Heavier BB will make your gun more accurate over the distance you're shooting at. Reason being is it's a heavier projectile and therefore and as anyone who has taken Gr. 10 physics can tell you is harder to make it deviate (via wind shear, and other atmospheric anomalies). That said, it will only do so much.
I'd say start off with .30's or .36's first before going up to .4's
Finally, I disagree with FOX on the "washing your BB's" thing. Sure it allows the hop up to work it's magic because the BB offers a more "grippy" surface BUT one of the dangers I have heard of why you shouldn't do that is because there is a chance that it might tear your hop up rubber. However for a sniper rifle that doesn't shoot that many rounds it's probably not a terribly huge problem but I'd be worried if it was done to BB's that would be put into an LMG or a regular AEG that might fire a lot more BB's than a sniper rifle. Personally I still wouldn't wash my BB's if I had a sniper rifle.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE