I personally would have it play out something like this
Commissar: "All present at this disturbance have been volunteered for special duties, Provosts load these.... people... into the trucks outside"
Have all the people present guilty or not guardsmen or not roughly bundled into trucks outside, any that resist get cattle prodded until it stops being funny, any weapons stripped off them. They get driven off to the front line for what seems like hours (if they start thinking about making a break for it have someone else try and have them get gunned down or flammered perhaps (and be very descriptive with them running screaming as the flaming petroleum sticks and burns melting skin and flesh, maybe a provost gets sickened and puts them out of their mercy ending up thrown in with the rest if you feel particularly cruel)). They get to the front and are bundled off the trucks and pushed into a rough line, the commissar getting onto a ammo box.
Commissar: "You reprobates are being given the chance to serve His Holy Majesty The God Emperor Of Mankind and expunge the crimes committed against HIM today and the many for which you undoubtedly have not yet been caught for, you will have your records expunged and those of you who survive will be allowed to continue to serve HIM in the most righteous armies of the Imperium, do any of you shirk this honor and wish to serve your sentence in another manner?" (Bold and caps for when he is getting particularly shouty/frothy).
Have one man put his hand up, a single shot rings out and he falls a smoking hole through his forehead (or a bolt pistol for scanner type head explosion if you would rather).
Commissar: "Anyone else?" I assume none of the players put their hands up "Very good, to the front"
Have them herded forwards past groups of guardsmen overhearing/seeing things like:
-A sergeant/officer explaining that "a few more waves and we'll be going in to clean up, the greenies have to run out of ammo sooner or later"
-A guardsman running book on whether there will be any survivors from this wave, giving odds of 100-1 (or higher) and getting no takers
-A religious guardsman mumbling a prayer for the dead and lost while making the sign of the aquilla over himself
-A guardsman making signs to ward off the evil eye as you look at him
Have them get to the frontline trenches and see the remains of the last wave fleeing back towards you, carefully emplaced heavy stubbers/bolters mowing them down. Scratched up and battered weapons thrust into their hands (poor quality or just not very good (go for a mix of things like lasguns of different types, auto pistols, stub rifles, shotguns, autoguns etc) and no other equipment (so I hope they brought their armour with them) and then they are driven forwards towards a large orky fort under the shouts of commissar cadets and the blows of provosts (think very much the opening battle scene in enemy at the gates), have people gunned down around them but the player characters as long as they don't do anything too stupid should be able to take cover in a shell hole or something (maybe a couple of comrades get killed) along with some others, I'd include some of whoever they were fighting in the bar brawl and maybe a few civvies as well, force them to work together to survive. Make it clear that going back isn't an option, (maybe a few from other groups try to slip back behind the front line and get gunned down) but have one of them spot a way inside (maybe a drain or similar) with them having to coordinate suppressing fie in their disparate little group to be able to run across from cover to cover.
They sneak inside the fortress maybe having to quietly dispose of some grots along the way. Inside they discover some ragged human survivors chained up (a sign saying "Slaevs/Emurgenci Eatz") being menaced by a sadistic slaver/mad dok, his "Aprentuzez" and grot "Oderliez", cue frantic gunfight/close quarter battle with every round of fighting giving a greater chance of Ork reinforcements (I imagine that a few rounds of wild gunfire probably wouldn't cause orks to worry that much but if it continued it might make them want to join in the fun (I mean if there is lots of noise then there must be lots of fun!)) assuming that the party survives the "fun" any surviving slaves inform them that there is a ammo dump further inside which the player party could sabotage blowing the fort apart. Depending on how well the party is holding together you may want to throw another firefight at them (maybe as they set the fuses to blow the ammo they have to hold off some orks/grots, holding for long enough to set the fuses and let them run enough that the enemy is unable to stop the explosion but giving them enough time to run (and for a action hero jump away from the explosion!).
This would give them a sign of what messing around with imperial "justice" can do to you, give them a hard fight, the possibility for some interesting replacement comrades (if you wanted to) and show them what happens to people if they don't concentrate on fighting the orks.
Sorry I meant for this to be just a few ideas but I ended up half writing an adventure session.... been writing a new campaign for one of my gaming groups and may have broken my brain a little!