you're using for teh muzzle flash and the exact naming of the Muzzle Flash boneobject.. We might be able to pin down what the cause is.. I think I know what it is but I'm not sure and since I've only seen it happen on one model so far cross referencing with yours would be helpful
