This is the 4^{th} article in a series on some new research on bullet jumps. Here is a recap of the previous articles in the series:
- Bullet Jump & Seating Depth: Best Practices & Conventional Wisdom
This article provides a comprehensive overview of what most professionally published books and reloading manuals suggest about bullet jump and seating depth when it comes to precision rifles. It also lays a foundation of what we mean by bullet jump, freebore, etc. - How Fast Does A Barrel Erode?
This article focuses on how quickly the lands of a rifle barrel usually erode, especially for mid-sized cartridges that are popular in precision rifle matches. It also covers how many shooters manage seating depth and bullet jump over the life of the barrel. - Bullet Jump: Is Less Always Better?
This article introduced some interesting, new research conducted by Mark Gordon of Short Action Customs on what the most forgiving range of bullet jumps are for the Berger 105 gr. Hybrid bullet over 10 different rifle/load configurations. It also provides an overview of the test methods and how to interpret the results.
In this post, I’ll share more research data Mark has collected related to bullet jumps for two popular bullets used for long range:
- 6.5mm Hornady 147 gr. ELD-M
- 6mm Tubb 115 gr. DTAC RBT
If you missed the previous post, I’d strongly recommend you start there – or this simply won’t make sense. I carefully explained the testing methods and data/charts in the last post and won’t be repeating that info again here.
Fundamentally, this research wasn’t focused on finding the specific bullet jump that provides the absolute best precision (i.e. smallest possible group), but was focused on finding the best precision over a range bullet jumps. We are looking for the window of bullet jumps that would continue to have a similar point of impact over 200+ rounds, without having to adjust the seating depth as the lands of the barrel erode. That means the rifle would be more consistent from the start of the match to the end of it or could shoot a particular kind of match-grade factory ammo really well for a longer period of time.
Here is a visual that illustrates what we’re looking for, which is the big green area that provides both very good precision AND is also very forgiving in terms of bullet jump/seating depth.
This research isn’t saying you can’t get good groups close the lands – many obviously do! But, could we get really good groups AND not have to frequently adjust the seating depth of our bullet? That’s the question this research is exploring.
6.5mm Hornady 147 gr.
ELD-M
The last post focused on Berger’s 6mm 105 Hybrid, but Mark has also compiled quite a bit of data on the 6.5mm Hornady 147 gr. ELD Match bullet. Before we dive into the complete results for that bullet, I’d like to start by looking at a unique aspect of the bullet jump tests Mark conducted with the Hornady 147 gr. ELD-M.
3 Loads, 3 Shot Orders: 1 Rifle
A portion of how Mark tested the 147gr ELDM was in three different ways using the same exact rifle. The custom rifle he used for these three tests was a 6.5 Creedmoor in a Defiance Elite action in a McRee chassis with a 26” Bartlein Heavy Palma contour 1:8” twist barrel and a TBAC Ultra-7 suppressor. They ran through the 20-shot bullet jump test, from 0.000″ jump (a.k.a. “kissing the lands”) to a 0.095″ bullet jump in 0.005″ increments, with that one rifle and varied:
- 3 different powder loads (41.0, 41.6, and 42.3 grains of H4350, all in Lapua small primer cases with Federal 205 Match primers)
- 3 different shot orders (shortest jump to longest, completely random, longest jump to shortest)
- 3 different shooters (Matt Stiner, Aaron Hipp, Mark Gordon)
So this was basically side-by-side testing for the same rifle and bullet over a 60-shot sample size and with some variation introduced. Mark obviously shoots a lot, but the other two shooters are accomplished PRS/NRL competitors who regularly place in the top 10 at major matches.
- Matt Stiner fired the 20-shot test with 41.6 gr. of powder in conventional order, starting with 0.000” bullet jump and firing sequentially up to 0.095”.
- Aaron Hipp fired the 20-shot test with 41.0 gr. of powder in a completely random order.
- Mark Gordon fired the 20-shot test with 42.3 gr. of powder in reverse order, starting at 0.095” of jump and ending with 0.000”.
Let’s look at the results for the vertical POI shift each of them recorded on Mark’s electronic target system at 600 yards based on the distance the 147 gr. ELDM was jumping to the lands of the barrel. The chart below shows the combined data for all three tests.
If this is the first
time you’re seeing a chart like this, I’d suggest going back to read the previous article
to understand what it represents.
Remember, on this chart the flatter the line the better. If a line is steep, that means there was significant vertical Point of Impact (POI) shift over a narrow range of bullet jumps (i.e. the bullet seems to be very sensitive to seating depth near that bullet jump). Mid-size cartridges like this can erode by 0.005” in just 100 rounds (read more on that), so where the lines are steep means you could experience measurable vertical stringing in 100 rounds or less. However, if we can find a section where the lines are flatter, that means the vertical POI didn’t change significantly over that range of bullet jumps. For example, look at the area around 0.055-0.065”. Even with different loads, different order of shots, and different shooters, all 3 tests show very little vertical POI shift over that window of bullet jumps.
