Me and a buddy were discussing extrapolating good vs bad velocity SD based on target impacts at distance. That brought us to the question of whether there is an approximate minimal distance at which you could shoot in order to infer whether a load will shoot at longer distances. I.e., if my load holds about 1” of vertical at 500 yards, is it safe to assume it will stay together at 1000? (Assuming no errant shooter input) I think 400 is too close to test. Is 500 far enough? Thoughts?
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Extrapolating SD
- Thread starter Dot3
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Do your load development at 100 yards assuming you have a good chronograph. Look for SDs in the single digits. At 400 and 500 yards atmospherics will hinder your efforts. If you can estimate your average SD for your load, you can then plug those numbers into your ballistic calculator and get a guess at your verticle spread for longer ranges.
very important: make sure your bullet, velocity, and barrel twist are suitable to stabilize your bullet for your intended max range (looks like you said 1000 yards). Use this Berger calculator. Preferably, the SG should be over 1.5
Twist Rate Stability Calculator
Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.
bergerbullets.com
“if my load holds about 1” of vertical at 500 yards”. If your loads HOLD 1” vertical at 500 yards you should enter some national level F-Class matches ASAP.
My example was only an example but I never thought of 1” of vertical as being superstar status at 500.
Good point about SG. I’m at sea level and my load’s SG is in the high 1.20s most of the time. Haven’t had any stability issues to 1k that I’m aware of. No keyholes or anything. But could cause a group to open up…?. BC variation is also a factor in whether a load will shoot at distance, regardless of how it shoots at 3-500
Good point about SG. I’m at sea level and my load’s SG is in the high 1.20s most of the time. Haven’t had any stability issues to 1k that I’m aware of. No keyholes or anything. But could cause a group to open up…?. BC variation is also a factor in whether a load will shoot at distance, regardless of how it shoots at 3-500
As far as velocity and such is concerned, the bullet can’t deviate from its initial vector (I’m excluding BC variation and environmentals).
So, if you develop a load @ 100yds and you know it shoots .5 moa……it can never shoot smaller than .5moa. So, an sd/es of zero would shot .5 moa out to infinity (just using infinity as an example).
Now, as long as your velocity is tight enough to shoot .5 moa it will continue to do this.
Now. Let’s say you know your sd is 5. And your avg velocity is 2850fps
68% of your shots will be +/- 5 fps from your average. 2845-2855
95% of your shots will be +/- 10fps from your average. 2840-2860
99.7% of your shots will be +/- 15fps from your average. 2835-2865
So, in theory, your shots should all be within that last range with an ES of 30.
Plug those numbers in your calculator and move distances until it’s .5 moa. Once the distance increases enough that it’s over .5moa, you can no longer use the group size you shot @ 100yds as your standard.
So, to do what you’re asking, you’ll want to get close to the distance where the SD takes you over the precision capability of you and your rifle, and then shorten the distance a bit to give you room for shooter error.
If your groups are now larger than that .5 and you’re making good shots, as well as monitoring the velocity to make sure it stays inside the prediction of the SD…….that means you have a bullet to bullet BC variance that’s dropping you outside of the precision capability.
Again, this is making a lot of assumptions. Such as no lighting or vertical dispersion from wind……no positive compensation if you’re a proponent and/or use that method, etc etc etc.
So, if you develop a load @ 100yds and you know it shoots .5 moa……it can never shoot smaller than .5moa. So, an sd/es of zero would shot .5 moa out to infinity (just using infinity as an example).
Now, as long as your velocity is tight enough to shoot .5 moa it will continue to do this.
Now. Let’s say you know your sd is 5. And your avg velocity is 2850fps
68% of your shots will be +/- 5 fps from your average. 2845-2855
95% of your shots will be +/- 10fps from your average. 2840-2860
99.7% of your shots will be +/- 15fps from your average. 2835-2865
So, in theory, your shots should all be within that last range with an ES of 30.
Plug those numbers in your calculator and move distances until it’s .5 moa. Once the distance increases enough that it’s over .5moa, you can no longer use the group size you shot @ 100yds as your standard.
So, to do what you’re asking, you’ll want to get close to the distance where the SD takes you over the precision capability of you and your rifle, and then shorten the distance a bit to give you room for shooter error.
If your groups are now larger than that .5 and you’re making good shots, as well as monitoring the velocity to make sure it stays inside the prediction of the SD…….that means you have a bullet to bullet BC variance that’s dropping you outside of the precision capability.
