I'm looking into getting a Surefire weapon light to mount to the narrower-than-standard rail on the frame. There's a ton of different models, but I'm not sure if any of them fit will fit this pistol. If anyone has put a Surefire (or similar) light on the non-standard rail P220 ST, which light model works (without an adapter)?

I don't know of any Surefire that will fit your P220 ST without an adaptor. I own one myself, in fact I purchased it within months after Sig introduced it just to be disappointed by not being able to find a light that would mate up to the older style rail.  I recently purchased a Surefire P104 to go with a P226 and found that the supplied adaptor will fit my P220 ST as well. Even better, the P104 with a little modifying with a dremel will mate up with a P220R that I purchased about 2 weeks ago. The P104 and P114 are actually designated by Surefire for the P226 with the P103 and P113 for the P220 but I found that the adaptor for the P104 works just fine with my P220 ST. Now that's a lot of "P" designations, going to "P" myself confused. The "P" series lights from Surefire are true weapons lights and when used with the supplied P61 bulbs are very bright, 120 lumen I believe.

Good info...Thanks tatsuosan1964!

Follow up on the P series fit for the Sig P220 ST.  If you are going to buy one of the P series Surefire lights for your P220 ST, I would recommend getting either the P103 or the P113 as it comes with slightly longer screws and the mounting plate that goes inside of the trigger guard is shaped to fit the P220 a little better.  Use the P104 or P114 for Sig P226 models.