When you say "in practise compared to on paper"
I'm guessing you're talking about groupings on a bit of A4 paper (although any size paper is acceptable) unless you're using that known phrase of "on paper"
Which then I guess we'd have to judge guns through there internal parts. Or what it says on the tin (another phrase used, which doesn't literally mean in the tin, but actually what the product claims it can do.).
Or are you saying, if a gun can plink cans in a garden very reliably, but is it any good when it comes to ingame skirmishes?
Either way, as a general rule of thumb, you do get what you pay for. E.G I own an ICS CXP-08 (£200 ish)and it shoots at about 320FPS with 0.2g BBs (completely unnecessary figure & fact, but I have a problem where I have to note down the muzzle velocity of a system when I talk about it) and that probably shoots up to 50m, but it won't be accurate at that range necessarily. However if I bought a 50 quid AEG, that's shot at 320FPS 0.2g BBs it probably won't be as good, won't have the same quality, accuracy etc.
About that on the tin stuff, I think you're new? I've seen your recent threads and you're relatively new to airsoft - I'm no expert, I just got told this when I joined Airsoft. But basically, when you're browsing guns and you see gun X with 300 FPS/.2gbb then you see gun Y with 330FPS/.2gbb, you may be swayed by what it 'says on the tin' however this determines nothing, so don't go with what it says on the tin, it's how the gun performs that's important (in this sense) so look for reviews.