I use a VM Hytec magazine block. I like it, I don’t think they’re made anymore. The are hard to find but you see them show up. It has good things and bad things about them. The good, it uses unmodified Uzi magazines. Uzi magazines are cheap and easy to load. The magazines are very secure and has no play. If the magazine in in, it’s in and not going anywhere. It works very well no feeding issues. They are easy to change out and go back to standard Ar 15.
The bad. The mag block is heavier than other magazine blocks. The magazines are tight, the magazine release button requires more pressure than others. The location of the magazine release button isn’t the best for quick magazine changes.
Break down, I don’t shoot competition or plan use in this in battle. When I shoot a magazine I take my time so the seconds longer it takes to load next magazine doesn’t matter to me, I would rather have the reliability. I like the mag block and would recommend if you don’t mind the weight.