Here keeping it simple is everything. You need to consider a few things, namely tube diameter which affects your rifle’s weight (for long hikes choose the most compact ones), exit pupil (in low-light conditions, keep the pupil 4mm above, and on a 2.5-10×50 the range is from 20 millimeters (2.5X) to 5 millimeters (10X), over adjustment is useless, mind your diopter (set it for your eye (look in your scope at a plain background not more than 20 feet) until the reticle is clear and leave it alone), parallax error is not a focus issue, choose FFP scope for taking advantage of the measuring opportunities of a Mil-Dot reticle and SFP scope if you are going to move it from one rifle to another, as for more flexibility.
The range of the devices for AR 15 varies, but if we are talking about the military, the first one is lightweight and compact SIG Sauer Tango6. Then there’s Aimpoint Comp M4 which has been used in the military for decades and has proven itself as the elite red dot optic. If you’re looking for proven performance with a successful history and combat experience, the Aimpoint Comp M4 is your choice. Then there are also Vortex Optics Spitfire – Prism Scope and Trijicon RMR – Backup Dot Optic.
There’s quite a wide variety of optics for M4 as well. But most often it’s Aimpoint M68CCO CompM4, EOTech Model 553, and Trijicon M150 RCO (mostly a few 4×32 versions, the TA31RCO-A4CP, TA31RCO-M150CP, and TA31RC-M4CP).
If we’re talking about long-range rifle scopes, they are (starting with the most expensive) Nightforce Optics 5.5-22×56 NXS Riflescope, Leupold VX5 HD 2-10×42, Vortex Optics Viper HS-T 6-24×50 SFP Riflescopes, Vortex Optics Crossfire II 6-24×50 AO, SFP Riflescope, Vortex Optics Diamondback HP 4-16×42 Deadhold BDC. Best budget scope is Leupold Vx Freedom (2-7×33).
Talking about turkey guns. You can use target loads at close range (10 yards) to know if your sights are aligned. If your pattern does not hit who you aim, you need to adjust your sights, to put the pattern on target. With rifled sights, simply move the rear sight in the direction you want the pattern to move (move it up if you want the pattern to move up). If the gun is on at 10 yards it will be on target at 40 yards. Don’t aim at turkey’s head, go for the wattles on the neck.
It all depends on your needs. Aim at the golden mean. Not too big and heavy and enlarged, not too small and weak. For hardcore hunting at 600 yards, 10X is more than enough. It’s totally OK to buy a 5-25x56mm or 3.5-18x50mm built on 30mm or 34mm main tubes. But don’t forget that such units are quite bulkier and heavier than weaker scopes with small objectives and less power. They can unbalance your rifle, make it slower to sight in, heavier to carry, and not that fun to use. Are you ready for that? Long-range rifles, in their turn, benefit from big, powerful scopes.
Here we are talking about ranges starting from 500 and ending with 1000 yards. This is the farthest the Army shoots with open sights. Consider models from Nightforce (Nightforce Optics BR Series), Leupold (Leupold VX-6 HD), and Vortex (Vortex Optics Razor HD Gen II) first.
The 4-12×40 scope with a 40 mm objective lens from Vortex is a great option for excellent deer targeting.
It all depends on your budget. Nikon Prostaff 7 5-20×50 can be a good option. Its objective diameter is 50 mm and the tube diameter is 30mm. The most useful power range for the all-around shooting is 5-25x or 6-24x. That goes for prairie dog shooting also.