There are ports on the Strike Industries King Comp underneath. So shooting prone will kick up some dust. However, these ports are not large like the angled side exhaust ports.
If the thread pitch is the same, this muzzle device would work with that caliber.
This Strike Industries King Comp uses 5/8x24 threads and is designed for .308/7.62.
Yes, this Strike Industries King Comp With Dual Chamber can be used with 300 Blackout. The Strike Industries King Comp With Dual Chamber has threads that are 5/8x24, so if your 300 Blackout is threaded for 5/8x24 the Strike Industries King Comp With Dual Chamber will work.
This Strike Industries King Comp is designed to work with 7.62 caliber rifles. You can find the 5.56 version by clicking on the link below, which will work with a Ruger AR556 as long as the threads on your barrel match the threads on the Strike Industries King Comp.
https://www.opticsplanet.com/strike-industries-223-king-compensator.html
The Strike Industries King Comp designed for .308/7.62 has a thread pitch of 5/8x24 TPI. The .223/5.56 version of the King Comp has a thread pitch of 1/2x28 TPI. Both versions of the King Comp are compatible with the Strike Industries Oppressor. You can find the .223/5.56 version using the following link:
https://www.opticsplanet.com/strike-industries-223-king-compensator.html
That rifle should have 5/8x24 and should be compatible with this muzzle device.
This compensator could be used with a 5.56 barrel, but the effect would be diminished.
Yes, the SI Oppressor should work with this muzzle device.
Yes, it is with the hole on the underside.