by B.B. Pelletier

Spring gun tuning: Part 1
Spring gun tuning: Part 2 – Building a mainspring compressor

Happy Birthday, USA!

Today, I’ll finish the mainspring compressor with details, dimensions and important tidbits.

The plank
The plank has to be long enough to accommodate any spring gun held between the vise and the tailstock. I used two 2″x8″ boards, one on top of the other, but the top one isn’t as long as the bottom, which has to could accommodate the vise. Look at the side view to see what I mean. The bottom board is longest (mine is 58″).

The view from the side shows how the vise fits on the plank.
The tailstock and bridge are both held to the plank by long bolts. I found it unnecessary to use nuts on the tailstock bolts because all the force was lateral. The nuts on the bridge bolts restrain the gun from moving in all directions. When the mainspring comes out of the gun, it can push in all directions.

The pusher cover
Make a wooden cap to pad the steel pusher ram of the vise. The wooden cover pushes against the end cap of the gun or against another pusher adapter that reaches inside a gun. For a BSA rifle, use a dowel to reach inside the mainspring tube. Cut a wide slot in the center to reach past a retaining pin. As you work on different airguns, you’ll create special tools to accommodate them.

The bridge
The bridge is three pieces of wood attached in a U-shaped pattern. It covers the gun laying on the plank and is held to the plank by long bolts. Make it wide enough for any gun to fit through it. Use pieces of wood inside the bridge to shim the gun tight once the bridge has been tightened to the plank. If the plank is 8″ wide, make the bridge almost as wide. Align the vertical attachment holes with the holes in the plank (see part 2).

The tailstock
Make the tailstock as wide as the plank. Stack three pieces of plank board to make the tailstock and position them with the edge of the grain toward the vise. The muzzle of the gun pushes against the tailstock, and the end grain cushions best.

The headstock/tailstock relationship
The headstock and tailstock accommodate a spring rifle action between them. When you disassemble the rifle, the headstock ram is pulled back (away from the tailstock) several inches to relax tension on the mainspring. Then FWB 124 and HW 77 need the most travel (about 4″), so factor that into your building plans. When you assemble those rifles, the ram must be allowed to travel the same distance in the other direction. It’s bad when you almost get a rifle disassembled, only to discover that your compressor won’t let you put it together because it doesn’t have sufficient travel.