Rangefinders
The laser rangefinder has probably been one of the best things to happen for shooters in the last 20 years. That said, there are pros and cons for these tools.
The price on a very, very good one is right up there. The best question is---what are YOU going to use it for?
You mention 0 to 800 yards. In most hunting situations, 80% of the game is shot at under 200 yards. Shots at varmints tend to be more, but if you want to put meat on the table, then you have to make sure of your shots.
I know there will be posts that state " I shot my Elk/deer/moose/mother-in-law" or whatever at 800 yards and I dropped them right on the spot. The percentage of animals wounded and never recovered because they are not hit right is much higher than anyone will admit.
Most rangefinders use infrared. Therefore, red or bright objects give better reflective readings than other colors. A stop sign can be ranged at well over 600 yards, but a brown deer may be hard to range at 400.
I have used both Bushnell and Leupold. They both work within their limitations. They are not ABSOLUTELY accurate, but give a very close reading on an object. Targets farther away are more difficult to range. Also, you have to have the time to use the rangefinder...a slow moving target can be ranged, but a spooked deer is almost impossible.
One of the best uses is if you are sitting in one spot. You can make up a "range card" by ranging several objects in front of you across your line of vision. For example, you might find an old tree on your left at 235 yards, a rock in front at 329, and a creek bank on the right at 195. If you have a small piece of paper and pen, you can draw a basic map with the ranges marked in, and if an animal comes out near one, you have a reference for range.
Shooting on the prairies is more difficult than in the Mountains or eastern woods. This is because of the lack of reference points, and distances seem much closer than they really are. Varmint hunters and prairie dog shooters usually have a rangefinder handy.
For me, it's another tool. I use one because it gives me a very close estimate of ranges under 400 yards. I do not shoot at game animals at extra long distances, and I think 300 yards is long enought to get accurate shot placement in a killing area.
Weather conditions also affect accuracy and rangefinders. Snow on the ground makes things difficult to range. Unseen trees or bushes can give false reading.
You sound like you want to range something, dial it in with the scope, and it will drop dead after the shot. There are so many variables in shooting that it is almost impossible to list. First of all, you have to have a very accurate rifle and very accurate ammunition to go with it. Next is your experience and technique. Now throw in wind direction and velocity (especially when it changes down range---anyone who has shot at Camp Perry will agree with this one) Add heat, temperature, light conditions, light direction, mirage, humidity, target elevation relative to your elevation, snow or rain, shifting cross winds, and all the rest of the goodies that make shooting interesting, and you will find that one solution does not fit all.
Another thing is scope accuracy when adjusted. Does it come back to EXACTLY the same place when you adjust it? Are the click settings accurate? If so, then treasure that scope, because most are not at long ranges. How's your barrel--first shot out of a cold barrel, or tenth shot at a target out of a heated up one? A big one is magnification of a variable scope. "Precision Shooting" magazine recently ran an article about moving a scope from a lower power (eg 3X) to a higher one (eg 7X) and shooting a group. Then moving it from the higher power to the lower one and shooting a group. Even with some expensive scopes, the groups did not hit in the same place. This was supposedly due to the way the spring tension affects the adjustments. For those interested, it was recommended it you want to go from a high power (eg 8 or 9X) to a lower one (say 6X), you should bring the scope all the way back to the lowest power, then come back up to the one you want. Again the spring tension issue.
So, this is my own observations. It is a useful tool, but it is not infallable.
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