My experience with arms mounts is not that they will always/ never fit on any particular type of Mil-Std-1913 rail, but rather there is a fair amount of inconsistency in the mounts themselves causing them to be too tight (not so common) or too loose (more common) on a variety of rails.
If you remember, an arms shill posted on this forum stating that the mount was designed to be "soft"- apparently (considering his status in the company) an official explanation as to why the arms mount slides to and fro on more 1913 rails then they would like to admit.
Additionally, i have seen enough broken arms levers to convince me that it is not an optimum mount.
All of my arms mounts were replaced with Larue mounts a long time ago. SF lights, optic mounts etc.
I have a choice, and my choice was to what works- not what doesn't.