Q. But military ammo has flash retardant, right?
Well, not exactly. This had a lot of us fooled too.
Back in the early Vietnam period M193 called for flash retardant components to be included in the round. Despite this no current specification any of us are aware of calls for flash retardant in M855 or other military small arms rounds. After some references from former procurement officers and contractors it's pretty clear that current standards don't call for it. It's a toss up if there is any flash retardant in your rounds. Surplus M193 might contain retardant, but fresh M193 probably does not. The best way to find out is to test it.
Opinion: According to one ballistic researcher of note:
"It seems that Picatinny Arsenal feels that flash suppressant might eventually cause a build up-in the gas tubes of M16 type weapons and cause the weapons to malfunction."
He goes on to note sardonically:
"...of course this will not happen since all the soldiers will be dead before this theoretical fouling problem occurs..."