If a ship's guns are aimed by a character with the Marksman talent, should he have no penalties for long range, as with ordinary combat?

I think he should. There are numerous examples of extrapolation from personal to ship conditions, like with the Concealment and Shadowing skills, and the talent description plainly states that the character should not be penalized for long or extreme ranges when making Ballistic Skill tests. When your character does the firing action in ship combat, he makes a Ballistic Test roll. Also, the talent costs 500 XP, and when it comes to straight aiming down the barrell, the same effect can be achieved with not hard to obtain gear and/or cybernetics. If the talent applies to ship combat, it still has something unique to it.

My GM disagrees---he thinks that whatever the textbook says, Marksman was obviously not intended for ship combat.

The talent costs 500 XP and when it comes to straight aiming down the barrell, the same effect can be acvhievd with not hard to obtain gear and/or cybernetics.

Unfortunately, I agree with your GM. Marksman is a talent from Dark Heresy that would be more helpful when you don't have hundreds of thousands of thrones, or are railroaded to Quaddis without equipment.

First, On Page 212 it even says that space warfare is very different from short-ranged firefights, and given that a Void Unit is approximately 10,000KM I don't think it matters how good a marksman you are, it shouldn't give you any advantages for ship combat.

Second, Marksman mentions eliminating penalties at long and extended range, whereas space combat does not have extended range

Third, the Turbo-Weapon Batteries ship upgrade was designed to eliminate long-range penalties, so it does seem to be a component limitation.

Its origin is irrelevant to me---in Rogue Trader you have billions of thrones and can get wares that do the same thing for a relatively easy acquisition test.

If shooting something in space doesn't have anything to do with how good a marksman, as defined in the talent description, you are, it shouldn't be a function of the Ballistic Skill stat in the first place. The name of the stat is 'Ballistic Skill', IOW, the stat is a quantitative measure of your ability to make things happen by the use of ballistics. Firing in guns in space is also within the realm of ballistics---the cornerstone of space travel physics is ballistics.

I can't really either see that the design choice of not operating with extreme range in space combat has anything to do with it.

You do have a point with regards to to ship components that eliminate long range penalties.

From page 219 of the RT core rules book:

"When firing a Weapon Component, the character directing
the fire makes a Ballistic Skill Test, adding in any appropriate
modifiers."

I think you are stretching here. As a GM I would not allow it. The modifiers in the book are the specific modifers for ship combat. I am not going to give you an Aim bonus for taking 6 seconds (a Full Action) to aim either. The modifiers should be those specific to ship combat only. You may disagree, but your GM would be smart to disallow the talent. In addition, you are trying to adapt a talent from Dark Heresy to Rogue Trader. That makes the inclusion of the Talent, and any interpretation, completely up to the GM. The GM would be in their rights to disallow the talent completely, as it is not a Rogue Trader talent. Also, the talent is written for a game where there is no ship combat. The talent itself was, obviously, not written for ship combat.

I don't think your analogy to the aim action holds water, frankly. Firstly, the timescale is several thousand higher in ship combat, what a character spends 6 seconds doing obviously doesn't matter. Secondly, actions are actions, and ship actions are ship actions and explicitly different from personal combat actions. Aim is not a ship combat option according to the rules, just like dodge and parry isn't. The Marksman talent, OTOH, isn't an action, it's about being a just as efficient shooter on long ranges as on short.

Now, the minutiae of aiming a gun in RT ship combat is not detailed very well. My GM argued that computers would be doing all the aiming. My counterargument is, like I hinted at above, that if aiming and firing guns in Rogue Trader has nothing to do with at least some of the following: hand/eye coordination, gauging distances, timing and motion prediction

...it shouldn't be a function of the Ballistic Skill stat. But it is.

You say it's a Dark Heresy talent, but it is also a Rogue Trader talent, just look it up on page 102 if you don't believe me. So, the adaptation has clearly been done by FFG, not me, and as far as I can tell, a large part of the overlaying design philosophy is just that: adaptation. Adaptation of the mechanics that were originally designed for small scale, personal conditions to grand scale encounters between gargantuan craft in space. Now, of course ship combat and personal combat can't be fought with identical rulesets---that would just be too silly for players to be able to suspend their disbelief. So, the combat actions had redesigned for ship combat(the order might not be correct here---I'm only familiar with the roleplaying games of the 40k universe), but the core of it is still the same: d100 rolled against characters' skill and/or stat ratings, after applying appropriate modifiers. In other words, no matter how huge your ship is and how many computers and other cool components it stocks, whether you succeed or fail always comes down to a character's ability, from a knife fight to two fleets of unimaginably large ships engaging in space. If I take the Talented talent and apply it to my character's Tech Use skill, my Aid the Machine Spirit rolls in ship combat will be more successful. If I choose to spend lots of XP on my Ballistic Skill stat, my shots in ship combat will be more successful. Ability does matter, and should matter, just as much in ship combat as in personal combat, and Marksman makes you more able, just like the aforementioned examples do. Why should it only be applicable in small scale conditions when scaleability is obviously a large part of the design?

If the Marksman talent was originally developed for Dark Heresy, with no thought for ship combat in mind, why shouldn't it be scalable like the Concealment and Shadowing skills, which were also designed before Rogue Trader was implemented yet have a much wider range of use in Rogue Trader, as they can be used to conceal and shadow space craft?

