You need the .7 difference. Here is a true 13 rail on a 13.7 barrel. .64-5 ish works for a surefire socom though…
Thanks- I may switch to an 11.5 barrel for the extra rail space
I run a 10.0 Larue on a 10.5. The rail extends slightly over the threads ~1/8". I run a Specwar suppressor; it fits nicely because the mount has about 1/4" behind the teeth which extends it from the shoulder. If the surefire mount does the same thing as the specwar mount and the fast attach lock ring doesn’t go too far back over the mount, you might be okay. A couple of shims might be able to make it work.
I had heard that it works with some SilCo mounts. Good info. Thanks. But the .5 is not enough for the Surefire SOCOMs. Shimming out a MD for a can .2 is not ideal. .2 doesn’t sound like a lot, but twist off your muzzle device almost a 1/4 of an inch… no thanks…
Disregard
What scout light mount are you using?
Arisaka Inline Scout Mount. 
Yessir! 2 thumbs up
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Bcm, centurion, geissele. I love the gen 1 geisseles, pricy but damn are they pretty, like a well made knife or something. Harder to find unfortunately. Centurion and BCM I like equally.
Like it 
Good lookin gun you have there sir. Jealous, I’m a bit jealous…
Personally I like Centurion CMR 9.5’s and Geissele 9.5 SMR’s for 10.5 and 10.3 barrels.
Centurion C4 10.0 on a 10.5" also a close second
