Reshimming is something you can do you do yourself with minimal gearbox knowledge. Basically all you're doing is taking the gearbox apart, and putting the little metal washers in a way that makes the gears fit together better. The way I do it is take everything out of the GB, put one "shim" on each gear, put the gears in (and only the gears) and spin them manually to see if theres any resistance. If there is I identify what gear is causing it and then add (or remove) a shim.
If I left anything out I'm sure one of the more experienced forum members will correct me, but thats how I did it with my friend's P90 and my M4
