Underbarrel Masterkey Shotgun – Spring Thunder/Silly Shell Compatible

Description

Underbarrel masterkey blaster using the Spring thunder shell ejecting foam blaster system. Created with MHP arms.

IMPORTANT: We do not currently offer silly shells, so if you purchase the silly shell variant, you’ll need to supply your own silly shells.

Hardware kits available via the dropdown, and files can be downloaded here.
And here: https://shellingtonlabs.com/wp-content/uploads/2025/09/bigUndermount_ProductionRelease_V1.zip
Full video Assembly instructions.

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. Please note what selection of shells you’d like to be included. Otherwise we will default to a split of Dart, Rival and MEGA shells.

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.

FAQ
Question: How do I get the blue handle grip?
Answer: It is not available on Etsy yet, but we will include a link here when it is ready.

Question: How does its performance compare to the Spring thunder?
Answer: The stock spring that comes with the purchase puts it at about 80% of the power level overall. But this blaster is a bit sensitive to stiff Rival balls when loaded 3 in a row. 2 and 1 Rival balls work fantastic however.

Question: Can this fire Silly Shells?
Answer: Yes, with the adapter you can 🙂

Updates:
August 24th, 2025 – Added a couple new color options. Silly shell adapter is done, and will be available soon.

May 27th, 2025 – Initial Release