I have used various magazine couplers over the years and have generally found them wanting. Most couplers work by clamping two magazines together tightly enough that they do not slip. The problem is that they eventually do slip or they are clamped so tightly that the magazine body pinches causing feeding problems. I eventually gave up on trying to find a decent magazine coupler.
I recently had my interest piqued once again in magazine couplers when I came across a new design that is all together different from anything else I have seen. The FASTMAG PMAG Couplers from Springer Precision work by replacing the baseplates of the magazine instead of clamping them together. That is not only innovative and interesting, it should completely do away with all the issues of clamp type couplers. It is obvious that a lot of thought went into this coupler.
The FASTMAG PMAG Couplers are machined from aluminum and then hard anodized for durability. It must be durable since it replaces the baseplate and it will take impacts from being dropped during magazine changes and from being used as a monopod. The magazines are not staggered as they are with some couplers. Staggering is usually unnessary and placing the magazines at the same level allows for a broader, more stable base for the magazine monopod technique.
Springer Precision offers 3 different models of the FASTMAG PMAG coupler: a model for .308 PMAGs, a model for 2 30 round 5.56 PMAGs, and a duty model for a 30 round and 20 round 5.56 PMAG. The duty version is very intriguing. It is designed to couple a 20 round PMAG with a 30 rounds PMAG. It weighs less and is slightly more compact than the standard 2x 30 round FASTMAG PMAG Coupler. It gives you the flexibility of having a reload on board with your rifle without the full weight and size of another 30 rounds magazine.
I am really impressed with this magazine coupler.