This gun is a fucking beast once you get used to it, you might do bad with it for the first few games. But trust me, get used to the double scope- learn how to quickscope with it in sniper only games, and you'll do fucking amazing. I've onnly been using it for 2 days and I love it, also- this is the MOST accurate no scoping ever. It's spread is tiny as well, you can jump down superman and scope in to kill someone while in the air.
Only downside is the portablilty, but IDK cause I got a heavy vest on and I don't feel a difference with it off.
The type of playstyle for this sniper is campy, but move around camping.. you know what I mean? Stay one spot, kill 2 or 3 people and then move, stay there and kill a few people, move- repeat. Also need sound with this.. you can't run away with this sniper on you, so you need to be able to know wether of not they're behind you.
Another sniper rifle, the TPG-1 Pro- is an excellent choice if you only want one sniper to be carrying around with you. It's versatile, not a double scope like the M107- but a higher magnification scope which is easy to adapt towards, and it beats the DSR-1 Subsonic IMO. Just basically because the DSR-1 Subsonic feels... somewhat, artificial- I don't know, that varies for you. DSR-1 Subsonic is for quick pickoffs when camping, if you're good with the L9- it doesn't mean you'll be good with DSR because they feel a lot different. I actually find the TPG-1 Pro to feel more like the L9 rather than the DSR-1 Subsonic.
For Assault rifles, well, you're gonna have to try yourself. Don't get the G36E Valk, it's the same as the GP variant. The M416 CQB for mid/long(ish), the REC7 for close, and for a versatile AR, get the M6A2.
Edit: If you're interested in SMG, reply and let me know. I'll tell you about the ones I've tried- because they do NOT feel like the GP variants AT ALL. Some of the GP variants are WAY better, and some are horribly worse. A good example is the P90 GP vs the P90 TR NX- a huge difference in feel, and the NX one beats it ANYDAY.
2nd Edit: A lot of people recommend the L85 MOD, no offense, but it's bloody horrible. I'm not soo sure about the GP version, haven't tried it. But I have the L85 mod perm on one of my accounts, and bloody hate it. The G36E is a lot better- you can actually use that in close quarters. L85 fails in long range because of it's sway during tapping, and its spread got nerfed and is nearly as bad as the AK 47 in close range now. Gets out of control with recoiil/spread real easily because of it's rate of fire.