MZ5 and I have both used them. They do seem to hold groups @ 500 amazingly well(I'm a 175 SMK guy). Can't complain about that at all. My 10fp(Painless) seems to have been born immune to copper fouling, until the Custom Comps came along. I hadn't cleaned the bore in well over 500 rounds. I shot something like 20 rounds and my groups opened, inspected the bore and decided a cleaning was in order. After a very thorough cleaning with a squeeky clean bore back out I went. I held good groups at 5-700 for around 60-ish rounds.
Had to clean again, as the copper build-up was substantial(groups also began to widen). I finished that box O bullets and went back to my SMK's.
Quite honestly I think they grouped every bit as good as the Sierra's, and velocities were as should have been expected.
Since my last cleaning, painless is back into the 100+ rounds fired with no appreciable coppering, nor groups opening up.
$0.02
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