Just a heads up for anyone considering a Holosun 509T. The mounting design is not picatinny / 1913. They are completely incompatible. The sides of the mount do not extend down far enough to grip a 1913 rail.
I was mistakenly under the impression that it was, but I got one in the mail today and that's definitely not the case. See photos showing the gap there.
Note the gap
The bottom of the "claws" (don't know what they're actually called) don't go far enough to latch on to the angle of the rail. In this case I was unable to use a 45 degree Haley Strategic picatinny mount.
You get a RMR plate, which is cool though. However with a Glock MOS for example, now you're stacking two plates on each other. Probably more useful for a pistol with a dedicated RMR cut slide. As a plus, you could remove the optic with a side mounted torx bolt, so if you shoot IPSC in both production and production optics, removing or replacing the 509T is really easy. Once you have that RMR plate installed it'll stay there, and you just remove the optic from that plate.
For a rifle or shotgun, it won't go on a picatinny mount, though if you can find a dedicated picatinny RMR mount, that might be a solution. But again you have to use the supplied RMR mounting plate as an intermediary.
Anyways, just figured I'd mention it so nobody makes the same mistake as me. I'll probably keep it as I do have a pistol it can go on, but that wasn't my Plan A.
I was mistakenly under the impression that it was, but I got one in the mail today and that's definitely not the case. See photos showing the gap there.

Note the gap

The bottom of the "claws" (don't know what they're actually called) don't go far enough to latch on to the angle of the rail. In this case I was unable to use a 45 degree Haley Strategic picatinny mount.
You get a RMR plate, which is cool though. However with a Glock MOS for example, now you're stacking two plates on each other. Probably more useful for a pistol with a dedicated RMR cut slide. As a plus, you could remove the optic with a side mounted torx bolt, so if you shoot IPSC in both production and production optics, removing or replacing the 509T is really easy. Once you have that RMR plate installed it'll stay there, and you just remove the optic from that plate.
For a rifle or shotgun, it won't go on a picatinny mount, though if you can find a dedicated picatinny RMR mount, that might be a solution. But again you have to use the supplied RMR mounting plate as an intermediary.
Anyways, just figured I'd mention it so nobody makes the same mistake as me. I'll probably keep it as I do have a pistol it can go on, but that wasn't my Plan A.
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