Easy experience on COD4?

whats the most easiest way to gain experience in COD4
meaning what type of game mode...headqaurters, deathmatch etc...

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if you are good, search and destroy

if you're moderately skilled, headquarters

what I've been doing is running up to 25-30 playing HQ, then switching to search and destroy until I reset my rank

I earn more XP playing S&D (anywhere from 500-1400 without counting challenges) but a lot of the things I like in my loadouts for that gametype aren't unlocked until later anyway