Velocity Calibration

[brant89]

I recently decided to switch my 260AI over from a steady diet of 140gr AMAX's to 140 BTHP's since the AMAX is discontinued and the BTHP is cheap. After two ladder tests I found a load that should be quite accurate out to 1000yds (though velocity was surprisingly low) and went out to confirm my velocity. I verified my 200yd zero and proceeded to move back in increments out to 1000yds. At 300 and 520yds, my ballistic calculator agreed that my velocity was 2650fps (I realize this is quite slow for a 260AI, since it is only used for target practice it is a non-issue for me since accuracy is quite good). Then I backed up to 700yds and I was hitting over a minute high, but still grouping very well. So I used that velocity to calculate my 980yd solution and I impacted about one minute high again. I saw this same phenomenon occur before with the AMAX, its like my BC is INCREASING after about 600yds (which is obviously not the case). I played around with the velocity, BC, and elevation correction (thinking the scope may not actually be 1/4 MOA clicks) but nothing produced a solution that matched my results. Conditions were 75 degrees, sunny, 8-12mph winds shooting across a bare dirt field with rolling hills. I can only think of two possible causes; either my scope (Vortex Viper PST 6-24x50) has a mechanical issue where I'm losing about 1MOA as I pass 0 on my turret, or I have a lot to learn about shooting in heavy mirage. Any thoughts on this?

 

[tbrice23]

What ballistic calculator are you using ?

Does it include parameters for twist rate, altitude, pressure, and temp ?

Have you actually chronographed your load ?

 

[bigngreen]

Mirage can easily have you shooting that high, when confirming ballistics you need zero mirage!!

 

[Barrelnut]

Might be mirage. It is something I struggle with...

For turret accuracy, try Litz's tall target test below. I have used it on my Vortex PST 6-24X50 and it is very telling. My Vortex tracks almost perfectly up to 24 MOA which gets me close to 1100 yards on a 300 RUM.

It's a good test to verify your scope, then you can look at other things.

[ame]https://youtu.be/12Wf0Cuwwi8[/ame]

 

[brant89]

I'm using the Shooter app on my iPhone and getting my atmospheric data from my Kestrel 3500. No I have not actually chronographed since I have found the Shooter app velocity calibration to produce better results (until now). I can chronograph my loads tonight but I don't have a very good chronograph. If it is mirage, would it make sense that im not seeing it until after 500yds? Would I still see that much vertical due to mirage even in 10mph winds when the mirage is basically horizontal?

 

[brant89]

I have tested to make sure this scope was tracking true vertically (which it was), but I don't think I actually measured to verify that the adjustments were true. I'll admit that I completely forgot about this test. I will do this tonight with the chronograph set up also. Thanks for the input guys, I'll keep you updated!

 

[brant89]

So here is what I've got:

Weekend range session:
(Distance,MOA)
200,0
296,2.5
519,9.5
700,15.25
980,27.25

Today:
Dialed incrementally from 0-35MOA, all adjustments consistently showed 5% error. Chronographed 10rds and ended up with Ave-2719, es-28, sd-9.13.

2719fps seems reasonable to me especially since the actual BC is probably less than the advertised in my calculator, but what happened to that 520yd group??? It's not like it was a giant group size or out of the ordinary for that rifle (3 shots @ about 3"). Thoughts?

 

[Barrelnut]

Just a couple thoughts. In the original post the errors started at 700 yards. The further you shoot, the faster the errors start to compound themselves. Maybe this is why you have issues at 700 and 980 but not at 519.

Ran the numbers thru shooter with a 140 A-MAX at 2719 FPS. The difference between 695 yards and 705 yards is exactly 1 MOA. Maybe the target is not exactly 700 yards? The bullet is dropping fast at that distance and a small error in the measured distance will start to show up.

Also, the error on your scope is 5%. At 700 yards that is 3/4 MOA. With the shooter app you can adjust that error out. Would be a good idea to try and do that. So 1/4 MOA of distance measurement error and 3/4 MOA scope dialing error and look where we are at. 1 MOA high. Throw the possibility of some slight vertical mirage in there and things could get even crazier at 900.

Also, my Vortex PST had less than 1% error at 24 MOA. That's quite a difference between our scopes...

 

[brant89]

I had originally thought the errors were at the longer distances, but if you look at those distances with the 2719fps velocity and a .95 elevation correction you will see that my drops at 296, 700, and 980 are all within 1/2 MOA of my actual drops but the 520 is off by 1 MOA, so therefore I concluded that the 520yd target is the oddball. I am pretty confident that the ranges are accurate since I ranged each distance three times with a vectronix plrf10c mounted on a tripod.