You can see that regardless of the shooter, order the shots were fired in, or the specific load used, there appears to be a sweet spot around 0.060” of bullet jump, because all of the lines on the chart appear to flatten out near that area. Also, we can see on the chart above that there doesn’t appear to be anything less than 0.040” of jump that seems to be forgiving, and 0.050-0.070” appears to be best for the rifle that was tested … regardless of shooter or specific load.
And while each of the 3 tests only included 1 shot for each of the bullet jumps, it is interesting to see the commonalities in the patterns. There seems to be very few places on the chart where if one of them is shifting up or down in vertical, the other two aren’t also doing that same thing, except possibly to more or less of a degree. The peaks and valleys may not perfectly line up, but they certainly seem to come close in a few spots!
I noticed a recent forum post on AccurateShooter.com of a discussion related to my previous article and Alex Wheeler, owner of Wheeler Accuracy and a respected gunsmith who has built some of the best shooting 1000 yard Benchrest & F-Class rifles, said something that seems relevant to these three tests. The discussion was around tuning loads and finding the ideal seating depth/bullet jump, and here is what Alex said:
“From my experience powder charge will not drastically effect the correct seating depth. Meaning if you change your powder charge the gun will not go from preferring a .010 jump to a .060 jump. It may move a few thousandths, but I think you can use any powder charge you want to rough in on seating depth.” – Alex Wheeler
It seems like the results here, which are based on 3 different loads, seem to corroborate Alex’s experience. If you would have ran through this test with any of those loads, you would likely land at a similar bullet jump.
There was one more quote from Alex in that forum conversation, which is a timely reminder:
“I talked with Mark about his testing a few months ago. Keep in mind he is looking for the widest tune not necessarily the tiniest group. That’s not to say you may not also find the tiniest group at those jumps. Just know what his goal was with this testing. I do think he did a very good job of achieving it.” – Alex Wheeler
Well said, Alex! By the way, Alex is the only other person I’ve seen present an accurate and repeatable method to measure the distance to the rifle lands on your barrel (watch it here), so I know he is someone who believes in the importance of tuning seating depth and has likely done a lot of precise testing himself.
A Wider Sample Size for the 147 gr. ELDM
Okay, that was just a sample size of 3 tests (60 shots total), so now let’s look at the complete data Mark has compiled so far with the Hornady 6.5 147 gr. ELD-M. We’ll include the 3 tests above, and we’ll add 6 more rifle/load configurations, including rifles chambered in 6.5 Creedmoor, 6.5 PRC, and 6.5×47 Lapua:
- 6.5 Creedmoor: SAC Alpha 11 action in
Manners T4A stock with 26” Bartlein Rem. Varmint contour 1:8″ twist barrel,
Mark Gordon shooting - 6.5 Creedmoor: Defiance Elite action in
MPA chassis with 26” Bartlein Heavy Palma contour 1:8″ twist barrel, Mark
Gordon shooting - 6.5 Creedmoor: Defiance Elite action in
McRee chassis with 26” Bartlein Heavy Palma contour 1:8” twist barrel, Matt Stiner
shooting with 41.6 gr. load - 6.5 Creedmoor: Same rifle as #3, but Aaron Hipp
shooting with 41.0 gr. load - 6.5 Creedmoor: Same rifle as #3, but Mark
Gordon shooting with 42.3 gr. load - 6.5 PRC: Defiance Deviant action in KMW
Sentinel stock with 26” Bartlein Heavy Palma contour 1:8” twist barrel, Mark
Gordon shooting - 6.5 PRC: BAT TR action in McMillan A5
stock with Bartlein Rem Varmint +1 contour 1:8″ twist barrel, Mark Gordon shooting
with 57.0 gr. load of H1000 - 6.5 PRC: BAT TR action in McMillan A5
stock with Bartlein Rem. Varmint +1 contour 1:8″ twist barrel, Mark Gordon
shooting with 58.5 gr. load of H1000 - 6.5×47 Lapua: SAC Alpha 11 action in MPA
chassis with 26” Bartlein Heavy Palma contour 1:8” barrel, Mark Gordon shooting
Let’s dive into the vertical POI shift measured for each of those configurations at 600 yards:
That same area around 0.055-0.060” of bullet jump appears to still be a sweet spot for most of these rifles/load configurations. Most all of the various rifle cartridges, loads, and rifle configurations have relatively flat sections near that window of bullet jumps. Compare what is happening there to the spikes and erratic POI shifts that seem to happen closer to the lands on the left side of the chart. There are several rifles that seem to have significant vertical stringing for those jumps that are closer to the lands.
