Are you one of those guys who has been reading this series of posts on bullet jump, and thinking to yourself, “Well, my 0.020” bullet jump sure seems to be working fine. Doubt this would be any improvement over what I’ve already got!” This is the post for you!
As Mark started sharing some of his bullet jump findings
with a few shooters, he met some skepticism – even from sponsored shooters on his
Short Action Customs team. Here is how Mark tells one of those stories:
“After we’d already done most of this bullet research, we had Solomon from our shooting team fly out for a local match, and he helped with some bullet jump testing. Solomon was dead set on Berger 105’s performing best when jumping 0.020″. So, we shot his load over our electronic target system at 600 yards, and it did shoot extremely well. I struggled to convince Solomon that there might be a better jump, especially with how good the 0.020” was already shooting for him. So, I was forced to dig a little deeper and formulate a test that would compare the bullet jumps in a ‘realistic’ range of jumps someone might experience as their barrel wears during a match and/or the jump tolerance they might experience with factory ammo.
I ended up creating a simple test where we shot three groups at 600 yards. Each group consisted of 18 total shots fired: 6 shots with a 0.015″ jump, 6 shots at 0.020″ jump, and 6 shots with a 0.025″ jump, shot in a round-robin fashion. Then we cleaned the bore, fouled the barrel with a couple of shots, and performed an 18-shot test for jumps at 0.050, 0.055, and 0.060 inches. Finally, we repeated the full cycle and finished with an 18-shot test for jumps at 0.075, 0.080, and 0.085 inches.”
Mark’s 18-Shot Jump Test
Method
Mark said this was originally a 15-shot challenge, where they shot 5 shots at each jump. But they eventually changed it to an 18-shot challenge by adding a 6th shot to each group because a slightly larger sample size was helpful to prevent false-positives and ensure they could have full confidence in the results.
For each jump, you’ll test a 0.010” wide window of bullet jumps. 0.010” is about much the lands of a barrel would likely erode over 200 rounds for most mid-sized cartridges that are common in precision rifle competitions (like Creedmoor, Dasher, or x47 Lapua cartridges). So unless you are going to adjust your seating depth every 100 rounds or less, you likely need to find a range of jumps that is at least that wide and provides a similar vertical point of impact (POI) across the entire range. That is what this challenge is designed to inform you about.
There are 18 shots per group, and you’ll end up firing 3
groups – for a total of 54 shots. But, at the end of those 54 shots, Mark feels
like you’ll have hard data on whether your shorter jump is really better in
realistic scenarios. Here are the groups and number of shots Mark recommends:
- Group #1: 0.015-0.025” Bullet Jumps
- 6 shots at 0.015”
- 6 shots at 0.020”
- 6 shots at 0.025”
- Group #2: 0.050-0.060” Bullet Jumps
- 6 shots at 0.050”
- 6 shots at 0.055”
- 6 shots at 0.060”
- Group #3: 0.075-0.085” Bullet Jumps
- 6 shots at 0.075”
- 6 shots at 0.080”
- 6 shots at 0.085”
Of course, if you’re current load uses a 0.010” jump, you
could change Group #1 to be 0.005-0.015” or 0.010-0.020” to test how “durable”
or consistent your current load is a 0.010” range of bullet jumps that you will
likely experience, if you don’t adjust seating depth every 100 rounds or less.
Within each group, Mark fires the shots round-robin style.
For example, in Group #1, they’d fire one shot with a 0.015” jump, then one
shot with a 0.020” jump, then one shot at 0.025” – then they’d repeat until all
6 shots are fired at each bullet jump. Then they’d clean the barrel, foul the
bore with a couple of shots, and repeat the tests for Group #2, and so on.
Two important notes before you do this:
- You must carefully measure the distance to the lands on your barrel with an accurate and repeatable method. How most shooters do that is not precise enough to be within +/- 0.010” – I know that was true for me. Click here to learn about the only two methods I know of that are repeatable to +/- 0.002” or less. If you don’t do this, you likely won’t be testing what you think you are.
