I have an ARCatch22-based setup which uses the same magazines as the M&P15-22 (albeit with ARCatch22's adhesive shims); they do not drop freely when I use a polymer lower, but do drop freely when I use an aluminum lower (bit of an experiment, that one - it's unclear how long the unanodized aluminum pinholes will last before recoil and other stresses egg them out - but I digress). These magazines weigh about 2.5oz which is quite lightweight for an AR-15 magazine, and my full-length 25-rounder (like my centerfire pmags) does drop freely because it weighs almost 5oz.
It seems to me that any polymer magwell has a trade-off. Too tight, and the friction with the magazine will apply to the magazine a normal force greater than the magazine's weight; too loose, and the incomplete contact between the magwell and magazine probably allows the magazine to wobble in-place even when not released. A smooth, flat, metallic surface on the magwell (or the magazine or both) probably breaks that tradeoff by allowing the magazine to move laterally to the magwell even while maintaining a large contact patch with both sides of the magwell.
As regards my own setup, I have two hypotheses.
- My particular polymer lower was designed for centerfire magazines which tend to weigh at least 4oz, or mine hasn't "worn down" enough yet, or mine has imperfect surface finish which has a higher friction coefficient than normal, or mine was designed to reliably hold magazines steady at the cost of not dropping lighter-weight magazines freely.
- Polymer lowers generally are not as smooth-surfaced as aluminum and therefore must be manufactured to impractically-strict tolerances in order to exert just the right amount of friction to hold a 2.5oz polymer magazine steady yet also drop it free when it is released. Because of that impracticality, they are not so manufactured so they generally do not drop a 2.5oz polymer magazine freely.
In the case that the second hypothesis is correct, I have been thinking about smelting and machining some heavier baseplates for these magazines. Aluminum bronze, notably, is relatively easy to smelt from scrap materials.
However, I don't (and won't for quite awhile) have a sufficiently diverse library of polymer lowers with which to disprove either hypothesis.
Thus follows the question I pose in the title of this post.
Before you go to a bunch of trouble try taping some coins to the bottom of that Mag and see if it will drop free.
Short answer to your question - yes, they all drop free.
Longer answer: 15-22 mags are the same dimensions as regular AR mags. The company that sells these products recognizes this and has several options that revolve around either applied plastic shims of one form or another, or oversized/different parts (ie bolt catch). You don't mention what specific parts you have, so it's hard to say whether or not the 15-22 mags will drop out of your setup.
Would a little judicious sanding of the high friction areas (just to the sweet spot) be a simple solution? This is something I have done successfully to 10-22 mags.