Again, this is making a lot of assumptions. Such as no lighting or vertical dispersion from wind……no positive compensation if you’re a proponent and/or use that method, etc etc etc.
If you provide me with actual numbers (avg velocity, SD, precision capability), be it real or theoretical, I can give you ranges in which should work as well as considerations for other factors.
ok so a 30fps swing gets me to ~800 before the velocity variation takes me beyond the theoretical .5 moa mechanical capabilities of my system. Just like you, I’m relying on a whole lot of assumptions for this theoretical “test” in order to extrapolate what my SD might be. So, to be clear, I have an old chrony alpha master from the early 2000s. I don’t necessarily trust its margin of error or accuracy. I’m new to the long range game but have subscribed to the method of finding a load that shoots at 100 then going straight to long distance to see how it holds up. Therefore, I don’t have any real ideas what my SD actually is because I’ve only used the chrony to be sure I’m within expected pressure limits. But I see your point that monitoring velocity with every shot and mapping where it lands is valuable in determining how much shooter input is involved, because the bullet won’t break the laws of physics.
The catch is, relying on the assumption that the shooter will give no input is faulty…at 800 yards, MY abilities fall well below the abilities of my rifle.
But for the sake of curiosity, what can be done with an avg velocity of 2850 and SD of 5, with .5 moa precision capabilities?
“But for the sake of curiosity, what can be done with an avg velocity of 2850 and SD of 5, with .5 moa precision capabilities”?
if your final load settles in at 5 SD and you can consistently shoot 0.5 MOA, then I think you are good to go. Now focus on wind reading skills.
Much depends on your goals. Are you shooting PRS? I don’t do this, but many on this forum do and I would expect them to advise practicing on different shooting platforms and situations.
I shoot F-Class and the rabbit hole going from 0.5 to 0.2 MOA is expensive and deep.
if your final load settles in at 5 SD and you can consistently shoot 0.5 MOA, then I think you are good to go. Now focus on wind reading skills.
Much depends on your goals. Are you shooting PRS? I don’t do this, but many on this forum do and I would expect them to advise practicing on different shooting platforms and situations.
I shoot F-Class and the rabbit hole going from 0.5 to 0.2 MOA is expensive and deep.
So this is relatively theoretical, however, it is a great representation of the variation muzzle velocity versus rifle accuracy.
First two images are 0.5 MOA with 5fps SD and 20 fps SD, the last two are 1 MOA and with 5fps SD and 20fps SD, respectfully.
First two images are 0.5 MOA with 5fps SD and 20 fps SD, the last two are 1 MOA and with 5fps SD and 20fps SD, respectfully.
I have no idea where my SD has settled in at due to not using the chrony. The wind is absolutely my weakest point no doubt about it. Doesn’t matter how good the vertical is if I can’t manage the horizontal. I will be shooting my first PRS match this weekend and fully recognize that my load development will NOT be my limiting factor.
This idea came about when my buddy was shooting a new load that seemed to print fairly tight at 400 yards but was ALL OVER the place once he got to 7-800. I wasn’t shooting so I can’t say what other factors were at play. It’s the epitome of a camp fire discussion over bourbon.
Good luck and just have fun.
BTW, was just watching this and thought you might like it. I know it’s a bit off subject. This guy is one of the top US F-Class shooters and one of the best at coaching wind shooting.
That’s where the monitoring as much as you can comes in.
The more variables you monitor, the easier it is to narrow down on the culprit.
Generally speaking, shooting well at 400 and not at 800 is shooter induced. It also depends on what “all over the place” means. I’ve seen someone consider shooting 1” groups @ 400, and then 6” @ 800 horrible. When it was just two groups that were well within the “noise.”
But, without watching the shooters fundamentals, knowing the load data and such, etc etc etc…….it means you have a whole host of possible issues. And it’s likely a combination of things.
I like how you gave two options: buy a good chrono, OR work more and buy a good chrono. Haha. It’s on the list. But it’s a long list.
As for my buddy, no comment other than he’s generally been repeatable in his process. And by all over the place I mean, missing a 12” plate by a good margin. Vertically. Rear bag issues? Maybe. But why not at 400 or whatever it was? Who knows.
@Marine52 thanks for the video. In looking for ways to not be dominated by mirage, I did not look for the keyword “blob.” Thank you
You’d have to have a pretty shitty velocity variance to miss ~ 1.5moa @ 7-800yds.
You can still hold like 1.5moa of vertical @ 1k with a 50es.
And the bullet isn’t going to be stable to 400 and then lose stability by 800.
