150 Khornate Berserkers running around our halls is still going to reap a bloody toll. Victus, and Santana might be able to go toe to toe with one of them head on, and I stress might considering the bloody life a Berserker leads. But I doubt we're going to see to many individuals on their own, and we're going to need to deal with packs of them marauding about. Coupled with the fact that many hallways aren't going to allow for massed firing lines of guardsmen, and we're talking a metaphorical Red Tide washing through the ship.

If it's khornate marines, your situation is dire. Your an Astartes so you should be good, yeah? Try saying that about the Guardsmen who are going to rout. Additionally, Assuming Khornates to be stupid is what got a living saint, an embodiment of the Emperors holy wrath, decapitated. It also got a chapter of marines almost entirely destroyed if not entirely destroyed, a contingent of Sisters of Battle mutilated, and a contingent of Guardsmen and armor torn to bits.

Do not doubt and do not belittle the Red Path.

You have a few advantages: first, your power supplies are enough for inner defensive systems to be active (I suppose there are some hidden in-built automatic turrets, for example?). Secondly, you have advantage in numbers and, given that lasguns are set to maximum power, this fact can halt khornates' assault a bit. Third, this is your vessel and, like Victus rightfully noted, you can block/isolate entire sections of the ship, halting the incoming assault.

To make this uneven fight a bit favourable for the imperials I'll make boarding torpedoes hit the ship's lower decks so that the assailants will have to cross quite big distances of the ship (it's still quite big, 3-4 kilometers in length) to reach the command bridge.

Well Victus has an Infernus/Inferno Pistol so he could potentially deal with 4-5 on his own without much trouble. Doesn't matter what armour they're wearing when a melta is involved.

No, all 300 bloody bastards made it . Such quite a big number of warriors is set up deliberately - if you kill them all, what will remain for Necrons, Dark Eldar and Eldar, after all?

So you're telling me that not a single one was in the entire front half of the ship?

I am generally unsure of the defenses of an Imperial Ship, so I'm not sure on the turret situation. All I can say is, reading through the books, you see Astartes dealing with boarding actions relatively well, so I imagine even the blood crazed berserkers are going to have an idea what they are going up against, and how to deal.

I can recall a situation in Betrayer where a World Eater gets shot in the head, clear as day by a mortal wielding a laspistol, and this does little more than irritate him. Lasguns are fine, but we're talking about fully armoured super soldiers. We also need to factor in that numbers are fine, but they can't be wielded in the manner to overwhelm the boarders due to the confines of the ship. Stuff a hundred guard into a hall, and most aren't going to be able to do anything until their buddies infront of them are dead. And since the Guardsmen need to head to stations across the ship, they are dispersed.

Controlling the ship is handy, but it's going to need a concentrated effort by the bridge crew. Going to be dealing with a rush through the lower levels, which considering that they are usually filled with undesirables, slaves, stowaways, even sometimes miniature populations, will only get them further pumped up. Good distraction however.

If you hit them of course.

I hate to the bearer of bad news...Some bits from a novel indicate that the pistol might let you down

Spoiler

" Roaring in fury at the desecration, she raised her inferno pistol and pumped two super-heated blasts into the back of the attacker. The first dissipated on the surface of the armour, but the second penetrated the ceramite and created a satisfyingly large hole towards the base of his spine. Alecia heard a howl of pain and flanked the abhorrence as it turned to attack her. She needed to move fast in order to finish the creature, and had just drawn the angle to fire another salvo when she felt a tearing sensation in her abdomen. "

and

" Gaul raised Acritus to strike at the nearest enemy, and felt a crushing force hit him from the side. As he was smashed to the ground, a chainsword plunged towards his head and Gaul kicked violently upwards with his knee, dislodging his assailant and sending him out of his view. Gaul was up in a heartbeat, and saw that the berzerker’s armour had been melted and burned at the base of the spine, inhibiting his movement. Turning with a roar of rage, the barbarian threw himself forwards with all of his might, his broken-horned helmet low. Gaul angled his relic blade down, allowing his attacker’s weapon to grate its way down its length. "

The above are tidbits from a Kharn the betrayer novel called " The Red Path ". The berserker in question while a part of the Dramatis Personae, had nothing really going for him. In fact, he was succumbing to the butchers nails.

You don't get to be a sniper in the Deathwatch if you're not a damn good shot. The advantages of such a close quarters environment work both ways and huge bloodthirsty maniacs charging you in a confined space... It'll be very difficult to miss.

They began escaping after the Sword of Orion first opened fire, even before you came up with an idea of ramming them