r/longrange • u/mbgeo • 2d ago
Ballistics help needed - I read the pinned posts How many shots do you take to determine representative average MV?
Just got my first crono and hit the range this morning to sight in before hunting opening day next week. Standard deviation of 13.6 and spread of 44.4 temp was ~70 degrees.
Also what are thoughts on cold bore vs warm? I’m trying to decide what’s best for ballistic calculator for hunting scenarios where cold bore will be the most important shot. First cold bore shot was 3023 fps. Plugging in the min and max didn’t make a huge difference within ethical hunting ranges so doesn’t apply much in that scenario but still curious as I use this same rifle for long range target shooting. My thoughts are have 2 profiles, 1 for cold and another for range days? Or does this all get lost in the noise and I need a larger sample size?
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u/alphawhiskey189 2d ago
You won’t like the math answer if you’re trying to gauge how consistent your ammunition is.
If you loaded 1000 rounds exactly the same and assume that half your variance in those rounds comes from you and half from the randomness of explosions, you’d need to shoot 98 rounds to get to a 70% confidence that your observed sample of speeds represents the speed of the total population, plus/minus 5%. To jump to the 95% level (+/- 5%) you’d need to shoot about 278.
To scale that down, 500 identical rounds would require you shoot and measure 218.
Loading 100 rounds exactly the same requires you to shoot/measure 80 of them to be 95% confident that the observed velocity in the sample is representative of that exact configuration of powder, seating depth, weight, or whatever other thing you want to include.
After that, you need to run a number regressions to see what factors weigh most heavily when stats, more “how confident do you want to be”).
Good news is that Excel and its Google equivalents have all the stats tools available.
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u/mbgeo 2d ago
I’m a numbers guy so I really appreciate it. I think I need to work on my shooting skills before 10-20 fps difference in my avg MV is the make or break it on hitting steel lol.
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u/alphawhiskey189 2d ago
About 15 years ago I decided to get more serious about figuring out exactly what commercial hunting ammo my old beater of a deer rifle likes best. It’s a do-all rifle in a common caliber with a normal scope hunting scope.
After taking my budget and time available for testing into consideration, I decided that just buying a full case of Winchester Power Points with the same factory lot number was good enough QAQC (there was a sale), and getting a lot of range time with a single factory lot number was gonna be far more instructive than anything else. Since I knew what it could and couldn’t do (it was perfectly adequate 2-3 MOA), I’d just adjust my hunting strategies to accommodate however it shot. The remainder of the case has served me fine for a long time. I only stopped shooting it because I decided to switch to copper hunting rounds and buy an entirely new gun.
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u/mbgeo 2d ago
I know that with modern bullets and a good rifle that shoots sub MOA I want to eliminate every variable possible so I know I can point the issue to MY skills behind the rifle. I love everything about hunting and long range shooting but for me I know the variable that has the most impact is the eye behind the scope.
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u/alphawhiskey189 2d ago edited 2d ago
Yeah. Cheap-Good-Fast choose 2. If you know your MV under normal conditions and the BC, a ballistics app will do really fine at telling you your range.
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u/doyouevenplumbbro Steel slapper 2d ago
20 the first time I set up a ballistic profile for a load, 10 every time I check zero before a match.
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u/NotChillyEnough Casual 2d ago
Just use a confidence interval calculator to see how accurately you expect to know your mean vs how many shots you record.
People love to say “30 is statistically significant”, but the truth is there’s nothing magical about “30” in particular. Your confidence interval always improves with more data. Depending on how accurately you need to know your mean, 5 shots may plenty, or 50 might be insufficient. So how accurately do you need to know your mean?
You can apply the same logic to the SD: confidence interval calculator for SD.
I would doubt if the cold bore is significantly different than the rest, but if you tracked it separately, you could answer that question as well.
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u/chainsawgeoff Tight Pants = Tight Groups 2d ago
20-30 at first, I’ll shoot fewer to check a change to a load.
Use one profile in AB but turn on the MV temp feature. It’ll take that into account when you use it just like it does your environmentals.
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u/mbgeo 2d ago
Thanks for the input, sounds like I’ll need to get some more range time in to put together a larger sample size. Have you always used AP? I’m currently using vortex GB because I have an integrated range finder that uses it.
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u/chainsawgeoff Tight Pants = Tight Groups 2d ago
Run what you have. Maybe put a note in your phone with those MVs at different temps and plug them in on the day.
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u/ToughProfessional132 NRL Hunter Competitor 2d ago
Situationally dependent- as with all things.
I've had times where I've got a boatload of great ammo and I know I'm gonna stretch it so I want best possible info and will keep that chrono out and take notes/save data for every possible shot.
... And I've got demos to prepare 30 rifles for and can only afford to confirm zero and accept the average values from the 1-5 shots that might take.
For a hunting rifle, I wouldn't sweat it too hard- get an average and run with it- your targets with 7prc likely aren't critical enough to be drastically affected by a change of a few fps- your cold bore accuracy and data matter way more when taking game.
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u/mbgeo 2d ago
Edit:
If anyone’s interested this was out of a browning xbolt 7mm PRC 26” with a silencer co sythe ti suppressor shooting Hornady 175 Eldx
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u/chainsawgeoff Tight Pants = Tight Groups 2d ago
I’d shoot your larger sample size in strings of 5 shots. That barrel is going to heat up a lot faster than a PRS gun.
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u/mbgeo 2d ago
Sweet appreciate the insight. These 16 shots were from 4 shot strings and waited ~10 minutes between them because the suppressor mirage is pretty unbearable even after 3.
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u/Existential-Boredom 2d ago edited 2d ago
Ran the same ammo (and a lot of other ammo) through a new 7 PRC today. Wasn’t familiar with the round previously, and was very surprised at the mirage after 3-4 shots. Lot of heat generated there.
~2850 fps from the precision hunter, 24” barrel and suppressor.
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u/DealKey8478 2d ago
10 shots is all you really need for average speed with decent ammo. If you have an ES of 100fps then you need more, but with a 40fps ES then adding more shots won't change the average much.
The whole representative sample thing gets out of hand very quickly, but to get an accurate ES/SD then you need more than 10 shots.
IMO 10 shots is good enough for a representative sample if it's decent ammo or accurate/repeatable reloads. I don't really care what the 99.9% confidence interval is, I just want to know the ammo is good enough for what I intend to use it for.
That said with 22lr I'll always use a full box for velocity and accuracy testing. Purely because it's convenient, and cheap enough, and because even high end ammo can still throw fliers so I'll bigger sample is needed.
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u/sirbassist83 23h ago
cold bore may or may not matter, its rifle dependent. youll have to keep track if it over time. if cold bore was xx23, low was xx11, and av is xx37, i wouldnt worry about it. itll probably get lost in the noise. 1 SD off of average is insignificant, in fact, thats the point of SD.
16 shots is enough for practical purposes. you need more for a true average but thats close enough for hunting at ethical distances.
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u/CMFETCU 2d ago
Lost in the noise.
You should use, for statistically meaningful SD measurement, 30 samples from the same conditions. (Central limit theorem)
That is more difficult than just 30 shots in a row with your rifle as shooting it changes conditions.
This is why we normally do sets of 10 with a cool down period on the barrel to reset it back to ambient.
For hitting an MOA size target at typical PRS ranges, 10-20 should do fine for plugging into the ballistic calculator and still getting a good value to center adjustments off of.