Jump or Case Volume

[wkdickinson]

In another thread (which I can’t find again) I read a post from a fellow who believes that in his rifle OAL is more important than jump. When I first read that, I thought, aren’t they really one in the same. I have always thought of OAL as it relates to setting the distance between the bullet ogive and the start of the rifling (jump). He put a link in his post to a series of articles in Precision Shooting about a study that suggested trying to find a wide “node” for jump, similar to ladder testing powder. Their proposition was that by doing that you don’t have to continually adjust you OAL to compensated for throat erosion (“chase the lands”).

But that was not what this gentleman was really talking about. His premise was that by chasing the lands we are continually increasing the case volume by moving the base of the bullet out. According to him his rifle has stayed in tune for about 1,200 rounds using the same OAL.

So, when we do jump testing, is it really the distance from the bullet ogive to the rifling that is important, or are we really fine tuning our case volume? I am pretty well read on “precision” shooting and reloading and can’t remember ever reading anything that really examined or discussed this concept and a quick search here didn't turn up anything either (but maybe I worded my search poorly).

 

[ckaberna]

grab some popcorn fellas. Good question though. Not sure how it can be proved with way?

 

[Ned Ludd]

I've had rifles that required "chasing" the lands. I've had others that didn't and continued to shoot with the bullet seated at essentially the same original optimized seating depth as when the load was first tuned in in a new [broken-in] barrel.

For every Lot# of bullets I use, I remove 10 at the start, number them, and take all the pertinent bullet measurements. This represents the "measurement set" for that particular Lot# of bullets and I use them to estimate the distance to "touching" as long as I'm shooting that Lot# of bullets. By taking regular measurements every 200 rounds or so, land erosion is relatively easy to document over time. When the lands have eroded sufficiently that the original seating depth tune might be in danger of going out of the window, I simply do another seating depth test. If the results indicate the optimal window has changed, I seat the bullets accordingly. If not, I leave them where they were. I haven't been able to get any good handle yet on why a particular rifle behaves in a certain way with respect to seating depth, which is why I determine optimal seating depth empirically at regular intervals. The nice thing about seating depth testing is that we don't have to understand every single variable that may be changing when we optimize it. We do the test and hopefully obtain a solid answer where the bullet wants to be seated. As a friend has told me repeatedly, "we also want to know why things happen a certain way...but sometimes we just have to settle for what works".

I would make a rough estimate that maybe 2 out of three of my F-TR competition rifles require chasing the lands, the other third does not. My guess would be that the bullet itself has a lot to do with it (i.e. weight, ogive profile, bearing surface length), as well as freebore length/diameter, barrel land/groove configuration, etc. For example, I can imagine that a tangent ogive bullet might appear to want to stay at the same seating depth for a long time. However, that could simply be because many tangent ogive bullets are far more forgiving with respect to seating depth optima than secant ogive bullets. It's hard to know for sure. You mentioned the change in effective case volume, but it's also worth noting that when chasing the lands, the relationship between the bullet ogive and the rifling is not the other only variable that is changing. Over time, the bullet will also have less and less bearing surface seated in the neck, which could also play some role in precision. The real question there would be whether moving the bullet a few thousands at a time would really change the grip sufficiently to cause a noticeable difference in precision. The same can be said of changing effective case volume. In fact, the effect of seating depth on effective case volume can be effectively assessed using velocity as a readout. But again, the characteristics of a given bullet, chamber setup, whether the bullet is being jammed or jumped, as well as a few other factors are what likely make this topic too complex to ever describe in any manner sufficient to draw some general conclusions that extend universally across a wide range of different rifles.

Edited to add: as an interesting aside, I recently had a couple of my .30 Win F-TR rifles re-barreled with .040" longer freebore than before, to allow a little more case capacity with Berger's 208, 215, and 220 gr .30 cal Hybrids. Previous barrels were all chambered with 0.180" freebore, which is just about perfect for the Berger 200.20X bullet favored by many F-TR shooters. The new barrels have 0.220" freebore, which still allows the use of the 200.20X bullet, although they are seated fairly far out the neck. However, it allows for a little more case capacity with the longer, heavier bullets. In the past with multiple 30" MTU contour barrels having the 0.180" freebore, tuning in previous Lot#s of 200.20X bullets was almost like clockwork. Seating depth would typically be optimal at .009" and .012" off the lands. This has happened with multiple barrels and Lot#s over a period of several years. The groups with bullets seated .015" and farther would start to open up again, whereas .003" and .006" off were not great, either. In doing some load development with one of the new longer freebore barreled rifles, I was surprised to find with multiple seating depth tests that the optimal window with the current Lot# of 200.20Xs starts at .006" off. The interesting thing is that this is a new Lot# of bullets I have only used in one other rifle having the much shorter freebore length, where they also started to come in at exactly .006" off lands. So two different 30" barrels, with .040" difference in freebore length, both tune in with a particular Lot# of 200.20Xs at .006" off the lands. I appreciate the sample size of this observation is too small to lend statistical significance to this observation. Nonetheless, I've never seen any other Lot# of 200.20Xs tune in their. It could be some small measurement error in determining the distance to "touching", but I don't think so because of the measurement procedure I use. My initial reaction to this observation is that perhaps the ogive profile of this particular Lot# of 200.20Xs is somehow a little different than previous Lot#s, and they therefore like to be seated a little closer to the lands. Realistically, I will never be able to "prove" this theory, but having the new Lot# of bullets tune in at exactly .006" off the lands even though the freebore lengths are quite different suggests something intrinsic to the bullet itself may be cause, rather than the chambers, which have a significant difference.

