4" universal Arca rail.
This size works well for an AR type rifle. I'm torn between this one and the SMOL for all purposes. I've tried the full length rails and I don't like how they feel when I shoot off hand so I switched to these short ones. I have the SMOL up front for bipod use and the 4" at the back for the tripod. I may try 2 SMOL's because I seem to put the tripod in the same spot every time and the extra length has not been needed.