Unicorn,
As my fellow BK brethern will attest I have been pretty much only a shotgun player in the BF series. In Bad Company is was almost insane with one-shot kills, no bullet drops and a 6x scope. Even as a happy shotgun user, it was a little insane. BF3 saw a huge nerf compared to Bad Company, but it didn't stop me and I still managed to hold very well with the shotguns.
So how has my BF4 experienc been? Well first of all I am actually surprised how well the QBS-09 is with slugs and a 4x scope and "accuracy" grip. You have to get through the horrible frustration of the snipers by using buck-shot, but once you unlock the full-choke it helps the spread and then before you know it you have unlocked the slugs. In fact I was so impressed with my QBS-09 setup that I used it only until I unlocked ALL shot-guns and racked up well over 500 kills in no time. I just unlocked slugs on the DBV-12 and I like the extra ammo and faster reloads (magazine) and it seems to work just as well with slugs.
What you DO have to get use to is bullet drop. There are a couple of 4x scopes that have different retinals that can help compensation. You can always head to the range and practice. I can one head shot fairly easy a stationary target at 100-120m out with a 4x scope. That makes it a pesudo sniper rifle, but one that fires FASTER. It is also great at CQB, because the suppresssion and rapid fire makes it easy to take down targets fast.
I just started unlocking carbines for fun and may unlock the DMRs as well, but as I use the carbines and need 3-4 shots to kill, I miss using that shot-gun one hit head shot or double body tap. I like the auto-shotguns better.
However, let me point out one IMPORTANT aspect about shotguns and my experience, I only play Hardcore and I only have been playing on the crouch server. I sometimes play on the run-n-gun 64 map server, but haven't played enough to form an opinion. I suspect the auto-shot-gun should fair just as well.
My only real dislike of the game is all the pseudo run-n-gun snipers that carry a sniper rifle around like a cqb weapon. I personally find (my opinion) that snipers rifles are a nob weapon of choice and to date I haven't used one in BF4. It is just too way way to easy to play with it. I unlocked it in BF3 and Bad Company and it just seem like I was cheating when I played with one if I treated it as a regular assault riffle or played with one in CQB. Of course you have to be quick to click first, but if you do it is a one shot kill. I gave up sniper rifles in Bad Company and only played with them a little in BF3, but again preferred shotguns because they were hard to learn to play with, made CQB more interesting and still had enough power at long distance. Don't get me wrong, those that play properly with sniper rifles I have no issue with, they are over 200m away, hiding, crawling, barely moving, and taking out soldiers one head shot at a time - now that is good stuff. I just get frustrated to see them on small maps, in a building, and using it to shot me at 10m away. Really?
Shotguns are certainly harder to learn to play in a diverse role (range, CQB, etc), but I like challenges.
I started a thread on shot-guns and my experience - you may find it interesting.
http://www.politeandfriendly.com/modules.php?name=Forums&file=viewtopic&t=15593
