So if I wanted to use for example:

ICS SIR rail for M4 series

Will it fit on let's say a G&P M4A1 or any M4 airsoft gun, GBBR included?

because it's not so straight forward...;)

a G&P WA system URX front end set will work on a G&P WA system...everything but the barrel will work on a WE system...everything but the barrel will work on an appropriate threaded AEG body.

a WE labled grip will work on a WA and RS...WA style will not work (without modding) on a WE. Neither will work on an AEG.

Lots of stocks will work on GBBR/AEGs the same, obviously stock tubes/buffers are very different.

So....sometimes "directly" is too simple...

Can you swap rail systems between AEGs and GBBRs?

Also a reminder that barrel assembly's, trigger assembly's and internals (not sure about recievers) are not compatible between different brands and models of GBBR. They are designed very specifically for the gun that uses them. Different manufacturers use different designs for GBBR and therefor parts are usually not interchangeable with them. Rail systems should be interchangeable but I can't say 100%.

not totally true, u can get some externals on, u just need to know wut ur doing. its been done, just look at Heatstroke's SPR. it an aeg with a gbbr ris n flash hider. but yes to avoid problems use aeg parts for ur aeg.

what about a KJW delta ring on a G&P GBBR body? Would that work or is the threading different?