- Adjusting your seating depth will affect your chamber pressure, so, as always, you should be careful when changing a load and watch for signs of excessive pressure. The Sierra Reloading Manual says that adjusting seating depth to match your rifle’s throat/freebore and maximize accuracy “is fine, but bear in mind that deeper seating reduces the capacity of the case, which in turn raises pressures. Going the other way, seating a bullet out to the point that it actually jams into the rifling will also raise pressures.”
Results from A Few 18-Shot Challenges
We wanted to share a few of the results from these types of challenges
that Mark has already collected using his electronic
target system at 600 yards.
6×47 Lapua with Berger 105 Hybrid
Below are the 3 targets from this challenge with an 18-shot group on each one. I added a callout beneath each target for the vertical extreme spread that was measured for each group by the electronic target system at 600 yards (also highlighted in the yellow box on the readout).
The Problem with Extreme Spread
While measuring the extreme spread of a group is very easy
to do and useful, one of the problems of only looking at extreme spread of the
group is that it really only takes two shots into consideration – namely the
two that are furthest apart. Where the other 16 shots hit in no way affects
the extreme spread, which means we’re ignoring 88% of the shots fired! For
example, if you look at the target above on the left you can see shots are evenly
distributed vertically. However, the target in the middle has most shots in a narrow
vertical window, except for one outlier, namely shot #1. In fact, if you excluded
shot #1 from the middle target the extreme spread would drop from 3.9” to just
2.1”. Now cherry-picking data to exclude is the exact opposite of good
science, so I’m certainly not suggesting we do that. But it goes to show how
drastically an extreme spread can change based on the placement of a single
shot. It simply doesn’t take advantage of the full sample size.
Bryan Litz discussed this topic at length in Modern Advancements for Long Range Shooting
Volume II. Bryan carefully explains this in Chapter 1 of that book, but
here an excerpt with his recommendation:
“Measuring the extreme spread of shot groups is quick and easy, but it’s not actually a very good measure of dispersion. What do I mean by a good measure? A good measure should give you useful information, which is information you can use to make good decisions. When you look at the extreme spread of a 5-shot group, that measurement is determined by only 2 out of the 5 shots. In other words, only 40% of the shots are considered in the measurement. Even worse, for a 10-shot group, a center-to-center measurement is only using information from 20% of the total shots. Since the extreme spread, center-to-center measurement, is determined by only a small portion of the total shots available, it’s just sort of an indicator of precision. There are many alternative measures of precision. You could measure the location of each shot and calculate vertical and horizontal standard deviation (SD), radial standard deviation (RSD), circular error probable (CEP), etc. You could get really carried away with statistical methods of characterizing precision. Taking a step back and considering options for measuring dispersion; we want something more descriptive than extreme spread, but don’t want to go crazy with statistics. It’s my opinion that the mean radius of a shot group is a well-suited measurement for this task. Mean radius, also known as average to center is self-explanatory; it’s the average distance from each shot to the center of the group.” – Bryan Litz
Average distance to center (ATC) takes all the shots into consideration, and therefore is more representative of the entire group and not simply the two shots that fell furthest apart. It’s not that extreme spread isn’t useful. There are shortcomings to ATC as well. You could have two groups with the same ATC, but one has most shots tight and one flier way out of the group and the other group is more well-rounded without a flyer, but the primary shot cluster is slightly larger. In this case, the ATC was identical, but the ES was different. So, I’m certainly not claiming ES is irrelevant or not useful. While ATC does include all shots fired in the calculation, maybe we should analyze both ATC and ES when looking at groups to keep a balanced perspective.
ATC is more difficult to measure and calculate by hand, but software like the OnTarget Shooting app can analyze your target and provide ATC and ES for you, along with other stats.
The chart below shows the equivalent data for vertical extreme spread taken from the targets above and converted to Minute of Angle (MOA), instead of inches at 600 yards. It also shows the average distance to center (ATC) that Bryan suggested as a more complete or representative measure of the dispersion of a group.
You can see 0.050-0.060” bullet jumps resulted in the
smallest extreme spread. In fact, the ES for the shorter jump (0.015-0.025”)
turned out to be 48% larger, and the average distance to center between those
is 100% larger! The 0.050-0.060” bullet jumps clearly produced the most consistent
vertical at long range in this test.