BTW, here are the talent descriptions from the respective books:

Dark Heresy Core Rulebook, p. 118:

Marksman
Prerequisites: Ballistic Skill 35.
Distance is no object with a gun in your hand, and you are just as adept at picking off far away targets as those nearby. You suffer no penalties for shooting at Long or Extended range.

Rogue Trader Core Rulebook, p. 102:

Marksman

Prerequisites: Ballistic Skill 35
The Explorer’s steady hand and eagle eye allows him to
keep crosshairs steady on any target, regardless of range.
Distance is no protection against his fire. The Explorer suffers
no penalties for Ballistic Skill Tests at long or extended
range.

So, clearly the talent wasn't ported verbatim from Dark Heresy to Rogue Trader. In Dark Heresy it is clearly written with shooting on a personal scale in mind. ("gun in your hand"; "shooting") In Rogue Trader it explicitly /does not/ limit its use to one specific condition ("any target"; "suffers no penalties for Ballistic Skill Tests").

If the devs were not rewriting the talent description to better fit Rogue Trader gaming, why did they not port it verbatim from DH to RT?

To answer the Talent part first, starship combat doesn't involve steady hands and eagle eyes keeping crosshairs steady... It involves Auger Arrays and guesses as to enemy ship movement and cogitator computations. The skills in predictiing enemy movement and relative velocities may be similar, but that is why it is BS instead of INT to shoot ship weapons. The Talent seems pretty obviously about personal combat. The Talents have almost all undergone some wordsmithing between different games. Read the same talent in DH, RT, DW, and BC and you will see some significant changes...even if it seems like it does the same thing.

You claim that the Aim argument doesn't hold true because of different scales. If that is true, why do you think a Talent that talks about 'steady hands and eagle eyes' keeping 'crosshairs steady' is appropriate to ship combat. Some of the choice to attatch shooting to BS may be because they both involve understanding ranged weapons characteristics, relative velocities, and being able to guess how a target is going to move. The other part of that choice is to keep the game consistant and ballanced. Realistically, you could argue that ALL ship actions require either INT or FEL. INT because you are figuring out what to do with the ship, or FEL because you are issuing orders that people need to follow. That idea leaves several of the players with nothing to do. Your argument that the core of ship combat is:

the core of it is still the same: d100 rolled against characters' skill and/or stat ratings, after applying appropriate modifiers. In other words, no matter how huge your ship is and how many computers and other cool components it stocks, whether you succeed or fail always comes down to a character's ability, from a knife fight to two fleets of unimaginably large ships engaging in space. If I take the Talented talent and apply it to my character's Tech Use skill, my Aid the Machine Spirit rolls in ship combat will be more successful. If I choose to spend lots of XP on my Ballistic Skill stat, my shots in ship combat will be more successful. Ability does matter, and should matter, just as much in ship combat as in personal combat, and Marksman makes you more able, just like the aforementioned examples do. Why should it only be applicable in small scale conditions when scaleability is obviously a large part of the design?

I agree with the first part, it does allways come down to a skill/ability check, after applying appropriate modifiers. I disagree with your belief that the Marksman talent affects your ability to hit targets at long range more easily using starship weapons (I do not feel that it is an 'appropriate modifier'). You also included two examples that actually raise your ability/skill, not change what modifiers should be applied. Note that you yourself said:

Now, the minutiae of aiming a gun in RT ship combat is not detailed very well. My GM argued that computers would be doing all the aiming. My counterargument is, like I hinted at above, that if aiming and firing guns in Rogue Trader has nothing to do with at least some of the following: hand/eye coordination, gauging distances, timing and motion prediction

...it shouldn't be a function of the Ballistic Skill stat. But it is.

Note that the talent is described as

Rogue Trader Core Rulebook, p. 102:

Marksman

Prerequisites: Ballistic Skill 35
The Explorer’s steady hand and eagle eye allows him to
keep crosshairs steady on any target, regardless of range.
Distance is no protection against his fire. The Explorer suffers
no penalties for Ballistic Skill Tests at long or extended
range.

You will note that the talent only includes 'hand/eye coordination' out of the aspects that you consider part of BS. It doesn't affect the parts of BS that seem to be appropriate to ship combat. It appears to only apply to those factors that I don't consider part of ship gunnery. On some ships you will not even be using your hands to interface with the ship gunnery station (MIU, which does provide a +10 BS bonus when shooting ship weapons).

You may choose to disagree, and that is your right. I think that you don't have a strong enough case to justify using the Talent for this and would not allow it in my game. Now, if you had taken the Talent for that purpose I would have warned you when you took it, or allowed you to get the XP back when I made the ruling. I hope that you take a step back and accept that your GM has made a ruling, and if it is appropriate ask if you can give up the Talent for XP if you took it thinking that it would apply here. Keep the discussion calm, and provide a compromise. The GM job can be tough, especially when you have to make calls that the players don't like. Make it easier on your GM, and you will all have more fun.

You may choose to disagree, and that is your right. I think that you don't have a strong enough case to justify using the Talent for this and would not allow it in my game. Now, if you had taken the Talent for that purpose I would have warned you when you took it, or allowed you to get the XP back when I made the ruling. I hope that you take a step back and accept that your GM has made a ruling, and if it is appropriate ask if you can give up the Talent for XP if you took it thinking that it would apply here. Keep the discussion calm, and provide a compromise. The GM job can be tough, especially when you have to make calls that the players don't like. Make it easier on your GM, and you will all have more fun.

Alternatively you could ask your GM if they would be willing to allow you to spend additional EXP to gain(make) a talent that allows for a ship scale version of marksman, possibly having marksman as a prerequisite.