Z-Michigan
Member
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.
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.