You just need to consider what skillset your character has when choosing what weapons to use.
I like my Vanguard character best. This character is maxed in reload speed, recoil handling, trigger happy and 2 points in weapon sway, so it’s a gunner build suited for KVM 59/89 use. Reduced weapon sway helps for sniping.
The 4 go-to weapons for my “main” character are:
6* KVM 59, iron sights, compensator.
The KVM’s 250 round magazine outweighs the slightly higher pr. shot dmg. of the AG4 when focusing on full-auto shooting, and the ammo is plentiful. I find the KVM’s easiest to handle with just iron sights, so try that if you haven’t already. Note that the 5* version has the same damage as the 6*, just without the lightning. The lightning’s downside is that it can hit you as well. (A sniper build with make 'em count perk should use AG4 with a scope instead.)
6* M/46 Kpist, iron sights, barrel extension.
Insanely accurate at short ranges, even on overdrive. This gun can take down a Hunter just as fast as the KVM’s, if you know where to aim. Extremely effective against a pack of Runners at close range, and even a few Hunters or a Harvester. (On my sniper character I bring a shotgun instead of this one.)
6* PVG 90, 6-12 scope.
Pretty much speaks for itself. Just do yourselves a favor and stick to the 6-12 scope, not the 8-16! 16 is more than enough for the range you can see enemies at, and 8 is too much for mid-close ranges. Also, quick scoping with this weapon is extremely precise and effective. With a little practice, you can use it at short ranges to make quick junk of machines that sneak up on you while you’re sniping.
5* .44 Magnus, silencer, handgun scope.
Basically a mini-sniper. I use it against Runners if they’re no more than 2 or 3 together. Also useful against Ticks and Seekers. Quick-scoping works really well with this weapon too. It’s actually really fun to try a few longer shots with it against unsuspecting Runners, as the bullet’s flight path is really curved.
(I sometimes like to bring the silenced Älgstudsare along, just to pop the fuel tanks on a few Runners and Seekers, because it’s fun.)