So much for the mismatch theory.
I took the right grip off a 1900 and made the test. As the action recoils and the breechblock moves back the leaf recoil spring flattens itself against the inside of the rear grip strap, "loading" the spring somewhat as the effective length and spring curve change.
Is the recoil spring original, or a new reproduction? Here's why I ask.
I bought and installed a new recoil spring for a 1900, and had exactly the same sticking-on-full-recoil problem. It turned out that the "hooks" at the top of the spring which loop over the connecting stirrup were a little bit long, sticking out a bit. At full recoil they stuck up against the bottom of the breechblock, and kept the action from returning to battery. In the process this left a long gouge on the underside of the breechblock.
The solution was to file the ends of the loops until they no longer contacted the breechblock at full recoil, being very careful not to file them so much that they no longer held onto the stirrup. Problem solved.
This may or may not relate to your problem, good luck.
--Dwight
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