You can't. Apparently they don't want people to have an "unfair advantage" which is admitting that higher level gear is better than lower level gear. So if you start playing the MP later then you'll be screwed, having to go up against people of higher ranks, with better weapons and more experience.
That's why I hate unlock systems.
Actually,this game is very fair.
Some of the best Assault Rifles are unlocked early(the last 2 are just bad..)
The AK74 is unlocked very soon after you start the Engineer,and from my experience it's by far the best of the weapons for that class.
The M95 is the worst high-powered sniper(imo),it has very slow bolting and has no advantage over the other snipers..sure it does more damage,but that just makes it a 1 shot kill from a little farther away,you don't kill people from close range enough with a sniper for that to even matter.
I have no idea about the LMG's,only time I touch the Medic kit is when I pick it up to revive team mates after a fire fight at a flag/bomb site.
Really,most of the later weapons for each class are more novelty then anything..there are some fun/interesting guns later on in the kits,but none of them are much better then the first gun you get.
The perks on the other hand are a bit unfair,you have to put ~20 hours into the game before you even get a variety to pick from,but it's not a big deal..most of them aren't too game changing any ways.
Speaking of "novelty" weapons,the VSS is so fun..it's like the UMP 45 and M21 EBR from MW2 had epic spraysniper babies.