Cabelas is a great place for equipment... except magazines. Those mags are probably made by Western Products or possibly Victor. Both brands in my experience are pure junk. If they work then you are one lucky person. Regarding the sloppy fit on the Triple K, My experience is that some slop seems to be OK. USA's seem to fit the best of the aftermarket mags, I have 2 new PMI 30's and they have some front to back play but very limited up/down movement. I had to adjust the lips for proper feeding but that just took a few seconds. Contrary to opinions posted on this and other boards, I bought 5 20's and 5 30's new USA mags about 1 year ago, all stainless, and they all work and fit perfect!
I may have used all my luck but I'm hard pressed to speak bad of USA since I have the only ones that work! I won't hold my tongue on Western or Victor... as stated, pure junk! Cabelas is good on returns, since you can't shoot for testing, CAREFULLY cycle by hand and you should be able to get an idea if they will work at all. If you have dummy rounds (no powder/primer) it is alot safer. If you use live ammo...
PLEASE make sure you are located where nothing will get hit if one should ignite (aim down into the grass). Good Luck, keep us posted, would love to hear that they work without a bigger headache!