Danger Space

Jun 8, 2008
666
72
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Colorado Springs, CO
#1
I'm shooting the HCP rifle match in May Its a team match requiring a bolt gunner and a carbine shooter. Without getting into the carbine I've fully assumed I'd shoot my 6SLR. Its pushing a 115 Dtac at about 3020fps. I'm running it at about 50K PSI; well below its max. For most matches all the target ranges are provided so more velocity for a given bullet really only gives a wind advantage. For this match however the targets are UKD. Shooting the same bullet faster may help when the tgt range is not correct. So I needed to perform some 3rd grade math to determine if running the SLR at 3150 was going to be an advantage over 3020. 130fps difference. Here's what a little ballistic analysis told me.

Assuming my median distance for the longer range targets is 600yds, the target size can be predicted at 2 MOA for these guys, in this part of the country. Therefore my target size is 12" or .6 mrads. This means if I aim center I have .3 mrads below and above my POA to still hit the tgt; .6 total. I ran the numbers on my AB 5700 for .3 above and below my 600yd data and determined what ranges equaled the .3mrad mistake.

If my data for 600 is 2.66 mrad then I can still hit the target, mathematically, at 2.33 and 2.93. A .6mrad difference; the tgt height in mils. So backwards engineering the yardage to those data's, I can determine how much yardage error I can have and still hit the target.

For a 115 Dtac going 3020 2.66 mrads is 559 yd data. 2.96 mrads is 642 data. So I can hit the tgt from a mistaken distance of 559 to 642 yds. Therefore my danger space is 83yds. Anywhere inside that 83yds I should mathematically still hit the tgt.

So going 3150 (assumed node) my 600 yr data is 2.38 mrads. .3 mrads on either side of that is 2.08 and 2.68 yds. My yardage data for those holds are 553 and 645 respectively. Therefore my danger space at that velocity is 92 yards.

So I gain 9 yards of error by pushing the cartridge 130fps faster. IMO not worth developing another load in the higher node for this particular match. I'll run what I got.

In the next post, I'll explain wind danger space in mph and determine the worth of 130fps. It will obviously be less so but we'll do the math anyway for diligence.
 

Attachments

Jun 8, 2008
666
72
28
38
Colorado Springs, CO
#2
Assuming the tgt is a circle, I still have .3 mrad left and right of my mathematical/ hypothetical POI. Using my Kestral 5700 AB I determine that a 5 mph wind at 600yds is a .3 Mrad. So I have a wind error of 10 mph for a .6 mrad spread. I could, mathematically, call anywhere between 0 and 10 mph and still hit a 2 moa that at 600yds. At 3020fps.

For 3150 fps a ..27 mrad wind drift happens for a 5mph ar 600yds. Round off to .3 and you get the same windage "Danger Space" as 3020fps. Again no advantage of 130fps for the DTAC.
 

Culpeper

A lungo raggio
Nov 25, 2006
1,413
89
48
59
Roswell NM
#5
With a .308 168SMK it is more like 2 MPH margin of error on the wind call. Keep in mind you also need to factor in rifle accuracy. If you 'think' you have a .25 moa rifle than factor in .5 moa and watch your statistical confidence level tank. And another .25 moa experienced shooter error and watch it go south real quick.
 
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Subwrx300

Mastery: its difficult for a reason....
Jan 15, 2014
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Cedar Springs, MI
#10
I would say that the theory is correct BUT it assumes each round hits exactly the same POA/POIbased on solution. You need to shrink the target or bracket your drop to include the size of your rifle system and precision level.

For the sake of easy math, if your rifle system and shooting position allow .2 Mil precision, you will only have -+.2mil up/down/L/R as a buffer for 100% hit probability. If a .3Mil system, you'd only have 1.5mil on either side for 100% hit.

You can use your inherent precision level to create safer brackets that will give you specific ranges based on all possible shots in your system. Kind of like WEZ analysis system from Litz but simpler version.

At least that's how I would see it. If you gain accuracy at specific node, could make sense to move up/down. YMMV. Good post! Very thought provoking!
 
Jun 8, 2008
666
72
28
38
Colorado Springs, CO
#11
No need to include independent and external influences to determine the advantage of only speed. Those other factors would apply to 3000fps AND 3150fps so like a common denominator they can be reduced to clarify the one variable that we're focusing on, the speed advantage. No need to clutter the comparison; would be like trying to do load development from the standing position. Was the 4 moa group due to MMS or the accuracy node?