Shooter BC entries for 300 Scenar and 250 Scenar?

[cali_tz]

I am not having good luck with Shooter app on my 338LM firing Scenars (250 and 300). Shooter predicts a comeup about 1.2 MILs short of the actual. Meaning I need to add about 1.2 MILs to actually hit the target... this is at a mile roughly (1800yds).

Is anyone using Shooter with these bullets successfully at ELR ranges, and if so what are your BC entries? I have tried using the Lapua radar data, Litz data and Lapua single figure G7 BCs. Probably I am entering the radar data wrong... in that I am interpolating the BCs incorrectly into the speed brackets as Shooter wants you to do.

 

[BryanLitz]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

Couple questions:
1) what is your muzzle velocity and how much confidence do you have that it's accurate?
2) how are you determining range to the target?
3) what kind of scope are you using and have you verified it tracks accurately?

There are many things that can cause a ballistic prediction to be off at such long ranges, BC is just one of them. I enjoy the process of helping diagnose what the specific error is.

-Bryan

 

[cali_tz]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

Bryan,

Thank you very much for your expert assistance.

answers:
1) I used a Comp Electronics (CED) chron to measure my loads at one certain atmo condition, with a decent statistical amount of data, about ten shot string. I use the CED's Average speed for my Shooter ammo entry. However, I tactically shoot at a different location with different atmo conditions (but not very different), but that should be taken into account by the Shooter model. Naturally, I don't want to have to chron my ammo every shooting location so I trust it does the math right. To answer your question finally, I would say I am confident that I am no more than 50fps off from the true MV at my CED measuring location.

2)range to target is measured, taken 3x to confirm, with a Vectronix Terrapin. The angle to target was reasonably flat, no more than 5 degrees from level, and I don't know if Terrapin provides true straight line distance or some angle corrected distance (using some Riflemans rule variant?). I assume it's straight line.

3)Scope is a S&B PMII Mil/Mil. I actually have not verified that it tracks correctly using the common tracking tests I have seen on scope reviews. I could do that. I am assuming S&B factory (this was new) stuff tracks well from the out of the box.

FYI, both the 250 and 300 bullets needed 1.2MIL more at 1805 yards. Interesting that they were both off by the same amount.

I will grab my logbook and reply back tonight with the actual vs Shooter predictions, and the BC's that I have entered into the Shooter program.

 

[CoryT]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

Just FYI, 50 fps would result in about 1.0 mil shift under IACO standard conditions at 1800 yards. +/- 5 degrees is .1 mil. I've got S&B's that run .102 per click to .097 per click, so they are not all perfect.

How big is the target, how big is the group and how many shots are in the group? Is there a 1/2 MOA or smaller aiming point, or ar you just centering a white steel plate? Did you triangulate the group and measure to the aiming point? It's clear you did not use the chrono while shooting the group, which is a good thing to do for calibration. Have you run the system at closer ranges for calibration, say 600 and 1200?

Right now, there is an awful lot of 'slop' in the system that we cannot account for which can affect the actual solution to your problem.

 

[cali_tz]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

Cory, thank you for the input, that's very interesting.

The target is 24x24", and I can't speak to groups at 1800 yards with cross wind, that's just not going to happen, as shots were to left and right of the target as much as +/- 10 feet depending on gusts. But vertically the shots were varying no more than a total of 3 feet around the center of the steel, even while varying left to right as stated above.

Using a chron ex post load development is something I had not considered doing, but maybe I should be rechecking speeds. But tactically we aren't going to carry our chron around all the time... so we have to get to a point where we rely on historical data for well measured static loads.

I have to get my logbook to see what Shooter vs actual did at shorter distances, but my recollection was that at 1600 and less, the Shooter predictions were pretty spot on.

 

[CoryT]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

Well, wind is another issue, as there is some vertical deflection with a crosswind as well.

When calibrating the computer, it pays to use the chrono whenever possible. Track each impact with it's velocity, that way you know as much as possible about the actual shot. This also provides more data points for a better average velocity and SD, which means you have a better prediction when you get to a pure field shot.

If you really had +/- 1.5 feet, that's +/- .3 mil right there, so it's now real hard to say where any error may be, if there is any error at all.

 

[cali_tz]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

I may have traced the problem down to incorrect DA entry into Shooter. I believe either my Kestrel is far too high in it's DA calculation, or I incorrectly calibrated it earlier that day. When I put in a DA that I had used previously at that location, then the actuals vs Shooter were only 0.2 MIL off.

 

[cali_tz]

Re: Shooter BC entries for 300 Scenar and 250 Scenar?

also after poring over the data I've collected on 250 Scenar and 300 Scenar at 2750fps, it seems that I should settle on the 300 Scenar especially for 1500+ yards where it maintains velocity better than the 250, even with the 250 at 3050fps.

With the 300 Scenar I use less powder (save $s and barrel life?), get better trajectory performance at 1500+ yds, and better wind resistance.