Gun Modification Question

Does replacing the hammer spring with a lighter one on a Sig pistol make it ineligible to compete in "Stock" in either IDPA or USPSA?

I can only speak for USPSA as I'm not familiar with IDPA's rules.

Generally, in Production division (the "stock" division in USPSA), you can make any internal modifications you want, with the exception of removing/disabling safety devices. Replacing the hammer spring is fine.

Appendix D4 contains the rules about permitted modifications to Production guns: http://www.uspsa.org/document_library/rules/2014/Feb 2014 Handgun Rules.pdf
 
No problem with USPSA as mlaboss states.

For IDPA it looks like no problem either.

Firearms - Divisions
8.2.1. Stock Service Pistol Division (SSP)
8.2.1.1. Handguns permitted for use in SSP must:

8.2.1.3.7. Internal action work may be used to enhance trigger pull as long as safety is maintained (no visible external modifications allowed).
 
Here's the entire SSP Modifications section, in case someone wants to question ipscdrl's short list above. It looks like you're good to go.

8.2.1.3. SSP Permitted Modifications (Inclusive list):
8.2.1.3.1. Sights may be changed to another notch and post type but slides may not be machined to
accept different style sights in SSP.
8.2.1.3.2. Grips may be changed to another style or material that is similar to factory configuration and
do not weigh more than 2.00 oz/56.7 grams more than the factory standard weight for that model.
8.2.1.3.3. Magazine releases, slide stops, safety levers, de-cocking levers, hammers, and triggers, that
are stock on one SSP legal firearm may be used on another SSP legal firearm from the same
manufacturer provided they are drop in replacements. Parts in this list must come factory installed
on standard production firearms. Special parts that are available installed only from a factory
custom shop are not eligible in SSP. Effective 04/01/2014.
8.2.1.3.4. Recoil spring guide rods and dual spring recoil systems made of material that is no heavier
than stainless steel.
8.2.1.3.5. Frames may be replaced with identical frames from the same manufacturer.
8.2.1.3.6. A slip-on grip sock and/or grip tape, skateboard tape, etc. may be used.
8.2.1.3.7. Internal action work may be used to enhance trigger pull as long as safety is maintained (no
visible external modifications allowed).
8.2.1.3.8. Internal reliability work.
8.2.1.3.9. Aftermarket extractors and pins may be used.
8.2.1.3.10. Internal accuracy work.
8.2.1.3.11. Replacement of barrel with one of factory configuration and original caliber.
8.2.1.3.12. Plastic plugs may be used to fill the opening behind the magazine well.
8.2.1.3.13. Custom finishes may be applied.
8.2.1.3.14. Stock slide cover plates may be refinished.
8.2.1.3.15. Slides may be engraved. Engraving is defined as etching into the slide of logos, letters, and
graphics no deeper than the original factory logos.
8.2.1.3.16. Stippling and texturing may be performed on readily replaceable parts of the grip frame
such as replaceable back straps and replaceable grip panels.
8.2.1.3.17. Aftermarket magazines may be used provided they do not weigh more than 1.00 oz/28.3
grams over the same capacity factory magazine.
8.2.1.3.18. Aftermarket magazine base pads may be used provided they do not make the magazine
weight more than 1.00 oz/28.3 grams over the same capacity factory magazine.
8.2.1.3.19. Magazine base pads may be modified by reshaping, texturing, or adding bumper pads
provided that they do not make the magazine weigh more than 1.00 oz/28.3 grams over the same
capacity factory magazine.
8.2.1.3.20. Magazines that are longer than stock may be used provided they meet all other division
requirements.
8.2.1.4. SSP Excluded Modifications (Non-Inclusive list):
8.2.1.4.1. Externally visible modifications other than those listed in the Permitted Modifications
section.
8.2.1.4.2. Aftermarket or visibly modified magazine releases, slide stops, safety levers, de-cocking
levers, and hammers.
8.2.1.4.3. Robar-style grip reduction.
8.2.1.4.4. Add-on magazine well opening.
8.2.1.4.5. Slide inserts to accommodate a different recoil assembly design.
8.2.1.4.6. A barrel of another caliber that is not offered in the original factory model. - 33 -

8.2.1.4.7. Customization of the slide by adding front cocking serrations, tri-top, carry melts, and high
power cuts.
8.2.1.4.8. Compensated firearms with non-compensated barrels installed.
8.2.1.4.9. Checkering or stippling on non-readily replaceable parts of the grip frame.
8.2.1.4.10. Aftermarket slides.
8.2.1.4.11. Removing material from the magazine well opening.
8.2.1.4.12. Aftermarket grip tang extensions or beavertails.
8.2.1.4.13. Disabling the slide stop.
 
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