Info on mounting M4SD-II's:
Basically once you have the groove located (something the stop washer will help you do) you drop the locking gate and the flat needs to be parallel to the top of the locking gate in order to fully seat the gate to the point the spring locks it down. A quick attempt to twist the suppressor will confirm or deny the locked engagement of the spring and then you can shoot provided the gate passed that rotational test.

In some really rare cases you might have an A2 that is out of tolerance- either too large of an outside diameter, or to narrow a groove to permit the locking gate to fit. That should be the extreme minority like 5% of the time. The solution then is a replacement $6 A2, or a proprietary mount guaranteed compatible.

As far as tightness of the mount, that depends on the tolerance of the muzzle device and suppressor. Our muzzle devices are OD ground for a very snug fit- the suppressor bores are jig ground for again a very snug fit with our muzzle device. A2's vary from an interference fit (incompatible) to .0035" of tolerance- they have a .004" tolerance range and we wanted the suppressors to be relatively snug. There is about 15 degrees of rotational play, but almost no, or no wobble at all. We tested one and it was shooting 1.75" groups at 100 yards magazine prone with a 4X Acog and 55 grain XM193, and POI shift was a minimal 1.3MOA down on the M4 barrel we tested. That's pretty much the accuracy of that barrel.

These aren't any less solid than suppressors like the KAC NT4 and M4-2000. They are snap on carbine suppressors and they operate similar to other options in their class.