Even with all the equipment given to you to build such an airsoft gun, you still need to design something of the sort. Unless you are trained and are Familiar with all the different gas systems that are available in the market right now right down to the core, you are so going to have a hard time creating such a gun.
Its obvious what sort of characteristics one needs in a GBBR. In terms of reliability it needs to be on par with an AEG. Since cool down is a huge factor in creating inconsistency with the power of shots in an GBBR. As Brian mentioned, Durable, Accurate and Reliable are the MAIN things that a GBBR needs.
Another thing other than NON leaky mags is super durable mags strong enough to be abused through dropping on the ground for example.
this is just my opinion.
VFC HK416 Custom
Custom Bowie Tactical x SAI Glock 17 (TM Base),
Custom SAI BLU Glock Build (Timberwolf Base)
Custom Zev Noveske Shooting Team Glock 34
Custom Zev Dragonfly G17 (Project) (In progress)