If you live in one of the restricted states, in this case meaning one that limits the capacity of the magazine used with a semi-automatic firearm, there are a few hurdles you have to deal with, other than the 10-round limit, especially when running an AR-platform rifle.
One problem is that 10-round mags are quite small when compared to the standard 30-rounders, and gear like magazine carriers and chest rigs are made for standard mags. There’s just no way to use the smaller mags with gear made for ARs.
One solution is limited magazines that use the same body as a 30-round mag, but are limited internally to only hold 10 rounds.
However, this might be a better option. It’s a coupler that neatly joins two 10-round mags together at the base, making them about the same length as a standard magazine, while allowing you to keep an extra 10 rounds readily accessible.
The MAG Coupler from RJK Ventures LLC allows you to connect two factory PMAGs without modifying them. The accessory is a double-sided polymer affair that replaces the floorplates of the two magazines its joining.
So, you have two 10-round magazines joined with the feed lips on opposite ends.
The AR15 coupler fits MAGPUL PMAG AR/M4 GEN M3 magazines and is available as a single coupler for $9.99 or in a 3 pack for $26.99.
RJK has options for AK and AR-10 shooters too.
The AK option fits MAGPUL PMAG AK/AKM MOE magazines and the AR-10 option fits MAGPUL PMAG LR/SR GEN M3 mags (all $10.99 each).
If your local laws prohibit you from owning standard AR magazines, these couplers are relatively inexpensive and provide an option that lets you use, train with, and compete with common gear and accessories and they don’t rely on clamps or screws that can loosen with use.