How many space marines need to board an enemy vessel to take control of it? Does it matter how well they are equipped? My standard 'Space Marine" wears heavy powered armor and is equipped with what the game calls a "crew served weapon". I see them as Warhammer 40K troops carrying Gatling weapons.
Using only crew-served Anti-Personnel (CAP) is not necessarily the best. The problem is that basically, a formation comprised solely of CAP is far more vulnerable to losses, since each CAP is 12 tons, compared to the 5 tons of normal PW. What I usually use is 250 ton "Marine Squads" with 1 CO, 24 PW, and 10 CAP. That way, you will lose firepower more slowly, since losing a PW only removes 1 shot, compared to losing a CAP removing 6 shots. Basically, I'm reducing firepower by 1/3 in exchange for having more health in the unit.
Effectively, assuming equal tech levels, and that marines have heavy armor and advanced gene mods (2x armor and 2x health), then each crewman has a 0.390625% chance to kill a marine when firing, so theoretically every 256 crew will kill 1 marine per combat phase. Against higher tech crew this rapidly decreases (ie if you have racial armor 12 and defender has racial weapon strength 15, then they only need 105 crew to kill a marine every combat phase.)
In comparison, a marine has a 12% chance per shot to kill a crewman, so a 250-ton formation with 1 CO and 20 CAP can kill 14.4 crew per combat phase. However, if the enemy has around 550 crew (which is about right for an undamaged 20,000 ton beam ship), then you will be losing around 2 marines every combat phase. A formation with 1 CO, 24 PW, and 10 CAP will only kill 10 crew per phase but can absorb those 2 marine losses per phase with less loss of firepower since it will only usually lose about 2 shots per phase, rather than losing 12 shots per phase.
CAP will always kill an enemy crew the fastest, which can sometimes be critically important. However it is true that they take heavier losses. What is not true is that deploying PW in your marine formations gives you any benefit, in fact the opposite as PW are worse than CAP in any relevant way. The correct weapon to use instead of PW is PWL, alone or in combination with CAP.
Consider a test case of a 250-ton marine platoon (for the sake of easy numbers, we'll say 10-ton HQ and 240 tons of soldiers) attacking a ship with about 200 crew. Ship crew is modeled as INF+PWL with half their racial armor value and 2.0x fortification value, so if we assume equal tech levels this equates to each crew member having 0.5 HP, 0.5 armor, 0.5 attack, and 0.5 penetration compared to a standard unarmored INF-PW having 1/1/1/1. For the sake of example we'll consider our Marines to have 1.5 armor and 1 HP each (basic power armor and no gene mod techs, the bare minimum needed for marines).
The enemy crew at the beginning of combat can fire 200 shots per round, with a 2.8% chance to score a kill with 12% accuracy (base 20% times 0.6 evasion for INF), which comes out to 0.67 kills per round.
240 tons of CAP comes out to 20 guns firing six shots per round which are guaranteed to kill if they hit, which they have a 10% chance per shot to do. This comes out to 12 kills per round; neglecting attrition this means they need 17 rounds to kill the entire enemy crew. Over those 17 rounds, to a first approximation (i.e. neglecting attrition) the enemy crew can kill 5.67 soldiers which is a full 25% or 30% of the attacking force. In actuality this and following estimates will be somewhat higher due to loss of killing power due to attrition, however the comparisons I make remain accurate.
240 tons of PW comes out to 48 guns firing one shot per round, coming out to only 4.8 kills per round, requiring some 43 rounds to finish the job. During those rounds they would take approximately 14 casualties, similar to the CAP. Thus the
PW offer nothing better than CAP.However, PWL against
equal-tech enemy crew only can score 1 kill per hit, and 240 tons of PWL comes out to a much more impressive 80 guns and thus shots per round. Kill rate goes up to 8 kills per round requiring only 25 rounds to kill the enemy crew. Loss rate will be approximately 8.33 casualties which is just over 10% of the force, a massive improvement. However note that due to the marginal stats of PWL, an enemy with superior armor tech will be difficult for a PWL-only formation to defeat.
Therefore against equal-tech only (or at least your weapons tech matching their armor tech), PWL is the most effective marine weapon - and this is not a difficult requirement to meet if you prioritize weapon tech, especially if you do as some players do and research plasma tech as a cheap way to boost your ground forces. However, if the enemy armor tech is superior to your weapons tech, PWL fall off very hard and CAP is most effective. I would recommend either an all-PWL marine force, if you are confident that your weapon tech can match the enemy armor tech, or else a 50/50 mix by tonnage of PWL and CAP which will take slightly higher losses than a pure-CAP formation but since half those losses will be PWL infantry the percentage of losses will be much lower. The latter formation is able to defeat superior-tech enemies if you happen to run into them where the PWL-only formation will suffer badly.
It is also worth noting that deploying infantry with more armor and HP mods drastically reduces the kill rates. Compared to the basic 1.5 armor, 1 HP marines I've modeled here, fully-upgraded 2/2 marines will take about one-seventh as many casualties, at which point the actual cost of replacing losses drops off significantly (especially if you use larger/multiple formations to assault a single ship) and the sheer killing speed of CAP arguably becomes more valuable.
