I replaced the "Striker Spring" on a 9 mm Sigma pistol last summer, and that gun had a terrible heavy weight of pull, around 16 lbs.
A call to Wolff Gunsprings got the parts we needed for the job.

The spring to replace is up inside the action, surrounded by sleeves of ballistic type plastic.
Care must be taken when disassembling, because those plastic parts are easily damaged if forced.

I went online to YouTube, and found a couple folks had already beaten a path to success, followed their instructions, and We were rewarded with a reasonable 7 lb weight of pull, and reliable primer ignition.

Do Your homework first if attempting to swap springs on the Sigma pistol, and exercise patience.
It's actually quite easy, once You've learned how to disassemble/assemble.

Hope this helps!