Keep in mind this doesn’t mean the rifles don’t group well at those closer bullet jumps. For example, let’s look at the first red spike, which occurs around 0.015” jump and is related to rifle configuration #8, which is one of the 6.5 PRC rifles. The chart isn’t saying the rifle’s groups measured 0.8 MOA at that jump. It might group phenomenally well at that bullet jump – maybe even under 0.2 MOA. We don’t know, because that isn’t what Mark was trying to test. What we can see in the chart above is the shot with a 0.015” bullet jump hit 0.8 MOA above the absolute center of all 20 shots for rifle #8. Then the very next shot, with an identical load in the same rifle but with a 0.020” bullet jump, hit closer to 0.2 MOA above the center of the group, meaning that shot shifted 0.6 MOA down with only a 0.005” of change in bullet jump. Now, for each configuration there is only one shot for each bullet jump, so we shouldn’t try to draw meaningful conclusions from that small of a sample size. But the chart above contains 180 shots/data points in total, and it appears that some of the erratic shifts in vertical are localized in few distinct areas. There are many spikes and steep lines up and down on the left side of the chart for multiple rifles, but there are relatively few on the right side of the chart for any of the rifles. This tells us that Hornady’s 147 gr. ELDM bullet seems to be more forgiving of changes in bullet jump when you are using 0.050” or more of bullet jump, and that appears to be true over a wide range of rifles tested.
Now let’s analyze the data a different way. The chart below takes the same data as above (180 individual shots in a variety of rifle/load configurations) but helps us visualize it in a way that is closer to the crux of what we’re looking for. To get the data below, we looked across all 9 rifle/load configurations shooting the Hornady 147 gr. ELDM and calculated the vertical extreme spread over a rolling window of 3 sequential bullet jumps. Each column represents a bullet jump window that is 0.010 inches wide, which is approximately what you’d expect the lands of the barrel to erode over 200 rounds for popular precision rifle cartridges. For example, the first column on the left below represents the extreme spread for the vertical POI over 3 incremental bullet jumps: 0.000, 0.005, and 0.010 inches. I calculated the vertical extreme spread over those 3 bullet jumps for each configuration, and then averaged those values across all 9 rifle/load configurations so we could see over-arching trends.
On the chart above, the shorter the column the better. For example, 0.050-0.060” and 0.055-0.065” bullet jump ranges were the two windows that had the most consistent vertical POI. Both of those bullet jump ranges averaged a 0.26 MOA vertical extreme spread at 600 yards over all 9 rifle/load configurations tested! That means they provide outstanding precision, and are very forgiving in terms of bullet jump and the distance to the lands. (It also means there were some very consistent shooters behind those rifles!)
Now let’s widen that window from 3 to 5 sequential bullet jumps, to see if there is still a similar pattern. Each column below equates to a range of bullet jumps that spans 0.020”, which would be about what you could expect to happen over around 400 rounds in popular mid-sized cartridges used in PRS/NRL style matches. How quickly the barrel erodes can vary even for the same cartridge depending on how hot your load is, the quality of the steel in your barrel, how long you allow the barrel to cool between strings, and a dozen other factors, so your mileage may vary. Your rifle lands might actually erode by 0.020” over 200 rounds or it could be 600, but I’m simply trying to give general context for what a 0.020” span of bullet jumps represents.
This chart above seems to tell a similar story compared to the previous one, although the “optimal” area we saw before around 0.045-0.070” is less pronounced after we doubled the size of the window of bullet jumps we were analyzing. Of course, we are still talking an extreme spread of 0.45 MOA over a 0.020” window. Again, that doesn’t mean groups averaged 0.45 MOA in that range, but just that average vertical POI didn’t shift by more than over that entire range for all the rifles/loads tested. It’s possible that groups at any point might be very small (or not), but the areas with lower numbers on these charts simply wouldn’t shift significantly as your lands eroded by 0.020” and bullet jump naturally increased by that amount.
6mm Tubb 115 gr. DTAC RBT
Now, let’s look at a very popular 6mm bullet, David Tubb’s 115 gr. DTAC RBT Closed Nose bullet. This is a bullet that David Tubb designed, and it is manufactured to his specs by Sierra Bullets.
For the 115 DTAC bullet, we have a sample size of 6 different rifles that were tested. One interesting aspect of the setups tested for the 115 DTAC is that all of them used a different action. Here are the details of all six configurations:
- 6mm Creedmoor: Curtis Vector action in
MPA chassis with 26” 1:7.5” twist barrel, Matt Stiner
shooting - 6mm Creedmoor: ARC Nuleus action in MPA
chassis with 26” Bartlein Heavy Palma contour 1:7.5” twist barrel, Mark Gordon
shooting - 6mm Creedmoor: Defiance Elite action in
MPA chassis with Bartlein Heavy Palma contour 1:7” twist barrel, Curt Geary
shooting - 6×47 Lapua: SAC Alpha 11 action in MPA
chassis with 26” PROOF Research M40 contour 1:7” twist barrel, Solomon
Manansala shooting - 6 Dasher Norma: Borden Mountaineer action
in MPA chassis with Bartlein Rem. Varmint contour 1:8” twist barrel, Mark
Gordon shooting - 6 BRA: BAT TR action in MDT ACC chass
with Bartlein Rem. Varmint contour 1:8” twist barrel, Mark Gordon shooting
The chart below shows the combined data, which is for 20
different shots in 0.005” increments from kissing the lands (i.e. 0.000”) up to
0.095” of bullet jump for each of those 6 rifles. So, it represents 120 shots
fired with the 115 DTAC.
