Working a Stevens Model 11 Little Scout. .22 LR single shot falling block design. Family heirloom being refurbished before passing on to next generation. Before beginning work, I test fired it a few times with no problems. Relined barrel, refinished metal, repaired broken stock. On first test fire after reassembly, the hammer was pushed back allowing the breech block to fall and the fired brass to fly out of the chamber. Installed stronger hammer / breech block spring with same results. I noticed the firing pin protrusion was excessive (.080") so I shortened the head of he firing pin to give about .040" protrusion. The breech block cannot fall unless the hammer is retracted about 3/8".

may the hammer is rebounding/bouncing back?

A friend had a rolling block that did the same thing, in the early 70s. He had lightened the main spring and it bounced off the block enough to let the block open and the case self extracted. It was scary at first, but it happened slowly and not the result of pressure. Weird.

This action is a pivoting block, that has a small section of the hammer supporting the rear of the pivoting block in the fired position.

Take a GOOD look at the mating surfaces of the shoulder/angle of the hammer verus the rear angle locking surface of the breech block.

If the angle is too steep, then the rear/downward thrust of the breech block will override the hammer and cause what you are seeing.

It is odd that it worked fine to start with, but is doing it now after refinishing, ?, any extra polish work to those areas could have caused it.

Also these old guns were designed to shoot standard vel ammo, not the current high speed ammo.

J Wisner