Simple question that ballistic apps can't answer -- I must be missing something.

Jump to latest Follow Reply
S

stovepipe

Member
May 13, 2024
240
227
Idaho
Hypothetically speaking, let's say I have an extreme spread of muzzle velocity of 30fps. Let's also say I'm sighted in at 50 yards. I want to know where the point of impact would be at 100 yards for the fastest shot and the slowest shot relative to the point of impact of a shot taken at the average velocity. I COULD just change the muzzle velocity in the app, but the result would not be mathematically correct because the app still thinks I'm sighted in at 50 yards and not missing at 50 yards -- but the fastest and slowest shots WOULD be missing at 50 yards and 100 yards.

Does anybody know how I could get a mathematically correct answer? Just an FYI -- I have strelok pro and the original hawke chairgun.

stovepipe
 
stovepipe said:
Hypothetically speaking, let's say I have an extreme spread of muzzle velocity of 30fps. Let's also say I'm sighted in at 50 yards. I want to know where the point of impact would be at 100 yards for the fastest shot and the slowest shot relative to the point of impact of a shot taken at the average velocity. I COULD just change the muzzle velocity in the app, but the result would not be mathematically correct because the app still thinks I'm sighted in at 50 yards and not missing at 50 yards -- but the fastest and slowest shots WOULD be missing at 50 yards and 100 yards.

Does anybody know how I could get a mathematically correct answer? Just an FYI -- I have strelok pro and the original hawke chairgun.

stovepipe
Click to expand...

A mathematically correct answer would depend on which mathematical model you use for how your projectile travels. The simplest model is that it travels on a parabola. That's pretty easy to work out.

But most ballistic apps take some amount of wind friction into account, and then do you account for humidity, altitude, cross-winds, projectile weight, BC, twist rate, etc? You'll mathematically get a wide range of predictions depending on which model you use.
 
  • Like
Reactions: Bandito, alejo, beerthief and 1 other person
Upvote 0
Unless your shooting 600 fps, the 30 fps spread _likley_ wont affect your POI that much.

YEs the program bases everything off your 50 yard zero (using an average of FPS derived from some number of shots...). So put the highest FPS you see in your app, and see what the POA for 50 and 100 is. then put the slower number in, and check POA at 50 and 100 again. Id bet they are close to identical. If not identical then split the difference and use that as your average FPS

IMO you'll never get an exact prediction...... but you should be able to get close enough to make hits (all other factors being equal)
 
Last edited:
  • Like
Reactions: Bandito and Treefrog
Upvote 0
What no ballistic calculator will give you is the effect of barrel harmonics on the POI. I found with certain rifles shooting certain pellets at certain speeds, if I lower the speed the POI would be higher and vice-versa. Lower it further and the POI would go down and lower the speed even more and the POI will go up again. The is all because of the change in barrel harmonics. That is to a point where the POI will drop a lot because of the low speed. And that is only about 20 - 30 fps difference in each setting. So I believe that tuning has a lot to do with barrel harmonics and finding the sweet spot around its POI. My rifle with H&N 18gr like 940 - 950 fps and up to 970fps, at lower speeds the groups get bigger.
 
  • Like
Reactions: bigHUN and Arzrover
Upvote 0
dan_house said:
Unless your shooting 600 fps, the 30 fps spread _likley_ wont affect your POI that much.

YEs the program bases everything off your 50 yard zero (using an average of FPS derived from some number of shots...). So put the highest FPS you see in your app, and see what the POA for 50 and 100 is. then put the slower number in, and check POA at 50 and 100 again. Id bet they are close to identical. If not identical then split the difference and use that as your average FPS

IMO you'll never get an exact prediction...... but you should be able to get close enough to make hits (all other factors being equal)
Click to expand...
So I guess I didn't miss anything -- at least not with any ballistic app I've used. The apps I've used simply don't have a function to answer "extreme spread" questions.

stovepipe
 
Upvote 0
Your point of impact is not going to change very much at 50 yards from a 30 fps change in velocity. You may not be able to shoot a small enough group to even see the difference. You would want to use a chronograph as you sight in to even know what shots you are using to establish the sight in. That sight in process could even be another way to know how much the velocity spread affects the point of impact. Presumably the higher velocity shots will hit a little high and the lower a little low. But the random nature of our guns shooting could affect this. Still there should be a trend of high velocity shots hitting higher. The more accurate your gun and your shooting the easier it will be to see the affect of velocity. Once you know the effect of velocity you could mess with the sight in distance to give an equivalent difference in point of impact. Using that new assumed sight in distance you could calculate the drop at the range you want.

So I think you can estimate it from the software but I would be a lot more confident with some shooting results to make sure the effect of velocity on at least the zero distance.

I got curious so I fired up Chairgun using a file for my P35-22 which shoots H&N Baracuda Match pellets at a little over 800 fps. I modified the file to look at 830 versus 800 fps velocity. My normal zero on this rifle is 30 yards so I just left that alone and looked at how much the impact at 50 would change for the velocity change. It was .22 inch. Then I looked at what zero point would give me the same drop at 50 yards with the 800 velocity. It was almost the same (within 0.03 inches) as a 35 yard zero. Then I looked at drop at 100 yards for 800/35 (20.74 inches) versus 830/30 (19.41 inches). I also calculated the drop for 800 fps with a 30 yard zero - 21.11 inches.

What I think all of that means is you have to shoot a group less than .25 inches at 50 yards to establish the zero or you won't be able to see the difference in the point of impact at 50 yards for 30 fps velocity change. Not a lot of guns will do that. The impact on drop at 100 yards for my different assumptions is about 1.5 inches maximum. I think the best thing to do if this bothers you is to rezero at 100 yards. The difference at 100 might be big enough to see but my groups at 100 have all been over 2 inches.
 
