I got this from Magnetospeed when I had some of the same issues. Hope it helps.
Hello. If you are getting to the 4 dashes but not getting any readings or error 2 or 5 messages (or any error with number message); then it is possible that you have the sensitivity setting not adjusted or the vertical alignment. Vertical distance or the space between the bullet flight path and the top of the sensor deck needs to be between .125 and .25" One of the two rubber V block pads should get you between that threshold if your barrel is between .5" and 1" in diameter. The horizontal spacing (the space between the end of your rifle and the beginning of the flat portion of the sensor deck) needs to be between 1" to 1.5" If you are shooting .30cal and larger; start with a setting of REG (regular). If you are shooting .30cal down to .223, start with REG and then try HI 1. If you are shooting .223 and smaller, start with HI 1, but you will most likely need to use HI 2 for smaller calibers like .22lr. A chart and additional information are on this link:
https://magnetospeed.com/mwdownloads/download/link/id/1147
Regarding it coming loose on the barrel: . Before any use or mounting the bayonet on the barrel, the tension thumbnut must be reset to the very bottom of the threads. If the thumbnut is indexed up the threads, slack will be put into the strap. With slack into the strap, the bayonet may appear to be tight on the barrel and recoil will move it. This is the number one user induced causes of loose bayonets. I recommend taking the thumbnut off the threaded stud and re-screwing back on 2 revolutions to ensure that you have reset it to the bottom of the threads. Also take the other rubber pad you are not using and put it on top of the barrel and bring the strap over it. This will help mitigate any slipping of the bayonet on the barrel.