AK74u and why its used ~ Its used a lot by players because it fits their need, it has minimal range and it is great in close combat. And the gun fits for most map layout(s), its great for defense or offense game-play.
Lag - The lag is definitely something that needs to be looked into, it seems as if I am red/yellow bar 2-6 of the games I play. But the thing that is getting on my nerves most is the PbP lag, where if I am in a close combat, I start lagging like there's no tomorrow.
I try out different guns each game, but the guns I stick to are AK74u (because I like to control spawns and kill my enemies, it helps control the outcome and prevents continuous rushing towards the bomb/flag site) .
Basically, top 4 guns:
Famas - Great at long distance and good in close combat (I'd recommend spraying)
Commando - Starting using this gun today and it has great power, really useful if your the defensive type, good strat is to use jumping techniques/drop shot.
AK74 - If you want to circulate spawns or control the outcome, I'd recommend this gun with Tactical Insertion.
Enfield - This gun is sorta minimum for me, but its great for Free-For-All, I'd recommend using warlord and equipped with red dot sight and a suppressor. If you're the campy type, this gun is for you.