I have a SA LW operator that is a sweet gun. Equipped with a SF x300u it is my bedside gun. As such I'd like to have as many rounds as possible in it. However, using CMC powermag 10's I can barely seat the mag with a closed slide and can't rack the slide at all when inserted. Anybody else have this problem? How about with other 10 round mags? Is this common?

I have one cmc 10 round mag that works well in a TRP and an Ed Brown SF.

I have a couple of Wilson 10 rounders that can be finicky.  They seat OK, but the followers bind up and don't cycle.

Use 7 round mags for reliabilty.

I've only used a couple CMC 8 rounders with no issues. How old are your mags? Do they just need some break in time?

I own a few Wilson 47T's. The 47T's (10 rnd) can be a bit hard to seat at first but they break in. I never had a problem with operating the slide with a 47T loaded with 10 rounds though.

I use 2 CMC 10 rounders in 2 different 1911's(a Kimber Ultra II and a Colt New Agent both are 3" guns).  I have had zero reliability issues in around 1.5-2 years always running 1 in the chamber and the full 10 rounds in the magazine.  I even carry one as a back up for my EDC they have been that reliable.  I did buy 2 WC 10 round mags too which imo seem to have less quality than the CMC's, however I have not fired the WC's yet and am only going on how difficult they were to load and the followers seemed to look cheap.  We will find out about the WC's but the CMC's have been my overall favorite mags thus far.  I also have the CMC followers in almost all my 7 round magazines to turn them in to 8 round magazines and love those as well.

leave mags loadded for a few days, then retry.

2nd compare the distance of the magazine notch to other known good magazines you may have to lower the cmc one a little bit, to much you can make the magazine no longer function.

Ya, I'll try leaving them loaded.  They are fairly new and have only been shot with a few times. Otherwise they feed and function just fine. I think I'll leave the notch alone because they seat fine otherwise.

are you using the push pull method to rack the slide?

10rnds dont have a lot of give and the CMC may sit to high in the gun. dyken the magazine and see if the feed lips are hitting the slide.

I'm racking it the right way. dyken???

http://www.grainger.com/product/DYKEM-Layout-Fluid-2GKW3