On my Slix-Shots, and some stainless ones I put on my Remington and 1860, that the CCI #11's are a good tight fit, with no miss-fires. I do press them on with the hammer, which is scary to some, but I make sure the gun is pointed in a safe direction. (duh!) and I do it with the attitude that it WILL go off, not MIGHT go off. I have never had a cap go off. Anyhow, the fit seems to be the opposite of other's experience...nice and tight instead of loose.???
Can you no longer order #11's directly from CCI? That's how I got mine, somewhat recently.
I found, that with my revolvers, that the Remington caps fragment a little better than the CCI's. For me, they fall off the nipples better. The CCI's seem to be more sturdy, and want to remain on the nipples. I prefer they fall off.
Pinching caps is not a good solution, I hope a better remedy is found. I wouldn't worry about making the cones too thin if turning them down, it will take very little metal removal to do that.