Last edited:
  • Like
Reactions: Browser1
Upvote 0
Obviously, what you need is a point mass trajectory model based on elevation angle rather than zero distance. I have a feeling one of them does have the option to keep a constant angle, but I cannot remember which one. It may have been Chairgun or MERO, but again I cannot see it at the moment. I have such a program as it is the standard system when modelling artillery trajectories, but it is not commercially available and uses purpose drag laws rather than BC.

You could get a rough answer by looking at the different times of flight to the target distances and work out the difference in drop due to gravity. Use a slightly lower value for the acceleration due to gravity to allow for drag reducing the drop, say around 30 ft/sec/sec. It will not be exact, but it will give an approximation of the velocity effect.
 
  • Like
Reactions: Airgunmann
Upvote 0
In the Strelok chart, turn on the absolute drop column.

Set your velocity to whatever you want. Look at your 100yd row. Note the amount of drop. Then if you set it 30fps slower, you will show a greater drop. The difference in drop is what you’ll see at the 100yd target.

That assumes that there are no harmonics at play. It’s possible to harmonically tune some systems to mostly compensate for moderate velocity variations at a specific distance.

I try to build vey stiff systems so harmonics are not a big factor

1719969304272.png
 
  • Like
Reactions: dgeesaman, Centercut, Airgun Al and 2 others
Upvote 0
Scotchmo said:
In the Strelok chart, turn on the absolute drop column.

Set your velocity to whatever you want. Look at your 100yd row. Note the amount of drop. Then if you set it 30fps slower, you will show a greater drop. The difference in drop is what you’ll see at the 100yd target.

That assumes that there are no harmonics at play. It’s possible to harmonically tune some systems to mostly compensate for moderate velocity variations at a specific distance.

I try to build vey stiff systems so harmonics are not a big factor

View attachment 477079
Click to expand...
Scotchmo, this is what I was looking for! Thank you. It would have taken me a month or more to find it on my own.

stovepipe
 
Upvote 0
I may be wrong but you would better work on that ES 30 and SD a bit more.
The App may calculate from your input (with right numbers or rounding errors) and predicted the lowest and highest POI, but you still need to work out the barrel harmonics.

No other way around but shoot a five to ten tins of chosen projectile (alone) and document to yourself, making a spreadsheet would help.

I can just assure you that at 100 Meters (50 is not enough to read the holes) if you shoot let say 10x5 groups...
and if you start with a speed of 880 (AV=average) and with each ring incrementally add a speed of 5 fps all the way up to let say 910... (btw this is your ES of 30 range you mentioned earlier)...
you will realize the pattern is closing at 885 and next is opening up at 895 and next is tightening again at 907 and the next is loosening up at per say 915 and so on... these numbers take as fictional only but your experience will be similar in nature. These phenomena is coming from the barrel amplitude of waves...
Ah, and don't forget about at least five "blank" shots in between shooting the groups, these are to cycle the springs and Reg - preparing for next groups, you can just dispose these in a backstop only.
So, prepare couple tins in front of you, a radar, pen and a paper, and go have some fun ;)
 
  • Like
Reactions: Bandito and Browser1
Upvote 0
bigHUN said:
I may be wrong but you would better work on that ES 30 and SD a bit more.
The App may calculate from your input (with right numbers or rounding errors) and predicted the lowest and highest POI, but you still need to work out the barrel harmonics.

No other way around but shoot a five to ten tins of chosen projectile (alone) and document to yourself, making a spreadsheet would help.

I can just assure you that at 100 Meters (50 is not enough to read the holes) if you shoot let say 10x5 groups...
and if you start with a speed of 880 and with each ring incrementally add a speed of 5 fps all the way up to let say 910... (btw this is your ES of 30)...
you will realize the pattern is closing at 885 and next is opening up at 895 and next is tightening again at 907 and the next is loosening up at per say 915 and so on... these numbers take as fictional only but your experience will be similar in nature. These phenomena is coming from the barrel amplitude of waves...
Ah, and don't forget about at least five "blank" shots in between shooting the groups, these are to cycle the springs and Reg - preparing for next groups, you can just dispose these in a backstop only.
So, prepare couple tins in front of you, a radar, pen and a paper, and go have some fun ;)
Click to expand...
You're right that I need to figure out the ES problem. It's actually more like an ES of 40fps and a bad SD as well. It could be a loose jam nut or a sticky valve or it just doesn't like the slugs I'm shooting or the speed I'm shooting them at. Maybe the cheap chronograph is just lying to me. Maybe I just need to clean the barrel really well. I'll figure out the ES problem. But the problem did make me wonder just how much that spread would affect groups at various distances and that's what I wanted the ballistic app to tell me, which it has now. Anyway, I'm not going to the range to shoot until I get the ES under control.

stovepipe
 
Upvote 0
stovepipe said:
You're right that I need to figure out the ES problem. It's actually more like an ES of 40fps and a bad SD as well....
Click to expand...
The large ES/SD is usually an indication that your muzzle blast is polluted greatly and that will effect the POI. Again if you versed enough you may be able to read the tune errors @ 50 but @ 100 will show amplified. A good start is to "rest" the gun steady on table to minimize the human errors and go from there with tuning first.
You better get that ES first below 10 for peace in mind and from there you tune the group sizes.
 
Last edited:
Upvote 0
recent chrony session showed a 50fps increase over the lst 6 weeks or so.... dont nknow why...
so I can speak to this a little better.....

comparing the "old" numbers to the most recent ones..... its really only at the extrreme ranges 10, 11, 12 yards. and 45 -55 that had any changes in hold, even then it was small.

so it does matter, but not much. Im only reaching out to 55 yards.
 
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu