Since a scope sight is mounted so high above the bore of the rifle, is there a point where the bullet crosses the line of sight? On most rifles it is usually at about 25 yards and then again at the point where you have it set, say 200 yards. But on the AR15 type rifle, if the bullet crosses the line of sight at 25 yards, it would be way too high downrange. Can you refer me to any helpful charts or info on this? Thanks.

A bullet crosses the line of sight twice. Once at close range (going up) and once at your zeroed range (going down.)

All of the ballistic charts and software will factor in the height of the sight above the bore when computing the trajectory and you will have a close zero and a long zero where the bullet crosses the line of sight.

A 300 yard battle sight on an M16 firing M855 ammo will have the bullet crossing the line of sight (with open sights) at 36 yards and 300 yards, which allows for center mass aiming at any distance from zero to slightly beyond 300 yards on man sized targets.

A scope on a flat top AR only sits .10 of an inch higher than most low mounted bolt guns.

Perry

quote:

Originally posted by perry:
A scope on a flat top AR only sits .10 of an inch higher than most low mounted bolt guns.
Perry

Not sure how you come up with that. Its a very general statement. Eg. My scope on my BAR sits 1.5" above the bore line, my Leupold CQ/T on my AR sits 3.0" above the bore line. That aint even close? To get the right check weld on an AR you will need all of that 3". Low mounts will be a waste of time.

That said. My 'kill zone' is a 3" circle on the AR. My loads can hit a 3" circle at 300yards if I do my bit. Point Blank Range is about 279yards with a 55gn VMax pill. So I can negate hold over/under up to that point. A 3" circle BTW covers most animals heads/kneck area. Shoulder shots are a breeze.

My 2cents.

This site might help you http://ballistics.ntinnovations.com/OpenLoad.aspx

I'll have to measure in the mornimg but for some reason 1.6 inches seems like the proper heigth on my flat top AR. I know the differences in low mount rings verses the high rings are less than an inch so I dont know where you are getting the 3 inches from. Does it have a carry handle?

Perry

Sounds like p67's scope is mounted on the carry handle.

Back to his original question: Whether the bullet crosses the line of sight twice is dependant on the range at which the gun is zeroed. If the zero point were the apogy of the bullet's arc, then it would only coincide with the line of sight once and at all other yardages both nearer and further, the bullet would be below the line of sight. I don't have any the resources in front of me at the moment to calculate what that zero range would be for a scope mounted three or so inches above the bore, but it is obvuiously greater than for a scope mounted much lower.

As ric says, with a typical scope height and at typical ranges for which a rifle is zeroed, the bullet crosses the line of sight twice. The lower the scope above the bore, the slower the velocity, and the greater the zero range, then the closer the initial crossing point.

BTW: On most of my sporting rifles zeroed for about 250 yards, the initial crossing point is more like 40-45 yards than 25 yards.

My flat top has Talley rings made for ARs and the center line of the scope is just shy of 3" above the centerline of the bore. I tried "high" 30mm rings and couldn't get low enough on the stock to use them. I believe 3" is about right. If you think about it, the scope needs to be in line with where the irons would and on an AR that is on top of the carrying handle or about 3".

Playing around with QuickTarget you get the following crossing points over and under LOS:

45 gr Berger at 3600: +35 yds & +200
62 gr ball at 3200: +75 yds & +200
70 gr VLD at 2800: +60 yds & +200

Of course, that was with a 200 yd zero.

I set my scoped AR to hit 3" low at 25 yards and that gets me real close at 100 yards.

Because there is no drop in the AR stock, scopes have to use high mounts for most people to see through them. The gun was designed with the rear sight on the AR's carry handle and the front sight on a pedestal. With the flat top receiver I use a tall scope mount ring combination: