Are your rifles bolt actions? If they are,
Just for the heck of it, take the bolt out and hang a shiny pie plate or something up a 100 yards out, now that the bolt is out glance down through the barrel from the back of course, and see if you can see your shinny target.
If you can then get something to stabilize the rifle a little be it sand bags or a gun vise in order to hold your gun solid then try to look through and get the shiny target centered in your view again looking through the barrel. Once centered have another person adjust the turrets until the cross hairs are on your shinny target. Glance back and forth to see if both look spot on. If this is the case you will end up within inches of a perfect shot at 100 yards (if that's where you hung your shinny target)
After a while you will be able to do this real fast aiming at a sign, pine branch, target face whatever that you have out about 100 yards to get your scope close for sighting in at 100 for zero.
No gadgets and fast plus it's just better knowing you did it yourself.
Cheers
oneshot.onehit