These Meprolight M21 Self-Powered Day & Night Reflex Sight are new unless you select a factory demo model.
Yes, this will hold up to .44 magnum.
No, it is a fixed reticle.
There is not another mount for the Meprolight M21 Self-Powered Day & Night Reflex Sight w/ Dust Cover. The Meprolight M21 Self-Powered Day & Night Reflex Sight w/ Dust Cover will mount to a picatinny rail on the receiver of the Benelli M4 however, it will most likely not co-witness with the front sight of that shotgun.
The MeproLight M21 Day/Night 1x30mm 4.3 MOA & 5.5 MOA Self-Illuminated Reflex Sights are strongest to hold up to the recoil of a 12 gauge shotgun. They can also handle the abuse of normal wear and use.
The Meprolight M21 Self-Powered Day & Night Reflex Sight model number 0626110 has the Bullseye reticle.
The Meprolight M21 Self-Powered Day & Night Reflex Sight will fit any rifle or pistol that uses a picatinny/1913 flat top rail on the receiver.
You can mount the MeproLight M21 Day/Night 1x30mm 4.3 MOA & 5.5 MOA Self-Illuminated Reflex Sights to a Mossberg Shockwave if you have a picatinny rail mount to mount it on.
Only factory new optics are available from Meprolight.
The unit itself weighs 230 grams.