Ranking up fast in MW2:How far should i go with one gun(how many kills)?

I want to know how far to go with each gun before moving on to the next(how many kills before moving on) to get the best exp, to prestige faster. 150, 300? Should i get holographic sights for each gun? Also note i will be doing all SMG and ault rifles, maybe some LMGs, and some secondary's, but not so much snipers(intervention nothing else).