Yes, it could be installed on your FM9.
Yes, the OpticsPlanet Exclusive Firefield Fringe 10in Carbon M-LOK Rail will work with a Ruger AR556 if it has a low profile gas block. If your Ruger AR556 does not have a low profile gas block and uses an A2 Front sight, this rail will not work unless the A2 front sight is replaced.
The diameter of the hand guard is 34mm and the part where it threads into is 30mm.
Yes, you could use it on a .22LR AR-15
it does work with piston systems
Please email us at techsales@opticsplanet.com so we can further answer your question.
No, this would be too small for an AR-10 fitment.
This is for the AR-15 platform.
Being a 10" handguard, this may cover most of the gas block but not all.
A fore grip is able to be attached if it is M-Lok or using an M-Lok picatinny rail section.