How you doing, I've got a cz 512 semi auto in 22wmr (515 is Australian but identical to the 512) hornady 30g I've found are great for accuracy.........the firing pin tends to gum up quite quickly which has an impact on the force required to strike the bullet case, 512 was misfiring really badly, out of 16 bullets only 3 fired......I found the problem, turns out the firing pin wasn't striking the bullet hard enough so I filed the firing pin where the locking pin holds it in place to give more movement........works like a charm, just remember not to take too much metal off the firing pin just a bit at a time till it gets better......as for extended magazines I'm only aware of 5 shot and 10 shot magazine's