I have found the 125 grain copper plated Rainier's I loaded to not function as I had hoped. I have been having issues with these loads creating excessive fouling and unburned powder in the GP100.
I took to the range 158 grain Hornady XTP's loaded with 14.5 grain Accurate #9 powder with CCI550 primers (magnum primers) to function flawlessly. The recoil over the 125 grain is more noticeable but very managable.
The 125 grain Rainiers were loaded at 12.4 grain Accurate #9 using the CCI550 primer too. They felt like a cap gun after shooting the 158's.
I have consulted the manuals and will work these loads up 13 grains of Accurate #9 to hopefully prevent this issue and make hopefully for an all around paper punching round. So stay tuned for these findings in the future.
Until then, have a great Christmas and New Year!