I'm sorry, I'm going to have to disagree with you all. The ViperTech M4 GBBRS are the MOST solid built GBBR out there.
-The body consists of 6061T6 aluminium which is really tough
-The internals are ALREADY steel parts.
-The Hammer spring is extremely stiff and strong.
-There is a variable stroke BCG that allows you to adjust ROF
-The Hop up system gives you an extremely crazy range.
-Uses WA system mags. ( the Gun only bolt locks on EB, and GHK magazines) Prowins, AGM, G&P, Beta Project, King arms, Western Arms magazines will fit and work, however their bolt does not lock back
-This gun is extremely gas efficent, affordable, and tough.
-It DOES NOT need any upgrades out of the box, nor will you need to.
I hope this video will change your mind
VIPER TECH M4 GBB torture test - YouTube
The only downside is that the gun has super unrealistic trademarks, and the ones with the realistic COLT M4A1 trademarks are for their domestic market in Taiwan.