In California, we are stuck with mags that are either 10 round or less, or greater than 10 round that we possessed LEGALLY in California PRIOR to 1-1-2000. Being 63, and having been an LEO and FFL for decades, I have plenty of USGI M1Carbine 15s (about 25 or so) and 8 or 9 USGI 30s going back to the 70s and 80s. All feed well. One of my 30s is an early commercial one, but is heavy steel and crisp edges just like GI. It is the only commercial 30 I have had that feeds 100%, though it seems virtually identical to my GIs. Back a few years ago when Chestnut Ridge Supply had NOS USGI 30 round magazine springs, I replaced all the springs in my 30s. I wish I had thought to buy NOS 15 round mag springs when the were available and cheap. A few of my 15s springs are pretty iffy.
I am also fortunate to have S&W, Browning, and Glock 9mm high caps, and stacks of AK, AK , HK, and other assorted high caps from when I did gunshows and sold in Shotgun News in the pre-internet days. USGI M1 Carbines (and Ruger Mini-14s, etc) are featureless, so I can use my mags in them.
I also run my AR, AK, and HK clone "California Featureless" (no flash hider, no collapsible stocks, and no pistol grips) so I can legally use my high-caps. I use muzzle brakes, fixed stocks and either kydex grip wraps or "Hammerhead" non-pistol grips.