I could imagen a space marine carrying a marine size Barret .50 laoded with autocannon shells.

I would use the following stats:

Astartes Precision Rifle:

Type: Heavy SP
Range: 300m
Rate: S/-/-
Damage: *
Pen: *
Clip: 5x2
Reload: 1 Full
Special: Accurate, Reliabe, Special
Wt: 32
Renown: Respected (or mission based)

Special: Cannot be silenced, comes with telescopic sight

Ammo*

Anti Personel Rounds:
Damage: 4d10+5 I
Pen: 5
Special: Felling (1), Devastating (1)

Anti Material Rounds:
Damage: 4d10+5 X
Pen: 10
Special: -

These entries are making me want to turn my Devastator into a monster sniper.

Santiago said:

I like this, though another special rule should be that the weapon can be fired beyond 3x at -10 each or something to a max of maybe 5 or 6x. because really a sniper rifle goes a lot further then 900m. I guess the range in the game is o reflect the TT so that is probably why the variation, though I try to keep it on a realistic scale.

Nimon said:

All the ranges in the game feel odd to me- the main battle rifle of the Guard only fires 100 meteres, 300 at max range? I think, as some people have mentioned, a lot of the ranges are drawn in to make melee viable. Giving standard battle rifles 5 or 800m ranges and sniper rifles 1500+m ranges dramatically changes the equation.

In this case, if you added the special ability, how would you modify the marksmanship talents that effect called shots and range modifiers? Our group applies flat modifiers, for example if you have no penalty for called shots, you really just have the ability to ignore -20 in size penalties. Shoot at something smaller than that, and you get modified penalties. An ordinary character would suffer a -30, but the character with the talent only suffers a -10 (ignoring 20 of it).

Charmander said:

Basicly just be another -10 at every range beyond 3x. You would have to have a scope just to do this, and some scopes could reduce the penalty, but you are right the range also is modified for the melee guys too. The thing with sniper rifles is that they are single shot though, so the additional range should not be to bad. A HB with the extra range could be extremly broken. So it would have to be specific to basic/exotic single shot weapons.

Nimon said:

Specifically I was referring to Marksman, which give the ability to ignore range modifiers when firing at long or extreme range. RAW would indicate the extended ranges would still have their big penalties, but it could make sense to say that the abilitiy essentially removed 30 points of range penalties, or if you're feeling generous alter it from 'long or extreme' range to 'at range.'

And my point for melee is if you have weapons capable of 1800m ranges, your melee guys are less attractive options. If all of the comat is happening close to 2km away, you can't really effectively participate. Though limiting it to a specific weapon, or class of weapons, would reduce this quite a bit.

I would also expect some enemies, notably Chaos and Tau, would likely have some sort of similar type of long range weapon, which wouldn't necesarily be all that much fun for players to encounter (an encounter based on 'find the sniper' could be quite interesting and exciting, being shot for 25+ damage pen 5 felling 1 in the noggin from 1800m could be a little frustrating). The more I think about it the more a weapon like that feels, to me at least, like a mission specific plot point rather than part of standard req.

Look I quite like the rifle I made, its strengths are that it is acurate and does alot damage to one target with the flexibility in ammo. The downside is that it is not a Horde killer and in very fluid combat would be inacurate i.e. when the user is being engaged in close combat or forced to fight on the move. When you guys read it like that does it sound all right?

Sounds about right, I noticed the Penetration of my version should be 4 and 9 but okay.
I wouldn't make it an Exotic Weapon but Heavy SP, for the rest....go for it.

Any new pieces of info on the topic of aiming with an accurate heavy weapon?
Or this special accurate heavy weapon?

I'm guessing it's a 'special snowflake' that is heavy and accurate, and so ignores the normal rules. Probably an easily-made oversight regarding the 'accurate' trait.