[Hectic]

The buffer tube will not withstand the stresses of a 400fps spring. The piston either, likely the gears and gearbox shell as well.
you want realism go gas, you want function, low maintenance, go electric.
realistic and aeg dont really belong in the same scentance IMO. Yes companies have made attempts at it (some prety good) but at the end of it all they are just adding more moving parts to break in systems that alrdy have alot of lil stuff that can go wrong.
now looks wise there are alot of realistic aegs, the vfc/umarex line of H&K guns comes to mind, LCT AK's, systemas (also have the lock on empty feature).
and i contacted echigoyaguns and they told me they would upgrade the gun but if they break it I buy it wouldnt even give the option of me talking with their tech to get measurements and specs and then i send parts then they build gun and send (thats how they did my vsr for me) basically tho you will need a custom gearbox shell to avoid the V2 type failiure at the front. Then a custom buffer tube, or at least a modded one. And then piston and spring Thats at minumum for it to stay intact at 400 fps for any ammount of time. Could prolly do without the shell but may as well go big if yer gonna do it.