I'm sure I'm not the first to post something like this but I haven't seen any tips suggesting this on the YouTube commentaries I've watched so I MAYBE I'm gonna help some people out here. I know it worked for me.
OK, I feel the best way to figure out what guns you like without messing up your K/D, W/L, SPM or any of that stuff is to create a custom game with bots. You can set their difficulty just like in the campaign (recruit to veteran) and stack the teams against your favor (2v8, 3v6) if you wish so you have more enemies. I played against them on Hardcore and I feel they resembled many pub matches in terms of difficulty. Veteran would be quite a challenge I would imagine because they would probably be a lot like they are in the campaign as far as accuracy and the speed of which they are targeting you but I digress, that's not the point of this though it might help some.
Anyways, when you set up a custom game like this you can create classes as you wish without having to unlock anything with tokens and no matter your current level, much as you can in league play. What I did was set the score to 400 kills for TDM and I set up classes of guns that were similar so I could switch classes when I died and really get a quick comparison of how weapons of the same class felt when compared to each other back to back. After the round was over, I would eliminate guns that I know didn't feel right and tweak the ones I liked (i.e. add a grip instead of a suppressor or see how much of a difference FMJ makes vs without).
There seems to be a lot of disparity out there as to which guns are the best in each class (besides the FAL lol) and that is no doubt because everyone has a different play style and prefers different aspects of the weapons they use. This is a good way to figure out what works best for your play style and the situations you find yourself in. Try it out and let me know what you think. Hopefully this helps some of you find a set up you haven't even thought of trying but fall in love with. Also, if you happen to put the bot difficulty on Veteran, post what it was like compared to matchmaking.
TL;DR: Set up different classes and play against bots in custom games to get a feel for different weapons/perks/killstreaks without sacrificing your K/D, W/L, or SPM. Good luck!