I must be blind; I have both the core rules and BF Koronus, but I can't find any entries. If you know where I might find them, let me know.
Regardless, they seem easy enough to create, set their skills, att's and armor in accordance with the skill of the crew:
Incompentent- The armsmen are mostly bilge scum and local toughs-with-something-to-prove themselves; poorly trained with little or no discipline, who act more like a gang than a legion of soldiers. These mooks are easily bribed or overrun and won't risk their lives for much besides their own lives and purses.
Compentent- The armsmen are decent and disciplined soldiers, at least the competency of regular Imperial Guard, who treat senior crew members with respect. They assail lower crew echelon members with appropriate amounts of intimidation, being trained well in both boarding and repelling boarders and dealing with mutinous situations. They may be well-trained or well-equipped, but are seldom both. They will only surrender when their Lord Captain commands.
Crack- The armsmen are either highly-trained professional soldiers or experienced veteran Competent armsmen. These soldiers are trained almost exclusively to repel boarders and to execute disciplined boarding actions. However, they have additional training in crowd control and mob pacification. They are both well equipped and trained. Any senior crewmember can depend on these armsmen to gladly lay down their lives or fight to the last man. They consider the explorers' vessel their home and defend it accordingly, fighting long after the ship is lost or the rogue trader has abandonned them to their fate.
Veteran- The armsmen are comprised of the Rogue Trader family's elite troops, or they may be even longer serving veteran Competent armsmen under the rogue trader's command for decades having been duly rewarded for their service and loyalty. They have been trained exclusively in void-ship based combat and have the experience of many police actions. They have received additonal special training in fighting xenos and have knowledge of their tactics. These soldiers also fight in small, highly coordinated units designed to maximize guerilla tactics. If they are house soldiers they may have also received loyalty conditioning that makes even thought of insubordination (to the rogue trader) or surrender impossible.
Elite- The armsmen are long-serving veteran Veteran armsmen or superlatively trained voidship commandos. They are the pinnacle of discipline, loyalty and training. Many are known on a first name basis with the rogue trader and probably consider him or her a family member (akin to a mother or father). These soldiers operate so well they are more like an single living organism that a legion of individuals. They effectively utilize guerilla tactics on invaders as well as the voidship's crew; often anticipating insurrections before they occur. They are superlative ship boarders and leaving their quarry permanently unharmed unless ordered to, which they do with terrifying efficiency, thus convincing the enemy to surrender rapidly. These armsmen are skilled with subterfuge and disguise, as well, and are able to mesh seamlessly with the voidship's regular crew in order to keep tabs on troublemakers.
The idea being that Armsmen are akin to marines (not Adeptus Astartes) from the Age of Sail; an onboard police force of elite solidiers for the intimidation of a conscripted crew, the security of the ship and boarding actions, when the poorly-skilled, undisciplined and mutinous crew just doesn't have their hearts into risking their lives for the enrichment of their own navy.
Let him who is with sin cast the first plasma bolt. - Warmaster Picklehauber