The Marksman barrel attachment increases range one band by default, and can be modified with up to 2 accuracy mods - therefore adding up to 2 boost dice.

The Sniper shot talent allows a shooter to spend a manoeuvre to increase the range of his weapon one range band per ranks. Innate talents from attachments/mods apply to said weapon only, and therefore the talent is wasted on this attachment; and the attachment has the talent listed as a modification option , which seems weird since this attachment can only be applied to blaster rifles, which already have effective range at Long range and with the base modifier of this attachment gains extreme range.

Well, you can use Sniper Shot on any gun, right? So if for whatever reason you need to be using someone else's gun, or need to make a long shot while your rifle is out of ammo and you're using a pistol, then it's useful... ish. But instead of the rules, I think the talent is entirely flavor. Would you rather be inherently good at shooting long distances? Or would you rather own a gun that does a lot of that work for you? Different people might prefer different options there.

You're missing the point. The Marksman Barrel attachment (page 190) that can be added to blaster rifles only, has a modification option that grants a rank in the Sniper shot talent - but page 187 under the Innate talent mods category tells us: "this mod grants the user the listed talent only when wielding this gear and only affecting this gear " (italics by me). The italicised text tells us that the Sniper shot talent, which you can find on page 142 bottom the page, provided by the attachment can only applied to the modified blaster rifle. The effect of this talent is to increase range, but the base modifier of the attachment has already increased the range to the maximum possible range for non-vehicular combat, from Long to Extreme. So its a meaningless modification, there is no reason to pay the credits or do the mechanics check to get it since it will have no effect at all.

Now, if you already have the talent, this attachment might not be so useful, since you can already spend a manoeuvre to increase the range of your weapon - although the accuracy modification options are nice, boost dice are always nice to have but you won't need it, you would perhaps be better off getting the augmented spin barrel, you get more damage and can modify to get accuracy and pierce.

My point is just that the attachment provides you with a meaningless option when it increases range, then it also grants you access to another way of increasing range, but since Blaster rifles are already Long range weapons and the attachment increases the range to extreme, the modification option (Sniper Shot) is ineffectual and pointless. Now, if the barrel could be used on Slugthrower rifles, carbines and other Medium range weapons, it would make sense, but it specifically states that it " may only be applied to blaster rifles ."

Yeah. I missed that it was a mod giving the talent, and that you were not just talking about the general merits of buying the talent.

It might be useful if there is another attachment that reduces range. Up it, lower it, then get it back with Sniper Shot.

Sure, but that seems an odd design decision, although you could combine the marksman barrel with the spread barrel... for that effect. It'd cost you all the HPs of the BR and you'd end up with a Long range weapon, cumbersome 2 with a Blast 4 quality. Then of course you could modify that to gain accurate, increase the Blast rating and get the innate talent for an extreme range scattergun effect

While its cool, and I'd love a weapon like that as a player, as a GM I'm just thinking it weird that you get the to have two barrels, one barrel that spreads and scatter your blasts, and another that focuses and makes them reach longer. To me common sense dictates that these two barrels cannot be combined.

As far as I can find there is no other attachment for blaster rifles that reduce range. Still, that forward thinking they did for weighted head and so on, perhaps there is some up-coming attachment or weapon that qualifies as a blaster rifle with shorter range. I still think its odd.

Short barrel reduces range as I recall but that might be pistols only. That said, I'm pretty sure the rules don't intend you to put multiple barrel attachments on the same weapon.

That's my thought on the matter too. And yes, the short barrel is for light blasters only.