Retro Arms CZ Billet CNC 8mm Ver.2 Gearbox Shell for M4 Series Airsoft AEG Rifles (Model: Split Gearbox / Hop-Up Chamber)

"Trust me... I REALLY wanted to give this a 5 star review, I wanted to say it was the best gearbox out there. But I can't. From day one I had problems. When It says, "Disclaimer: Please use discretion when installing custom parts as fit and compatibility may vary. Some modification may be required. Professional installation recommended." It should say, " doesn't work with anything." I have had problems with the cylinder, a TM cylinder fitting. The cylinder head pegs are too small which make the assembly wobble. The trigger trolley and contacts won't fit, Semi won't work the spring guide is press fit and can come undone. Some motor grips won't line up. I had the sand down the area for the motor grip to actually fit over. Then the whole shell wouldn't work with itself! literally the two haves never seemed to line up... I wish I had my money back....

It has great features like QD spring guide and large areas for wires. But I'm having problems with it that a 10 buck crappy china shell wouldn't have problems with. For 100-120 bucks I'd say it is MORE than fair so have better fitment with parts. I've tried many brands and all of them didn't work. I really wanted this to work but I can't recommend Retro Arms. Go with a G&P gearbox. I'm glad I didn't buy a m249 shell. I wonder what other problems would occur....