Assuming the accuracy of gun is from mostly BBs of quality, the hopup and the tightness of barrel and the smallest barrel just after the BB stabilized itself and is about leave the gun's barrel. They sure influence on hits. However, high-quality stock guns might have small problem of it but still fixable.
For what the FAMAS, i am well aware and i wouldnt get it nor bother about it because this gun is so old, his parts are even impossible to find.
As for upgrades, i have try first the gun and see what this airsoft gun needs upgardes by shooting it and playing in a match.