I've since developed a technique that works a little better and it's easier using either .45 ACP Mec-Gar or .38 Super Mec-Gar mags for the Witness...http://www.czforumsite.info/index.php?PHPSESSID=076d1cea8749a9a18b3a12643d6996fe&topic=34651.0
All you do is remove the spring and follower from the Mec-Gar mag (.38 super and .45 ACP happen to use the same follower) and re-use the factory magazine body. Heck the follower alone might do the trick, I just don't have any factory springs on hand to confirm. So far not range tested, but will hand cycle hollow points. It also hand cycles with .40 S&W FMJ.
No, I haven't had a chance to fire the pistol in 10mm, yet. It's been away at the gunsmith for some other work. I got it back recently and immediately went to tinkering with the 10mm mags, even though I had just bought a .38 super conversion.
I will tell you that the extractor was not an issue in my pistol. The problem with my pistol was magazine related. Namely the rounds were nose diving in the magazine. You'll find lots of reports of the same thing all over the web, just do a Google search.
I could duplicate the problem with the magazine OUT of the pistol. Just push the rounds out of the magazine and you'd see them nose dive and catch on the mag body. (Yes, I tried filing the mag body down. It helped, but did not stop the jams.) The problem can be traced an improperly designed follower.
All of the factory 10mm mags I purchased wouldn't even chamber or hand cycle if filled with more than 5 or 6 rounds. (The .40 S&W mags weren't any better!) If filled to capacity the mag would lose control of the top round as well. Not acceptable.
Anyway, you could buy the Mec-Gar .45 ACP mag and give it a shot. Best case, you fix the pistol. Worse case, you convert to .45 ACP and have a high quality magazine to rely on.
I am testing the pistol this weekend in .40 S&W, .38 Super and 10mm.
Right now all I have is a 16 lbs Wolff spring for the 10mm/.40 S&W combo. I'm a little concerned about beating up the pistol with 10mm loads. However, I did install an 18 lbs hammer spring which does give some extra resistance over the factory 16 lbs hammer spring.
My loads aren't too hot. 180 grain bullets chronograph from my Glock 20 at an average of 1185 fps. (10.2 grains blue dot, Winchester primer, plated bullet) Henning recommends a 16 lbs spring for 10mm, 18 lbs for heavy 10mm and 20 lbs for very heavy 10mm.
If the brass goes a little too far for my taste, then I'm not going to push it this weekend with the 10mm loads. I'll post some results on Saturday or Sunday night.