Try something that is labeled as a penetrating oil. I just do not have much faith in WD40 for much of anything. These tip-up revolvers take two fingers placed on either side of the latch and push up fully then tip up the barrel. Use penetrating oil on the top hinge as well, since that could be bound up. If you plan on any disassembly, I would take the stocks off and soak the gun in a mixture of ATF and Acetone for a few days either outside or in your garage. ATF works great at penetrating threads and acetone helps dissolve hardened greases and oils.

The gun appears to be in better shape than many and shows very sharp bottom edges of the rosewood stocks, indicating that it did not have a hard life. Hope all goes well with your cleanup project and post some after pictures.

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Gary
SWCA 2515