Range Report Excessive elevation adjustment to zero?

[bgweed]

I finally got my Viper PST mounted on my .308 AR. After clearing up a few random minor issues, I proceeded to zero it in at 100yd.

When I had successfully zeroed in at 100, I was disappointed to see that out of 19mrad of total elevation adjustment (which I had checked on my particular scope already), I was left with a mere 6 mrad of "up" left over. I had apparently used 3.5 mrad to get from mechanical zero to a 100 yard zero.

Does this seem excessive to anyone? I am using a GG&G FLT mount which I have used previously with my Zeiss Conquest to great success. I ran the data through Shooter and it predicted a 1.4 mrad adjustment to achieve 100yd-zero. This confirms my estimate of 1.5mrad that I had estimated based on my previous experience with this rifle. The extra 2mrad may seem trivial but I really want the extra 125 or so yards it gives me before I have to holdover.

 

[Rob01]

Re: Excessive elevation adjustment to zero?

How much windage did you use to zero? Using excessive windage will eat up your elevation.

 

[bgweed]

Re: Excessive elevation adjustment to zero?

Windage was within a few tenths of mechanical zero.

I understand that if I dial extreme windage, my total elevation adjustment range would decrease. What I'm commenting on is that I used more elevation adjustment to reach 100yd than I expected.

 

[phillietimothy]

Re: Excessive elevation adjustment to zero?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bgweed</div><div class="ubbcode-body">Windage was within a few tenths of mechanical zero.

I understand that if I dial extreme windage, my total elevation adjustment range would decrease. What I'm commenting on is that I used more elevation adjustment to reach 100yd than I expected. </div></div>

Is it possible that you did not start from the center of the adjustment range?

 

[bgweed]

Re: Excessive elevation adjustment to zero?

Before shooting, I checked top-to-bottom adjustment range to check it against the specification. I then set it to halfway between the "top" and "bottom" to get it on paper. I measured almost 20 mrad of total adjustment (at the zeroed windage), so even if I had started a bit off of vertical center, I expected more than 6 mrad left over. I suppose I can check again... tonight I may take off the turret, pull out my zero-stop shims, and check the adjustment below and above my zero.

Also, my load is a 175 FMJ-BT over 47.2gr of CFE 223, so I'm not shooting a slow-ass load that's got a really low POI. I confirmed this with some Nosler custom comp 168gr, which had a similar POI to my loads.

 

[AMG04]

Re: Excessive elevation adjustment to zero?

I ran into a similar issue when mounting a PST on a flat top AR. It took nearly all the elevation travel to zero the scope.

Since then I have mounted the scope on a bolt action with a 15 MOA scope base. It required much less elevation to zero at 100yds.

A canted scope base may solve the problem.

I did a quick search and I could not find any GG&G mounts with built in cant.

Here is an example from Larue:
http://www.laruetactical.com/larue-tactical-20-moa-psr-scope-mount-qd

 

[bgweed]

Re: Excessive elevation adjustment to zero?

My first reaction was to consider getting a QD canted base... but I do like my GG&G, and I already own it... shame to not use it.

According to Shooter, I should only need 1.5 mrad to zero it... I guess maybe my rail is 0.12 degrees out of parallel with my barrel, which uses up that extra 2 mrad. When I put it that way, it seems kinda reasonable that the machining was off by only 0.12 degrees! haha

 

[mdavlee]

Re: Excessive elevation adjustment to zero?

I had the same problem on a 338 with a 20 moa rail. I only had 10 mil lwft after a 200 yard zero.