I just wonder if it is a combination of "hard primers" and the 226 firing pin hitting a little on the light side, because I've read about 226 light primer strikes in the past, and I've also heard of Underwood having harder primers. I'll say this...the same ammo is 100% reliable in my other pistols...Glock, Ruger, etc. But when I put it in my 226, it's maybe 50/50 on first strike fires. All other ammo works just fine in the Sig. So that leads me to believe that yes, the Underwood primers are hard, but other pistols don't have an issue with it...just the Sig. I ordered a heavier hammer spring from Wolff springs, but the spring I received was like 2X the length of the factory spring. I couldn't even come close to installing it.