I have to agree on the 5% error, probably should have used something more accurate than a tape measure since it's only precise to 1/16 which is 6.25% error right there. LOL. My bad.

 

[Greyfox]

I would recommend using a chronograph to determine an accurate velocity. The velocity calibration is meant to help true up your points of impact but the velocity output in the program, in my experience, is not precise enough should you want to QC your loads for actual velocity and ES, change components, or get a new lot of a particular component.

 

[brant89]

Greyfox - I usually do chronograph my loads but I thought I could skip a step this time and just get my velocity from the calculator. I thought it might be more accurate than my el-cheapo chrony but I'm seeing that is not the case. Maybe eventually I will invest in a better

Barrelnut - Now that I'm not half asleep while typing I believe that the 5% error must be fairly close to actual (1/16th is only 6.25% error for 1 MOA of adjustment). I agree that it is pretty high error but maybe it is within Vortex's acceptable error or it possibly slipped through their QC. Sorry for the momentary brain fart.

I will try to get out and shoot from 200-700 again this weekend and see what I come up with. Thanks for the input guys.

 

[WildRose]

I recently decided to switch my 260AI over from a steady diet of 140gr AMAX's to 140 BTHP's since the AMAX is discontinued and the BTHP is cheap. After two ladder tests I found a load that should be quite accurate out to 1000yds (though velocity was surprisingly low) and went out to confirm my velocity. I verified my 200yd zero and proceeded to move back in increments out to 1000yds. At 300 and 520yds, my ballistic calculator agreed that my velocity was 2650fps (I realize this is quite slow for a 260AI, since it is only used for target practice it is a non-issue for me since accuracy is quite good). Then I backed up to 700yds and I was hitting over a minute high, but still grouping very well. So I used that velocity to calculate my 980yd solution and I impacted about one minute high again. I saw this same phenomenon occur before with the AMAX, its like my BC is INCREASING after about 600yds (which is obviously not the case). I played around with the velocity, BC, and elevation correction (thinking the scope may not actually be 1/4 MOA clicks) but nothing produced a solution that matched my results. Conditions were 75 degrees, sunny, 8-12mph winds shooting across a bare dirt field with rolling hills. I can only think of two possible causes; either my scope (Vortex Viper PST 6-24x50) has a mechanical issue where I'm losing about 1MOA as I pass 0 on my turret, or I have a lot to learn about shooting in heavy mirage. Any thoughts on this?

I had serious tracking problems and/or failure to hold zero when dialing with 3 of 5 Vortex Viper PST's which is why I no longer own them.

Vortex customer service treated me great and I had all of them back or got a new replacement scope within a week or 10 days but that was just too much aggravation for me to put up with. It's bad enough when shooting at paper or steel but it can also result in a severely wounded animal wondering off to die.

 

[brant89]

Return to zero has been perfect. I will say 5% error is a bit disappointing but since I can account for it in my ballistic calculator I can deal with it. What did you end up replacing them with?

 

[WildRose]

Return to zero has been perfect. I will say 5% error is a bit disappointing but since I can account for it in my ballistic calculator I can deal with it. What did you end up replacing them with?

I got a heck of a deal on two Lupold VX6's 4-24x52mm with CDS popup dials.

Best money I have ever spent on rifle scopes and that's saying a lot.

If I had to get rid of all the scopes I have on rifles currently and replace them all with one thing, that's the scope I'd put on all of them and that's really saying a whole lot since it would include me getting rid of my IOR Valdada Tactical Scopes which I dearly love.

They won't take the same abuse as the Mark VI and Mark 8 but there's no way I'd ever need them too either.

The "tacticool" bug has really bitten a big chunk out of the sporting rifle and optics industry over the last decade and a half but the truth is very few of us will ever have an actual need for optics built to mil spec standards of abuse and on high end optics you're talking about an extra thousand or more to buy the tactical models over the hunting versions in many cases.

If that's what a guy wants to spend his money on I sure won't say they shouldn't but I don't mind saying that for all but a very few of us, it's a waste of money because it's just not necessary.

Why spend the 1,800 to 2,500.00 or more extra on one scope when you can buy two for the same price or less and still have everything you will ever need or want in a scope? Or think of it this way for what you pay for the Mark 8, or high end Swaro Tacticals or others you can buy a semi custom rifle and stick a scope on it that meets the same criteria without the "tactical features".

Nobody will treat you better than Leupold on warranty and service and that's the one brand I've never had a scope failure with of any kind while all of the others I've owned have let me down one way or another so barring something really ugly happening out of the blue they've got me for life.