Pistols are excellent short to mid-range weapons as well as being a rapid fire weapon. A skilled pistol user can even out shoot a sniper (seen it happen and have had it happen to me). Having 150 kills required is no hard feat for them.
Shotguns are all short range weapons (with the exception of the KSG, which is better suited for mid-range). The necessity to get in close with them to make a kill means that it's going to be a bit harder to get those 50-100 kills (depending on whether or not you do one then the other or both at the same time). The pump action of 2 of the shotguns and the brief pause of another makes the quick kills just a bit harder.
Sniper Rifles are INTENDED as long-range weapons. They are never meant to be a quick-scope kill (despite how game mechanics work). They are based around the user being relatively stationary and homing in on his/her target to make the kill. 2 of the sniper rifles also use a bolt-action system that slows down shooting. If you play Core, then the other 2 need a bit more concentration to get the kill on the first shot (not Core haters please). Even on Hardcore, the bolt-action sniper rifles may not kill on the first hit depending on whether or not you had to shoot through something and where that bullet eventually hit.
Overall, I believe the kill requirements are appropriate for the weapon classes. Some of the other Camo requirements of certain weapons bother me though, but we aren't here to talk about that.
I kind of understand it, seeing as they are in their nature niche weapons, but I think that the secondary challenges should be changed for more than just pistols. I say it would work best like this:
50 headshots required for pistols. 50 kills with no perks/attachments for pistols. 5 bloodthirsties for pistols. Replace "Destroy aerial vehicle" challenge for RPG. Change Executioner challenges from headshots to one shot kills.
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I guess the snipers and shotguns have only 50 kills because they're not the "best overall" primaries to use. Pistols should really have only 50 kills without them, too. 150 is ridiculous. Ironically, snipers and shotguns are the easiest weapons to use without attachments bar none. Honestly, attachments do not affect their performance very much at all.
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