6BRA with Berger 105 Hybrid
Mark also ran this test with a rifle chambered in 6mm BR
Ackley Improved (a.k.a. 6BRA) firing Berger 105 gr. Hybrid bullets at the same
jump ranges: 0.015-0.025, 0.050-0.060, and 0.075-0.085. Let’s take a look at
those results:
You can see the group with the largest jump (0.075-0.085”) produced the smallest extreme spread. The shortest jump had the largest ES at 0.84 MOA, which is 47% larger than the 0.57 MOA ES achieved with the 0.075-0.085” jumps. When it comes to ATC, all three groups were very similar. If the ATC is similar but the ES is not that means there must have been a few significant fliers in the shorter jumps, but the other shots must have clustered reasonably close together to still achieve a low ATC. On the other hand, the lower ES of the farthest jumps means it didn’t have fliers as significant, but the main cluster must have been slightly larger for the ATC to end up the same. Overall, the load with the 0.075-0.085” seems to be the best overall performer for this rifle.
6×47 Lapua with Berger 115 DTAC
Finally, Mark did a similar 18-shot challenge at 3 different
bullet jump ranges with David
Tubb’s 115 gr. DTAC RBT Closed Nose bullet. For the middle range on this challenge,
Mark chose to test 0.030-0.040” instead of 0.050-0.060” that was used on the
prior two tests. Here are the results:
On this one, it looks like the shortest jump actually had
the smallest extreme spread – although it was only by 0.01 MOA! Obviously that
middle bullet jump range of 0.030-0.040” had a very similar extreme
spread, but it also had the lowest ATC of any of the groups. So, if I were
trying to pick between these jumps, I’d go with the middle group because the
ATC is 13% smaller than the 0.015-0.025” results, even though the ES is 2%
bigger. Once again, the Average Distance To Center (ATC) takes all 18 shots
into consideration, while the Extreme Spread (ES) only takes 2 of the 18 shots
into consideration when doing the calculation. So, if you’re going to weight
one metric more than the other, I’d suggest ATC.
I do have to wonder if this test for the 115 gr. DTAC would
have favored a 0.040-0.050” jump range, if they’d have tried that for the
middle range. I went back to the bullet
jump research data for the 115 DTAC, and the chart below shows where the
sweet spots were over the 6 rifles Mark collected data over:
You can see that 0.030-0.040” had an average vertical spread of 0.62 MOA over the 6 rifles that are represented in the data above, which is EXACTLY the same as the results in this 18-shot challenge. Isn’t that crazy?! The average over the 18 shots fired here with one rifle and the average over the 6 rifles tested with 3 shots in that range each, which we covered in the previous post, both ended up being EXACTLY 0.62 MOA! While the fact that they are identical to the hundredths place is likely a coincidence, it gives me a little more confidence in the how repeatable and reliable this data is. You can see above that it was 0.040-0.050” that produced the smallest vertical ES at just 0.31 MOA at 600 yards. I wonder if that would have ended up being the winner if they’d done an 18-shot challenge for that range of jumps instead. Of course, they may not have had all that data when they originally did this 18-shot challenge, since this testing has been a progressive thing over more than two years. If I were using 115 DTAC’s, I might try out that 0.040-0.050” range of bullet jumps.
The Challenge & Up
Next
If you’re one of those guys who already has a great load, but it uses 0.020” of jump or less, Mark wants to challenge you to go run this test for yourself. After 54 rounds, he bets you might find something that works better than what you’ve been using. That has been the case each time he’s ran through these tests with other shooters! With COVID-19 stay-at-home orders in most places and rifle matches canceled, maybe it is a good time to try to tune on your load to be ready when things start back up!
Up next, I’ll wrap up this whole series and offer a few suggestions on how to integrate bullet jump into your load development process. While the 18-shot challenge is a good way to test a couple of further bullet jumps against your current load, what is the best way to go about this to develop a load from scratch? If we now have another variable we need to tune in load development, how do I find a good load without having to shoot 100+ rounds doing load development? Do I have to test every 0.005” to find the best bullet jump? What order should I do this in, powder charge or bullet jump first? Great questions! In the next post, I’ll share some personal suggestions and advice from other top shooters in the next post to try to help you get to a good load in the fewest rounds possible.
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Source
Mark’s 18-Shot Bullet Jump Challenge! is written by Cal for precisionrifleblog.com