I'm a shotgunner, so of course once I hit 10th went on the path to get all of them gold. Previously to that I had only used the striker from 1st-10th prestige (got 22k kills and 2k headshots with it), although, I would never spray it which is why it was such a hated gun.
The first gun I got gold after the Striker was the Model. And it was a pain. Once you get damage on it it becomes slightly more bearable, but before then I would have to resort to dropshotting to be able to get a cock in before the other person shot me. Even then I'ld only survive 50% of the time if the first shot hitmarkered... of course then theres the times when you would get 2-3 hitmarkers in a row -.-
You probably need aproximately 500 kills with the gun to get damage on it, I'ld say go for it. It is a good challenge and once you get damage its a good pump-action shotgun which is, IMO fun to use. From there getting the other few hundred kills to get it gold is a breeze. One of the main problems with the model is the lack of attachments, which made it all the more harder to level up, but I say you should stick at it
However, out of all the shotguns, I *personally* found the KSG-12 hardest to get gold, I just found it got a lot more hitmarkers than the model or spaz and was a pain to use. Now I have them all gold though, so going for 2500 kills with them all
Keep in mind, those were my experiences before the shotgun buff/nerf which happened yesterday, I haven't tried them out with their adjustments yet :/