AR vs Bolt rifle sight height

mcseal2

New member
I had a mystery solved for me recently, you are all probably smarter than me and have this figured out but I thought I'd share it.

I had hit several coyotes high and missed a few shots at smaller critters with my AR rifles, kinda had me turned off on them for a while. I just seemed to shoot the bolt guns better. For predator hunting the AR's have some advantages so I kept shooting them some, giving them a try. I talked to a guy who hunts with AR's almost exclusively and he said my issue was sight height. Most AR rifles a scope is mounted about 3" above the bore and most ballistic programs show data for 1.5" as a default. It isn't a huge difference but it is a difference. With my AR 223 zeroed 2.5" high at 100yds it actually shows a 260yd zero and is 3.6" high in the middle of that. That's to much arch for where this rifle gets used. I might have to shoot between limbs, fence wires, etc. I adjusted it before my predator practice shoot a while back, but wanted to shoot paper at various ranges to make sure I was perfect as I could get.

I went and shot paper at 200, 250, and 300yds with it tonight to perfect my zero. The DPMS Prairie Panther shot pretty good. This rifle had never shot great until my gunsmith recommended lapping the barrel. After that it's still picky, but shoots an affordable remanufactured 55gr V max load very well. It's running 2962fps from my 20" barrel. I was shooting off a sandbag on the hood of my truck with a variable wind of 5-15mph, and have a 2.5-10x scope on the rifle. The wind and magnification played into my groups at longer range a bit, but I was happy. The three groups at 200yds were all under 1.5", the 250yd groups opened to 2.25" average, and the 300yd group was 4". Subtract one flier and it was 1.75" though. All the groups were significantly smaller vertically than that, most of the distance between shots was horizontal with the wind.

After adjusting my scope on the 200yd target, I was 1.5" high at 200yds, 1.5" low at 250yds, and 6" low at 300yds. I figured that was about as perfect as I could get. The wind died down to nothing as it got later, and I went back and used my final 3 bullets at 100yds and 165yds to confirm that I wasn't to high there. I shot one at 100yds that hit 1.7" high, and two more at 165 that hit 1.2" and 1.6" high. Those two really should have been a bit higher from what the ballistics program says, but I was happy with where they fell. My windage was perfect too, so the horizontal drift was due to wind earlier. Coyotes beware!

Incidentally for some reason, maybe my shooting, my 204 AR didn't fare as well in the wind. I was getting nearly double the drift consistently with the 45gr SP loads at 3325fps as I was with the slower 55gr V max. The BC's aren't that much different. I think maybe the forend on the DPMS worked with the sand bags better, but decided to figure it out later. The 204 can sit out for now as well as the DPMS is shooting.
 
Thats a really high zero. I run 200yd zeros on all my AR's. with the .223 thats about an inch high at 100yds, 9" low at 300yds, 25" low at 400yds. 0-200yds is point and click, beyond that is where i compensate.
 
I zero 1.25 to 1.5 high at 100 which is basically dead on at 200. This way I can hold dead on out to 200 and when they are a little beyond that I know I just have to hold on the side but still on the kill zone since I know the poi will not be above the point of hold. At 300 I just plan to hold over the back a bit. I wouldn't mind a miss at 300 near as bad as the 75 to 125 yard average shot. Not to mention I keep calling if they are over 150.
 
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The key to setting an optic on an AR is determining your exact distance of the center line of the optic and the center line of the barrel bore....Not all ARs will set the optic center line at the same height to the bore center...The old Colt SP1, with the fixed carry handle and the Colt 4x scope was more than the normal 2.5" above the bore center...Rifles with flat top mountings will be dependent on the optic mount system, as well as the size of the optic tube...Measurements need to be a accurate as possible...

When using a ballistic program, the information entered needs to be as exact as possible to arrive at a meaningful output...I use the JBM Ballistic tables and have to be very careful to select the right bullet, as well as the rest of the information required...Normally, the output is pretty close to reality..

Your wind drift between a 45gr and a 55gr bullet has to do with actual bullet weight/design and not the firearm used...Heavier bullets buck the wind better than light ones when each has the same exact configuration...There is a noticeable difference between flat based bullets and boat tail bullets
 
Good advice everyone. I like my zero that is a bit higher than most probably because I get some longer shots. I killed 3 with the AR last Saturday at 240, 42, and 150yds. It worked fine on all of them.

The 204 45gr bullet shows a .245 BC, and the 55gr V max from the 22 cal shows .255. I was a little surprised that the 363fps difference in velocity didn't show up more on the wind drift due to similar BC's and the shorter time of flight. Maybe it was more my fault than I think, me not shooting the 204 as well.
 
2.5" is too high for a varmint gun in my opinion. I try to go 1-1.5". I would run a 2.5" high zero at a 100 on a deer rifle.
 
I agree with Dustin - MPBR is determined by your target size, and varmints are smaller targets. No problem at all to go +/-3", or even +/-4" with a deer rifle, but for varmints, so much variance might be the difference between a hit and a complete miss. Unless you're holding for range (defeating the purpose of MPBR) within your MPBR, then dialing outside, +/-2.5" is too much.
 
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