A few quick notes here:
Physical mask is the next step, if you need one. You're already probably using it to disguise your weapon (a gauss rifle and a sniper rifle are both practically impossible to conceal through mundane means). Physical mask can alter sound as well as sight and smell. There's no reason why your firearm discharging should sound anything like a firearm. Nor is any reason it has to be anywhere near as loud.
Right, so you cast Physical Mask on your
Guass Cannon and mask its noise to sound like silence. Now, what happens when that assault cannon round hits on the other side? It explodes like the main tank gun round that it is and causes a ruckus from hell. I'm not saying that a SR round hitting a target is the quietest thing in the world, but its a hell of a lot more subtle.
Now in defense of your point, you could technically cast Physical Mask on each bullet too , but you either A.) have to do this in advance which is going to result in one hell of a sustaining penalty or B.)have to spend complex action + simple action (assuming smartlink) in between each shot doing this (free to drop the clip, complex for spell casting the physical mask (you have to touch the subject, ie bullet), and simple to load clip again). All in all, it gets rather cumbersome. Of course, in addition these spells can make you very obvious to any sort of astral security.
Now there is also stealth and silence, which used with each other can provide a decent coverage (stealth on the user and silence on the target area), but silence is fairly noticeable when it goes off. After all, we're used to hearing background sounds near us all the time and it's pretty noticeable when they stop...all at once. Keep in mind that Stealth will not take care of everything by itself because you are firing
explosive rounds with an assault cannon. Unless you're stealthing the gun and the ammunition, but you'll still need to see the above issue with physical mask. Fortunately it's not as big of a problem because a simple slit in the mag will allow you to cast without needing to drop/reload clip each time you fire. It's still cumbersome. Silence will work by itself
if you are close enough for you and the target to be within range, but I'd hardly call it a sniping situation at that point.
Okay, apparently my GM was doing it wrong for the walls... and making them too effective. He was just straight-up subtracting the Barrier Armor from the AP and DV before the bullet gets to the person, rather than wiping out AP, comparing the remaining value, and adding the rest of the BA to the target's Armor if it gets through.
No, I was taking his AP out of the Armor rating and adding the left over to the baddies armor. I tracked his DV to make sure it was exceeding the armor rating (otherwise it bounces off instead of going through).
Mechanically the Guass Rifle is better for firing through walls, but as a GM I'd toss a pretty hefty modifier on attack rolls to represent you're trying to aim so that the shrapnel (from the round that exploded when it hit the wall) hits the target rather than a solid round penetrating through.
Soo... we're back where we started like 2 pages ago, with "if you don't care about effectiveness, just pick the coolest looking sniper rifle. If you do, use Heavy Weapons."
But that's not the actual true statement at all. The real statement would be "if you don't care about having the maximum DV/AP just pick the best sniper rifle. Otherwise use heavy weapons." DV/AP are not the only things to take into account when it comes to a weapon's potential/effectiveness.