I don't think it is the mainspring that is so heavy, If you have ever seen the inner workings of this model, it is different, and there is a flat spring to a lever for the trigger rebound in stead of a coil spring and slide. This shorter flat spring is most probably the cause of the problem. The odds of finding one are like very very slim. Perhaps our local 1899 expert Mike will respond, and maybe he can shed some light on what you can do. Often just a good cleaning and light lube will help a lot. There are a few tricks to disassembling this model, so be careful if you are not familiar.

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H Richard
SWCA1967 SWHF244