They are pretty simple.
Looks like you have the more common capped top mounts rather than the screw top.
Clockwise = down and left.
For cap top zero.....
1. Focus rear ocular with scope head on infinity setting to get clear reticle on white background.
2. Set parallax to your distance using scope head. Don't expect markings to be exact but you will find them close.
3. Zero as usual. Id suggest getting on paper at 25 yds.
4. Set zero stop on drum. Look for two small straight blade screws on the numbered elevation or windage drum. Loosen those two small screws and rotate the drum to your zero. Retighten tiny screws. Don't Mongo the screws.
5. Shoot and enjoy.
The USMC removed the recoil spring on the tube. Their belief was that with heavy recoil (30-06) they wanted to limit the forces the scope was exposed to.
With no spring when the rifle is fired the scope sits in space while the gun moves rearward under it. After firing the shooter has to pull the scope back into battery.
It sounds like a good theory to me and besides using my Unertl on a USMC clone gun I removed the spring as insurance against harming the scope. You could get the same effect by taking all tension off the spring.
What caliber is your rifle. Looks center fire.
My .22 Unertl
My 30-06 Unertl
My other 30-06 in need of a Unertl
