Compact piston

[Technician1002]

If the chamber is barely larger than the barrel OK, the volume will be small and the flow choked. The way to get a small ratio with a larger chamber is to have the piston in a smaller piston cylinder that is smaller than the chamber. My Dragon and Mouse Musket are both built this way.

The Mouse Musket has a 2 inch chamber with a 1.5 inch female adaptor in the breech with 1.5 inch pipe inside it to hold a 1.5 inch piston.

The Dragon has a 6 inch chamber with a 3 inch piston (2.5 inch pipe cap) with a 2.5 inch barrel.

This type of narrow ratio valve works well.

Muzzle of the Mouse Musket. Layers from outside in.
2 inch coupling
2 inch to 1 inch reducer
1 inch barrel