As far as I can tell, there is no actual bug/glitch with the weapon against armor. It seems to be working properly, it just sucks against it.
Part of that might be the third shot that would be needed to break the chest armor hitting the head armor instead, knocking off neither piece.
It's something we're definitely looking at as a potential "buff" to the weapon, but we've got some other issues to deal with first for our next patch. We'll do a balance update after the next big patch (Hopefully).
Burst Rifle headshots are 3x damage.
If it's 5 bullets to down, and headshots are triple, then 5/3 = 1.8, so 2 bullets to down as headshots.
Burst Rifle doesn't sound so bad anymore..