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We recommend removing your muzzle device and attaching this directly to your barrel to insure the boresight is centered as the flash hider A) might not be magnetic (aluminum for example) and B) may not be perfectly even with the front of the barrel. You should not fire the gun with this attached, the boresight mounts to the front of the barrel. Maximum sighting distance would essentially be as far as you can still see the laser.
Will work with 223. Best practice would be to remove the flash hider. You do not fire the rifle with the rifle with the bore sighter attached to the barrel.
A laser boresighter is supposed to make the sighting in process easier not harder. If I've got to remove my flash hider then replace with new crush washer and realign well... Its already 10 times more difficult then using a few extra bullets.
Per the pictures, you will have to remove the muzzle device to insure this is centered (the flash hider may not be perfectly concentric). You cannot fire the gun with this attached, the laser aperture is in front of the barrel.