Scope issue with 25yd vs. 100yd zero

[Z-Michigan]

I was at the range yesterday with my 16" (heavy barrel, 1:9 twist) flattop AR-15 on which I had recently mounted a Simmons pro-diamond 2x32mm shotgun type scope. (I know this is a cheap scope, no need to remind me.) The scope is mounted using extra-high rings on top of 1/4" YHM risers; height over bore is about 2.75 to 3". I was shooting exclusively Centurion brand .223 Rem, 55gr FMJ ammo. This ammo has had excellent reviews for accuracy but chrono's rather slow on tests I've seen at AR15.com. I would guess it would chrono around 2700fps out of the 16" barrel.

I sighted the scope in at 25 yards and was shooting groups of about 3/4" at that distance, sitting at a bench. I then went to the 100 yard range and was not even hitting the paper (12x12" targets). After a lot of fiddling and help from a friend I managed to get it sighted in at 100, cranking the elevation knob up dozens of clicks. (Actually I can't remember for sure if I was cranking it up or down, but either way, I had to move the group about 15" from the 25 yard zero). It seemed to be sighted in at 100. Then I returned to 25 yards and shot a nice 3/4" group - which was about 12" lower than what I was aiming at. Windage didn't change much if at all.

From looking at a ballistic calculator I would expect the gun to shoot about 1.75" low at 25 when using a 100 yard zero. Obviously my actual result is way off. Can someone tell me what is likely happening? The scope and mount are 100% secure to the rifle, and I was shooting in the same way (sitting at a bench, hands holding up the rifle) at both distances. I suspect this may be an issue of scope parallax, but I don't fully understand that, and I could be wrong.

I used this calculator:
http://www.handloads.com/calc/index.html
Temperature 70 F., elevation 900', assuming a ballistic coefficient of .25.

 

[JDGray]

To shoot on at 25rds, with a 3" rise, you would be way high at 100rds. Zero at 100, and call it good

Parallax is easily noticed with a rested rifle that is on target, then without moving the setup, move your eye up and down, and side to side. If the crosshairs move off target, you have parallax. Scopes without an AO or side focus, will only be parallax free at one distance, set at the factory. You can minimize parallax, by being consistant in your cheek weld, or shooting exactly the same way each shot.

Your issue is definatly scope height related. My Varmint AR has the scope centered 3" over the bore centerline, too, but have never shot at 25yrds with it.

 

[ArmedBear]

55 grain .223 is good for over 3000 fps at the muzzle, even with a 16" barrel. That means it doesn't drop an appreciable amount at 100 yards. It shoots pretty much flat.

So, if you sight in at 25 with the tall sight tower of an AR, you are actually shooting for a 3" rise at 25 yards, and a 12" rise at 100 yards. It'll shoot almost a foot high at 100 yards.

A .22LR, at about 1/3 the velocity, does drop measurably at 100 yards, so it would be about right when sighted in at 25.

 

[Z-Michigan]

I appreciate the replies. Two comments though. First, all the testing of this ammo on AR15.com indicated a somewhat lower velocity, in the range of the 2700fps I noted. That would have only a small effect on trajectory, but a little. This ammo is nowhere near as hot as M193.

More importantly, when I run all the info through the ballistic calculator, a 100 yard zero (including the 3" height over bore) for this load should have resulted in hitting 1.8" high at 25 yards, which is far from the 12" low that I actually experienced.

 

[rcmodel]

I think it's your shooting.

There is NO WAY it can actually be shooting 12" low at 25 yards and be on at 100 yards.

If it was 12" low at 25 yards, it would hit the ground at about 50 yards.

With a 3" sight height, the bullet would come out of the muzzle 3.0" low,
still be
1.9" low at 25,
1.02" low at 50,
.38" low at 75,
and zero at 100.

rc

 

[rangerruck]

here is the prob; the scope , center of the glass- if you drew a straight line, out the glass of your scope, and then drew a straight line out the center of your bbl bore, I bet the difference between the 2 lines, would be at least 2.5 inches apart, if not 3 inches. So if zeroing at 25 yards, that means that your bullet, has to leave the muzzle, and travel upwards 3 inches, in just 25 yards, while going 2800 feet per second. At 100 yds, this type of rainbow trajectory will have you shooting over your paper, and probably at the sun.
If you are going to keep this setup, zero it at 100 yds. or even 200 yards, if your range has a 200 lane.

 

[wditto]

rcmodel and rangerruck have it right on - dat's a fact