Not too bad, but I wanna point out few things that caught my eye. ( Sorry if I'll sound dick-ish )
Why is a 25 year old guy considered a veteran? 40k humans have a well expanded life-span compared to ours and even people of 50-60 years are considered relatively young. Not that it matters that much, but just pointing it out.
The ambush in start, does it happen at the beggining of the short story or middle? Somehow I find it hard to swallow that Infilitration experts like Alpha legion would get caught so easily in an trap sprung no less by mere Arbites. The way they fight back also suggests gun blazing all over the place, instead of trying to silently dispatch the arbites? maybe silenced bolters, etc? To fit with the theme of secrecy?
Second thing is your main character catching a bolter round with the lower part of his helmet. Im pretty sure that would have completely blown up a part of his head, unless that helmet is made of starship armour.
As for my story, I've yet to submit it.
Third, for some reason everybody wants to write Arbites? Why? Im genuinely interested as to why. Are they that interesting?