Just thought I'd pass on what a buddy of mine and me found out about the right fitting caps. We tried CCI #11, Winchester Magnum #11, older Remington #11. They all fell off the nippless just tipping the gun backwards.
He was going to try #10.
Then I picked up some new Remington #11 at Cabelas for my rifle, and I told him it says "new geometry for a tighter fit" on the package and said he should try them. He says they fit good.