Fitting New Handguard

[Luke-airsoft19]

Recently brought a new rail system, to replace my old one. Never tried fitting one before and wasn't sure how to do it, there's a slight gap between the top rail on the handguard and the upper receiver, about 1mm maybe 2mm yet I don't feel I can get the barrel nut tighter without damaging something, just wondered if this was normal, or would I be fine trying to get the barrel nut on further?!