We recently received the 20- and 30-round magazines Brownells put out in celebration of 10 years of being an AR-magazine manufacturer. In the box with the standard-capacity models were these two, lesser-capacity magazines.
Good for localities that ban standard magazines or in certain hunting applications, the 5- and 10- round magazines have an oversize plastic floorplate that allows the small magazines to be inserted easily. The 10-round magazine is very close in size to a standard 20-round mag, where the 5-round magazine is slightly smaller.
Functionally the magazines are identical to the 20- and 30-round magazines. The 10-round magazine fits into the magazine well such that the plastic floorplate is completely exposed, whereas the floorplate of the 5-round magazine fits about halfway into the magwell. Both magazines load and drop free easily.
While lower capacity magazines keep ammo consumption down at the range, there's another upside to the smaller quantity. When testing rifles for accuracy and velocity, we run 5- and 10- round groups per protocol. Having 5- and 10-round magazines for this purpose helps keep track of where things stand.