I've never had a Sigma apart (a buddy's daughter has one - it seemed better suited as a wheel chock for his truck), but if they're using the same drop safety plunger setup, a similar treatment might be quite simple. The issue being how DIY it is.... Same with a sear swap. The springs (not denying the engineering involved in getting the right springs into the mix) are probably the easiest part.
Apex probably isn't doing the Sigma 'cause, in a fit of common sense, CA won't let 'em into the state
....
Regards,