Rifle Scopes Puzzled?

[tansinator]

Scope: Nightforce NXS 5-22X56
Rifle: Apa .308
Rings: Seekins(properly torqued)
Action: Surgeon with integral base
Load: Berger 185 BT, 44.9 Varget(hot), Win and Lapua cases

Problem: Some days when I shoot 600 yards it takes 13.75-14 MOA from a 100 yard zero. Some days, nothing else changes but weather, it takes 9-9.25 Moa. Since sighting in the rifle over a year ago the 100 yard zero has not changed and is always dead nuts on. Also, ballistic calcs tell me that I need 4 moa to get to 300 yards. Some days it is 4 MOA. Other days it is 2.75 MOA. Any ideas of what could be going on?

I have contacted Nightforce and they have never heard of such a big poi shift. Craig was very helpful and asked that I send it in and they would go through it and see if they could figure it out.

Just to clarify, this is not a bashing Nightforce thread. Just looking for ideas of what could be going on.

Thanks

Eddie

 

[KNIGHT11B4]

Re: Puzzled?

Have you box tested to confirm that you NF is tracking?

 

[tansinator]

Re: Puzzled?

Not since this started happening.

Eddie

 

[KNIGHT11B4]

Re: Puzzled?

Might not be a bad idea.

 

[poison123]

Re: Puzzled?

Have you checked the ammo your shooting? Last time I looked the case capacity was different between the Winchesters and Lapua's.

 

[Drifter™]

Re: Puzzled?

Take a chrono each trip. You might be surprised at velocity variation on different days, with different powder lots in handloads, different lots of factory ammo, etc. 100 yards will not show much discrepancy, but longer yardages will.

 

[tansinator]

Re: Puzzled?

I understand that case capacities and weather can effect things, but it is always 14 MOA and 9 MOA. This just does not add up. I have chrono'd these loads at 100 yrds and they are very close with the different brass. I also, unfortunately, have nowhere to shoot them at distance (600 yrd) other than matches. I don't think the range officers would allow me to set up a chrono during a match.

Eddie

 

[KeithR41]

Re: Puzzled?

Parallax?

41

 

[tansinator]

Re: Puzzled?

Parallax is set before I will pull the trigger. Are you stating that something may be wrong with the parallax?

Eddie

 

[psinclair]

Re: Puzzled?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Drifter_1</div><div class="ubbcode-body">Take a chrono each trip. You might be surprised at velocity variation on different days, with different powder lots in handloads, different lots of factory ammo, etc. 100 yards will not show much discrepancy, but longer yardages will. </div></div>

5 moa difference?......

Box test your scope.

 

[BobinNC]

Re: Puzzled?

Just to frame your problem for your:

-4.3 MOA of drop @ 300 yds and -14.0 MOA @ 600 yds equates to a MV of around 2700 FPS.

-2.7 MOA of Drop @ 300 yds and -9.3 MOA @ 600 yds equates to a MV of around 3200 FPS.

It is a physical impossibility for your load to be going 3200 FPS from a 308.

So one of two things must be happening:

1) Your scope has gone bad.

2) On some days the target you think is 300 yds away is really only 240 yds away (-2.7 MOA of drop) and your 600 yd target is really only 480 yds away (-9.3 MOA of drop).

No other explanations fit the circumstances. But if I had a vote, it would not be #1. And here's why:

Exactly 20% of 300 yds is 60 yds, which is exactly the drop difference between 240 yds and 300 yds. Oddly enough, 20% of 600 yds is 120 yds. which is exactly the drop difference between 480 and 600 yds.

The odds of your scope being off on some days and not on others by exact amounts is almost nil. The odds that your targets are at different ranges on different days could well be high.

Occam's Razor comes to mind.......

 

[tansinator]

Re: Puzzled?

I agree with you Bob. The difference at 500 yards is also about 20%. I know the distances are correct.I am sending it in Friday for repair.

Eddie