The Valkyrie Assault Rifle

I dunno, but mine will kill a Centurion trough a wall on a Human Infiltrator and I don't even use the 25% extra damage barrel on my BW, because it makes it look ugly. BW also synchronizes with Cloak perfectly. You can get off 3 shots in Cloak's damage window, reload and be ready to Cloak and fire again, while with Widow - Cloak, fire one shot, reload and maybe get one off still in damage boost, but only if you quickscope. 

BW is also much more useful in CQC in smaller maps like Glacier or if when you're getting overwhelmed. You can force 3 enemies to dodge or back off or to simply kill them, before you gave to reload. Also you can pop a thermal pack and have 3 shots ready again instead of 1. It's also not as costly to miss with BW when your screen shakes, the enemy get's staggered or whatever may happen. You simply adjust your aim and have 2 shots to finish something off, instead of having a big empty stick in your hands. 

As for Claymore - it's certainly more powerful. But Talon is a better mook killer (especially Phantom killer) Talon will 1 shot most things with a headshot and again - it's not as costly to miss 1 or 2 shots in clip (and you will miss, like it or not - because husks, Nemesis, Phantoms, Goons are crazy and don't know what they are doing, they moon walk, change directions, flip around and so on. 

you both need to L2Headshot

I prefer weapons that have several shots in the mag for a few reasons. Particularly, as was mentioned previously, that it's not as costly when you miss. And weird moving husks/aboms/etc can be difficult to hit.

With a multi-shot weapon, you could spray some fire at husk-types, which stops them for a second, and even prime them for a Fire Explosion in the meantime, as you roll backwards then hit your Detonator Power. Or you could spray a few rounds at Dragoons/Marauders to halt their advance, trigger their roll, then line up easy headshots as they stand up. And again with the Fire Explosion, toss your Proxy Mine or whatever have you at them as they are rolling, then headshot as they stand.

With a single-shot weapon, you can be stuck reloading (especially problematic if you get hit by something that either staggers you, or makes you just stand still and unresponsive to commands for a second), use a power (if you have one worth using and it isn't on cooldown), switch weapons (takes time you might not have), or use an Ops Pack and go down anyway, followed by a MediGel and Rage Rocket. That's hardly efficient.

The Valkyrie isn't the best gun, but you can make it work. As has already been said multiple times here, Turians make this gun work. I like to put it on the Turian Sentinel from time to time, works really well on Gold. I regularly alternate between the Hurricane, Phaeston and this gun on that kit.
The continuous clicking becomes tiring very soon, though.

Good gun once you've fixed its recoil and accuracy issues, but horrid off the shelf with every second bullet missing it's mark.

Note: fixing the issues doesn't actually require a turian, it's just the easiest method.
The scope+stabilisation mod option works just as well at getting it to shooting straight, although I wouldn't recommend soloing with a scope.

Also, drill rounds are probably the best ammo choice, getting multihits on bosses outweighs the minor difference in armour damage reduction IIRC.

Yes, as spraying fire at advancing melee enemies to prime them with fire ammo and proc a roll animation to detonate as they are recovering requires no headshot skill whatsoever.  The headshot at the end isn't even necessary, a bodyshot at that point will do just as well.

1) Put it on the Quarian Marksman
2) Get headshots
3) ??????
4) Profit

FTFY

And I didn't even mention how fat lolturian stability is always broken anyway, so have fun not even getting any advantage from your morbid obesity except maybe handicapped skycar parking.

CS is lol. I'd literally rather have Sabotage, as at least I might use it once or twice a game vs a turret or prime that's in the way (as opposed to never because I'm not playing some garbage cryo soldier or whatever).

Tacticool Scan goes through walls as well (it even sees through them), and it also does damage (enough to kill swarmers, which is all you need). With Valkyrie, using Tsol's PM on multiple enemies is pretty much pointless because you'll have 4 seconds of no MM (thus 50% less deeps, even less because un MMed Valkyrie has horrible bloom and misses everywhere, and then even less for no headshot bonus) and then only 4 more seconds of debuff.

Actually, it's pretty useless on bosses with this weapon for the same reason, in that any 20% gain you make in the last 4 seconds it's active is cancelled out (and then some) by firing 50% slower for the first 4. Tacticool scan by contrast will decrease boss TTK as with most weapons, exceptions being something retard heavy like the shitfire that tanks the cooldown far past the animation. PM's really only useable as an individual debuff with very lightweight weapons.

Great power if you wanna help debuff and serve your rightful role as a support class for a superior character like Quarksman, tho in that case I'd suggest you play the much better Volus Engineer.

Guess what my last two UR upgrades have been? I swear to God that store hates me.

As for the lively Quarian v Turian debate, I can vouch for Xen's capabilities, but for the less skilled among us the TSol would still be the preferred option. Quarksman, especially with the Valkyrie is for people (and Admirals) who no longer need to Git Gud, imho.

A tangential word or two in defence of Concussive Shot though - I've played a TSol with an Executioner Pistol and had an absolutely great time managing to use all three of his powers to good effect. You can mow through mooks reload-cancelling with CS in a way I find much more useful than Marksman, on occasions anyway. I know you can reload cancel with MM as well, obviously, but I sometimes find myself exposed after the second shot has gone and I have no other powers at my disposal and I'm too Fat to dodge.