[ujiro]

You can use standard V2 gearboxes, but you would require the non-standard lengthened hop up unit, and lengthened cylinder head and nozzle. I believe you must purchase the entire gun in order to have these, so yes, you need the G&P proprietary long gearbox.

If you want an SR25, wait and find it in stock somewhere. The metal body kit is outrageously priced. And you could just do what I did; buy the G&G one, which is exceptional quality, and get the proper trademarks engraved yourself. Save money, similar product, and it will be more unique.

EDIT: read Tys' post below mine! He lays it out well.