I've compared identical loads across my chronograph and didn't see any significant difference between large and small primers.

10mm shoots relatively heavy bullets and has a relatively large case. You'll want to keep your chamber pressures up above 15,000 psi to make sure the powder burns cleanly, therefore you should use faster powders. Of what you say you have I'd recommend TiteGroup, Red Dot, Bulls Eye, or Clay Dot (Sport Pistol works well too). You won't find hand gun data for Clay Dot since Alliant says is only for shotgun, but they also say it mimics Hodgdon's Clays, which is a poular handgun powder. I use a 165 gr lead bullet for competition with 3.5 grs of Clay Dot which make a 125 PF; you can go lower; the same load of Clays gives similar results.  Both are touchy powders, 0.1 grs can make a 40fps increase in velocity, but it burns very clean. I've used TiteGroup, Clays and Clay Dot in my 10mm revolver for softer loads, they all accomplish my goal.

4.2" Revolver: LRN 200gr. Poly coated, 12.5gr. AA#9 - chronographed 1146fps

4.2" Revolver: GDHP 200gr., 9.4 Blue Dot - chronographed 1161fps

I ran your load, as best I could given the information you posted, through "Quick Load". Your load was calculated to be about 31,000 psi, but there are a few caveats:  1) there were only two 220gr bullets in the program and I don't know how close they are to Rim Rock bullets;  2) It was assumed that your COAL was 1.260 ;  3) the results were calculated from an algorithm, not actual tests so there is a certain amount of error implied.   Your load seems to be good where it is, but if you want to go higher go slow and see if you can find some data from something actually fired.

1) Is the cartridge you are comparing your reload to a factory Hornady XTP JHP?
2) Does the COAL of your handload equal the specified COAL in the Hornady manual?

If your answer to #1 is no, then that is your problem, different bullets have different profiles. Many JHP have a radiused Ogive while Hornady tends to be a tuncated cone, that'll look different.

If your answere to #2 is no, then set your seating die to give you the COAL specified by Hornady.

Seating a bullet too deep can raise pressures a significant amount, but I'm talking about hundreths not a few thoundsandths of inches ( I usually seat my 10mm rnds a little long, but I have a revolver). The important thing is not the COAL but the setting depth (how far the bullet is sticks into the case) it's just that COAL is easy to measure, seating depth required arithmatic.

Any brass from the Vista Outdoors companies: Federal, Blazer, & Speer. Since Remington Ammunition was bought out of bankruptcy by Vista, it remains to be seen if Remington is converted to small primer. I have a 10mm revolver and moonclips are fussy with regard to brass, Starline and Federal work most foreign brass won't.  My chronograph hasn't shown appreciable difference between primer sizes, but I suppose it is best to keep the brass segregated. I find that the operation of seating primers separates the cases: If you are seating large primers, they won't seat in Federal cases - case separated; if I am seating small primers, they won't seat in a Starline case - case separated. That way I'm not going through brass separating by headstamp.

Standard de-capping/resizing dies usually squeeze the case down smaller than SAAMI specification to give you increased neck tension, therefore you solution may be making the cartridge diameter too small. Try using a Lee Factory Crimp Die it has a carbide ring at the base that resizes your case to SAAMI spec for a loaded cartridge as you push it up into the crimping area. This works well for plated and jacketed bullets, but with lead bullets it may down-size your lead bullet more than you want. But since you are using plated bullets, it may be worth trying, they are not expensive.

I don't have a SIG, but I have slugged a few barrels. Slugging a semi-auto pistol is easy: you remove the barrel, lube it well and drive the slug down from the chamber. Revolvers are a little more of a pain. I would make the assumption that a Sig will have a properly sized barrel and start with .401 bullets.

Ammo manufactured by those companies now held by Vista (Federal, Blazer, Speer) all have small primer 10mm brass. It remains to be seen if Remington goes along with this trend. In my tests of reloads I haven't seen appreciable differences in velocity between Federal and Starline. Of course this is based on the bullets and powders that I  used and your results my vary.

Your primers still have rounded edges, so I don't think the pressure is too high. As The_Shadow suggestes, I think it is a matter of the firing pin hole in the breech face being a little too large. My Beretta 92 has shown similar marks for about 20,000 rnds and still runs fine.

Continuing fltbed's comment: jacketed bullets create more friction in the barrel, lead bullets less friction, and polymer coated lead even less, therefore greater velocities with lead and poly-coated. Every gun shoots differently. Minor barrel diameter differences can affect velocity without affecting accuracy, chamber diameter may affect velocity even more since you'll loose pressure expanding the case. I have two .357 Mag. revolvers, of the same barrel length. The Taurus has tighter chambers than the Ruger (to the extent that a casing fired in the Taurus won't fit in the Ruger chamber) and, at full magnum loads, the the Taurus will run about 150 fps faster than the Ruger. I would strongly suggest that you don't increase powder charges chasing some arbitrary velocity. Even though you are 100fps below your "goal" nothing you hit (especially paper and steel) will notice the difference.

Vista Outdoors subsidiaries (Federa,l Blazer, & Speer) use small primers in their 10mm. I have mostly Starline brass, but also 100 pcs of Federal. I've shoot the same load over my chronograph using both primer sizes - didn't record an appreciable difference.

You cast the chamber, for bullet size you want the barrel (or with revolvers, the cylinder throat). Lead bullets should be .001-.002 larger than the barrel. Most casters have .401" for their standard sizing, that works for me.

Yes, Ruger can have mediocre triggers, especially the newer ones. my GP100 10mm was a little rough, but it was fixable. About $35 for springs and shims and a couple hours of bench time polishing parts and installing said springs and shims and the trigger is just fine.

The Lee FCD for pistol calibers has a carbide resizing ring in the base. It resized the cartridge before it crimps it. With lead bullets this ring MAY resize the bullet down to jacketed diameter. I knocked the carbide ring out of my 38 Spcl die, but was unable to do that with my 10mm FCD. I use a standard taper crimp die, SAAMI chamber specs will allow a bullet sized to .401, I don't know about .402.