Underbarrel Masterkey Shotgun – Spring Thunder/Silly Shell Compatible

Underbarrel masterkey blaster using the Spring thunder shell ejecting foam blaster system. Created by shellington and MHP arms.

Files here: https://shellingtonlabs.com/wp-content/uploads/2025/09/bigUndermount_ProductionRelease_V3.zip

Assembly video: https://www.youtube.com/watch?v=dfbuEUc4zUY&feature=youtu.be

This is a spring powered human primed single shot system, that loads Spring thunder shells in an under barreled fashion via standard Picatinny Rail system. Fits great on the Kirin or the (also not included) grip with picatinny. Its clamped position also makes priming a bit easier. 

Performance varies by quality of foam darts, but you should see 90-100fps on singled half length darts. Triple rival performance varies on ball quality and fit, MEGA and single darts fire great, double darts are also pretty amazing, but quad half lengths are hit or miss depending on dart fit.

Hardware kit includes:
Required screws and orings
Plunger padding
K25 spring