Mag infoBack to Kel-Tec stuff
The P-11 was designed to use and accept S&W mags. They designed the frame and grip to accept original S&W mags, they did not copy the internal dimensions of the S&W guns. There is a slight but important difference there. I'll give one example that I personally am aware of. I came into a couple of eagle 17 round mags. They would not even fit into the P-11. They would probably work in a S&W because I believe it had some extra room around the top area of the mag well. The stock S&W mags were narrower at the top and the back corners more rounded. In this case the P-11 hugged the S&W mag very closely (trying to keep everything compact) and therefore did not allow the slightly larger and more squared off (at the top) Eagle mag to seat correctly. I did however get them to work with a little working over (a little bending here, some grinding there..).
Bottom line, even if it is a quality aftermarket mag, the mag is made for the S&W, not necessarily a COPY of the S&W mags, and may or may not be dimensionally equivalent to them. You can try them out and they may work fine. Your odds will improve dramatically if you can test fit it into your P-11 before buying to insure it will at least fit. Your odds go up even further, if you can load a couple of snap caps into the mag and cycle them through your P-11 to check feeding. Your odds are still the highest yet of getting a functioning mag if you stick with real S&W mags.
BTW all of my S&W mags also needed new springs to feed properly (which I used Wolff 10% extra power springs).
For more on mag springs see Wolff's Faq (pay special attention to #4 and #5).
The 59 series hold 15 rounds, and the 69 series holds 12.
Mag sleaves to make the grip better on the longer mags are avalible from KT here.
I carry my P-11 with a stainless S&W 69 series mag with a modified base plate.
Here are some pics showing the height comparison of some different mag setups:
And just a few more mag pics:From left to right, P-40 Mag with +1 ext, P-11 mag with finger rest, 59 series (15 round) SS S&W mag with KT mag sleeve, 69 series (12 round) SS S&W mag with KT mag sleeve.