I was going to go into the whole primer, powder and projectile...

I was going to go into the whole primer, powder and projectile scenario but thought it was a waste of time after the exploding bullets comment, I'm sure most soldiers who've used machine guns of different variations on full auto at night have seen glowing barrels and know how hot both the barrel and chamber can become, obviously the biggest risk is of fire or excessive heat on the newly designed polymer casings being used instead of brass, there's an endless amount of ways live ammunition can come into contact with excessive heat and flames, I sure as hell wouldn't want a burning plastic shell casing dripping onto my skin, as you've said, we've all had hot brass inside our boots or uniform before, not very nice!

To most it's obvious that FR'ing a polymer round of ammo has nothing to do with how a round operates, but everything to do with the plastic casing.

Cheers.