Systema claims 6.04 is the best size and that anything smaller causes the BB to ricochet alot down the barrel, despite the fact when I replaced my systema 6.04 with a prometheus 6.03 my groupings improved
Now keep in mind that all depends on how good your BB's are!
A perfect 6mm BB will work best in a 6.01 barrel, but I'll stick to my tried and proven 6.03 barrels