I’ve been looking at these kinds of charts for a few months now, and it is still tough for me to decipher patterns in this one. I highlighted a couple of ranges that might be slightly flatter than other portions of the chart. The only obvious thing is there seems to be a lot of vertical shift around 0.020-0.040″ for a few of the rifles.
Let’s look at the other types of charts that are based on the same data, and maybe they’ll give us a clearer picture of what windows of bullet jumps seem to be the most forgiving in terms of vertical POI shift.
The chart above makes a couple of ranges of bullet jumps pop out. 0.040-0.050” bullet jumps appear to have minimal vertical shift. It also looks like most bullet jumps beyond 0.070” also provided consistent vertical POI over a wide range of bullet jumps. However, there are a couple of big spikes in the data that show an average shift in vertical of 0.6 MOA or more. The data appears to be saying the 115 DTAC is more sensitive to seating depth, at least across the rifles that were tested here. However, once you get beyond 0.070” of bullet jump, the bullet seems to become very insensitive to seating depth. In other words, the 115 DTAC only appears to be sensitive to seating depth when seating it very close to the lands, but not when you’re jumping it 0.070” or more.
Now let’s look at how consistent the vertical POI is over a window of bullet jumps that is twice as wide:
That “sweet spot” that appeared around 0.040-0.050” bullet
jumps in the previous chart seems to have vanished, meaning the sweet spot must
not have been 0.020” wide. So while you might not experience vertical stringing
in that range of bullet jumps over 0.010” of change in bullet jumps (around 200
rounds of barrel wear with popular mid-sized cartridges), if you pushed beyond
that without adjusting your seating depth, the data suggests you’d be more
likely to experience a vertical POI shift.
Honestly, you can see that by 0.030-0.050”, it drops off and pretty much gets smaller as you move further and further from the lands. Once you get to a range like 0.060-0.080”, you have an average vertical shift that is less than 0.5 MOA over a 0.020” window of bullet jumps. Remember, that doesn’t mean the rifle ever shoots groups that big, but just that the center POI of your groups wouldn’t likely change by more than that over 300-500 rounds (i.e. it is more forgiving in changes in seating depth or the lands eroding over time).
Wrap-Up & Next Post
While Mark has done preliminary testing on a few other bullets, the Berger 105 Hybrid, Hornady 147 ELDM, and Tubb 115 DTAC represent the ones he has compiled the largest sample sizes for so far. The data Mark collected for these three bullets helps us understand how these “sweet spots” can vary based from one bullet design to another. It also seemed to show that for a particular bullet there seems to be commonalities in terms of what range of jumps are most forgiving even over several different combinations of rifles, cartridges, loads, shot order, and shooters.
While there are slightly different patterns between each of the bullets, it does appear none of the bullets showed to have a forgiving bullet jump under about 0.040”. The most consistent vertical POI over a wide range of bullet jumps usually appeared to be closer to 0.060”. Jumps that have been traditionally seen as absurdly long, like 0.080” or more, actually seem to produce less vertical shift in POI as the barrel wears than when the bullet is seated very close to the lands.
Of course, if you tightly manage your seating depth and adjust it regularly (e.g. every 100-200 rounds), then you can still get extremely precise groups seated close to the lands. That is a really important point. None of this research is trying to say you can’t get tiny groups jumping 0.020” or less, or even seated into the lands. Those minimal jumps could produce smaller groups than if you were jumping 0.060” or more, although that isn’t necessarily a hard and fast rule either. What this research seems to show is that when you are seated close to the lands, your load may not be as forgiving in terms of changes to the bullet jump as your barrel wears and you could experience a vertical shift in your zero if you don’t regularly adjust your seating depth.
The next article will be the final post in this series, and will include some information from other shooters on their experience and how they’ve integrated this into their load development process. Stay tuned!
Find this interesting? | ||
Subscribe Be the first to know when the next article is published. Sign-up to get an email about new posts. |
or … |
Share On Facebook |
© Copyright 2021 PrecisionRifleBlog.com, All Rights Reserved.
Source
More Bullet Jump Research! – PrecisionRifleBlog.com is written by Cal for precisionrifleblog.com