I will try to make this short and to the point, I have used one for the past few years, Do they work YES, you have to make sure you have the right model for your cartdrige, then have to get a ballistic program that take in for account for all varibles, baro pressure, temp , altitude etc. then look in the shepherd book and match your bullet speed to their reticle. I have had one on a 300rum and it worked great but I now have out grown it, much easier to load for the rifles accuracy then dial for correct MOA with all varibles accounted for. You can get a scope with a MOA or mil hash marks on the reticle then just hold over or dial which ever you prefer, scopes like the NF, PST etc all have those reticles, svae yourseld some time and get one of those and you wont outgrow it. Remember that all bdc reticles or the custom knobs require all the varibles to be the same all the time for them to be accurate. I may hunt at 6000 feet one day then the next day find myself at 9000 feet, to much varible for a bdc to be accurate. Hope this makes sense and pm me if you have any more questions