Odd AR scope issue

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Elkins45

Elkins45

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I have a DPMS flat top AR upper that had a 16" heavy .223 barrel installed. I had a Leupold 2X7 scope mounted and sighted in at 100 yards. I recently removed the scope, replaced the barrel with a 16" 300AAC Blackout barrel, then bolted the scope back on. Now I don't have enough vertical adjustment in the scope to raise the point of impact onto center. I swapped the rings for a lower set and still can't crank the POI up high enough.

What's up with that? The scope worked fine with the .223 barrel. How could installing a new barrel mess up an upper? I'm thinking I may have to shim one of the scope rings but I would prefer to avoid that if possible. Anybody have any suggestions for me to check before I start shimming?
 
AR-15 rings on a flat-top upper are exactly the same height front & back anyway.

Changing ring height from high to low has absolutely nothing to do with anything as to how much scope adjustment you have.

My guess is, the barrel extension / barrel nut is not seated straight in the upper, and the new barrel is pointing off whopper-jawed to the flat-top rail.

rc
 
[QUOTEMy guess is, the barrel extension / barrel nut is not seated straight in the upper, and the new barrel is pointing off whopper-jawed to the flat-top rail.][/QUOTE


That is the problem IMO.
 
100 yards, supersonic.

I took the barrel off and I don't see that there is enough slop in the tolerances for it to be seated crooked, but I reinstalled it and retorqued the nut. We'll see tomorrow after work it that makes a difference. It was very firmly and squarely seated against the receiver and the barrel nut screwed on without any resistance or feeling like it was cross threaded.

rcmodel I have a lot of respect for your opinions but I'm afraid I have to disagree that the height of scope rings won't make a differnce regarding scope adjustment. As an extreme example (to illustrate the point) imagine a 12" tall set of scope rings that hold the scope 13" above the bore. There's probably not a scope on the planet with enough internal adjustment to bring the axis of the scope inline with the axis of the bore at a reasonable normal distance. Yes, I know that's a ridiculous example, but if height matters in an extreme example then it matters in a smaller one, just to a lesser degree.

Anyway I will shoot it tomorrow and report back. Hopefully taking it apart was enough to fix it.
 
As an extreme example (to illustrate the point) imagine a 12" tall set of scope rings that hold the scope 13" above the bore. There's probably not a scope on the planet with enough internal adjustment to bring the axis of the scope inline with the axis of the bore at a reasonable normal distance. Yes, I know that's a ridiculous example, but if height matters in an extreme example then it matters in a smaller one, just to a lesser degree.
Click to expand...

You know that is only 13MOA at 100 yards from being parallel right? If I am doing the math correctly it would only be 6.5 moa at 200 yds. Most scopes should have that much adjustment.
 
I would think the comment about ring height was intended to be confined to actual scope mounts that would be encountered in the real world, not fantasy 12" mounts. The differences in real scope mounts will be measured in milimeters....not feet:what:

One can always come up with a sufficiently outlandish scenario to illustrate a point but whether or not one actually proves one's point with such a scenario is highly questionable.
 
do you have a canted base/rings perhaps....bigger drop with the 300AAC along with a base/rings wrong way around would tun the scope out of adjustment at 100
 
I initially removed and reinstalled the scope by just loosening the Weaver mounts. I didn't take the rings off the scope. I tried to sight it in and that's when I discovered it didn't have enough adjustment even though the exact same configuration worked with .223. I only changed the rings after the original setup wouldn't work. It would be quite a bit of a coincidence if both were canted.

I suppose there's always the possibility the barrel is tilted relative to the extension or that it's just plain old' bent. If I can't get it to shoot tonight I'll chuck it in my lathe and put an indicator to it to see if it isn't concentric.
 
HoosierQ said:
I would think the comment about ring height was intended to be confined to actual scope mounts that would be encountered in the real world, not fantasy 12" mounts. The differences in real scope mounts will be measured in milimeters....not feet:what:

One can always come up with a sufficiently outlandish scenario to illustrate a point but whether or not one actually proves one's point with such a scenario is highly questionable.
Click to expand...
I won't swear I'm right about anything, but if a foot makes a difference then so does a millimeter. The farther a scope is from the center of the bore, the more internal adjustment that needs to be made to bring the two together. A scope that limits out with high rings might have barely enough with a lower set.

I don't think this is an idea I just dreamed up on my own. I'm pretty sure I have read about this from multiple other sources.
 
You could try something like a 20moa base. Also what kind of scope is it? You'd be surprised how little adjustment can come on some of the lower end 1" scopes.
 
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Natchez just ran out of stock on the Bushnell Elite 3200 3-10x40; they were on clearance for 159. But I got one of them on my Rock River Coyote Carbine, and it has 85 inches of adjustment. But the new model; named "Elite" and argon purged instead of nitrogen; has 90 inches of adjustment. It's 299. It's probably the most adjustment you'll find in a one inch scope; just wanted to mention it in case you do end up trying for a scope with more adjustment.
 
Elkins45 said:
I won't swear I'm right about anything, but if a foot makes a difference then so does a millimeter. The farther a scope is from the center of the bore, the more internal adjustment that needs to be made to bring the two together. A scope that limits out with high rings might have barely enough with a lower set.

I don't think this is an idea I just dreamed up on my own. I'm pretty sure I have read about this from multiple other sources.
Click to expand...
You are correct in theory, but in reality if its that close at 100yds, something is horribly crooked with the relationship between your scope and the bore of your rifle. Something isn't mounted straight. It could be that whoever made your barrel did a poor job of threading it to install the barrel extension or the barrel extension is bored out of concentricity. It is also possible that your rings aren't seated correctly.

As an example, I've got a Marlin of fairly new manufacture and a scope mounted on it with 96moa of total vertical adjustment. With 2 different mounting setups neither one higher than absolutely required to clear the hammer, I lacked enough adjustment to zero the rifle at any range. The reason is that the receiver it bored crooked and the barrel is angled down with relation to the top of the receiver. And so you realize, this way common with these rifles so stuff like this is a lot more common that you would like to think. I would have returned the rifle but when I've got a levergun that puts 3 shots into a nice cloverleaf with all holes touching, I'm going to figure out a way to make it work...and I did.
 
So I shot it today and it now has enough adjustment to sight in at 100 yards with some to spare. I don't know if removing and replacing the barrel did it or if it was the shim I added to the rear scope ring, but I guess it doesn't matter as long as it's working.
 
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