I will take part in this mission, for our chapter's honor and to bring His wraith upon these xeno scum
I thank that we should have both grunts take a corridor, the flammer in the north room and the captain in the south to keep the genestealers from getting in. The two bolter brothers take overwatch and fire as the xeno come closer, and once the tyranids get into assault range or if there's a large number of them, they retreat back and let the other two take stage, the captain blocking the genestealer's retreat and the flamer torching any in the middle cross road.
Having one grunt take each corridor is a bad idea, there is no position in the corridor which allows both corridors to be covered without having at least one marine in a position to be eaten without one turn worth of warning. Keep in mind that genestealers get six ap, and can turn without penalty during movement. So a marine in the center space can be hit by one close combat attack the turn a stealer breaks cover. If there are two marines standing back to back, one of them (the off center one) will be subject to 2 attacks. Keep in mind that if the genestealers entering from the farthest entrances move first, the grunts bolter can jam before the closer stealer moves.
having one marine cover a corridor is an acceptable risk for a round or two while the rest of the team moves. It's not a good idea to count on one marine to hold a short corridor for many turns. If the marines hold the horizontal corridor, we will lose at least one, and most probably at least two marines.
Ninja edit... huh I didn't know who could spend cp during the genestealer phase. Can you give us the mechanics for causing a weapon to jam? I want to run the odds about how long it will take for a bolter to jam twice (i.e. how probable is it that on a turn when we only have 1cp, there will be more than 1 jam).