 

[Hunting 2019]

Google Mark Gordon,that might be it.

 

[Hunting 2019]

PrecisionRifleBlog.com

 

[wkdickinson]

Google Mark Gordon,that might be it.

OK, I give - in what context, I get something about him being Governor?

 

[Straightshooter1]

In another thread (which I can’t find again) I read a post from a fellow who believes that in his rifle OAL is more important than jump. When I first read that, I thought, aren’t they really one in the same. I have always thought of OAL as it relates to setting the distance between the bullet ogive and the start of the rifling (jump). He put a link in his post to a series of articles in Precision Shooting about a study that suggested trying to find a wide “node” for jump, similar to ladder testing powder. Their proposition was that by doing that you don’t have to continually adjust you OAL to compensated for throat erosion (“chase the lands”).

But that was not what this gentleman was really talking about. His premise was that by chasing the lands we are continually increasing the case volume by moving the base of the bullet out. According to him his rifle has stayed in tune for about 1,200 rounds using the same OAL.

So, when we do jump testing, is it really the distance from the bullet ogive to the rifling that is important, or are we really fine tuning our case volume? I am pretty well read on “precision” shooting and reloading and can’t remember ever reading anything that really examined or discussed this concept and a quick search here didn't turn up anything either (but maybe I worded my search poorly).

Apparently, you're talking about my post.

For quite some time I was "chasing the lands" since there seemed to be such an emphasis on jump distance at the key to an accurate load. It was indeed a little frustrating as would make a concerted effort to maintain a particular jump that seem to work best. But it didn't take long until I had to redo the powder load to get the accuracy back with the jump that appeared to work best. I record details of my loads on a spreadsheet along with the measurement of the lands after each barrel cleaning. So I was keeping on top of my throat erosion.

Then I came across Eric Cortina imploring people to STOP chasing the lands and that it was "stupid. Well, I hadn't really heard anyone but him talk about it that way. So, one day I decided to give it a try and see how long I could go before losing the accuracy I like. As I said in that post, I went well over 1,200 rounds using the same load with the same bullet and the same seating depth (NOT OAL, but ~ the same CBTO). And though the lands had eroded over .030", I was still getting the accuracy of well under .05 MOA.

So, I was puzzled as to WHY this was happening and trying to think what was going on. I got to thinking about how when people are tuning their loads by adjust seating depth to fine tune their loads for the best accuracy. If people are adjusting their seating depths, they're changing the volume in which the powder works in an effort to find what pressure match the timing for harmonics to get the consistent accuracy. I can't think of anything else that might explain the results I got.

As Ned Ludd mentioned before, I think the design of the bullet is a factor, as I was using 168 SMK's that tend or be very jump tolerant. I don't feel bullets with the secant ojives are as tolerant in keeping the same seating depth for as long as I did. I haven't tested them, so I have no idea how they might compare. I just know I can go a long time without changing seating depth though the lands erodes substantially.

 

[wkdickinson]

Apparently, you're talking about my post.

Could be, if so I found it very thought provoking. I hope people who have a lot more understanding about this stuff than I share their perspective. But thank you and I am sorry I didn't give you credit for the idea.

 

[std7mag]

In most of my rifles (hunting) i can reach the lands, and do distance from the lands.

Except for my Mauser action in 284 Win.
The throat is long enough that i go by COAL, and use a jump forgiving bullet. VLDs don't shoot worth squat out of it.

Of course i haven't tried 180s or heavier either.

 

[VA_XTC_Shooter]

I don't have an answer for you, but do think it's not as simple as chasing the lands. My recent barrel (6BR) I have been shooting 105 VLDs from day one. The initial seating depth test for the new barrel found 0.005" off the lands to be the sweet spot.

After 1,000 rds I was using the same set up. Barrel had barely eroded, only 0.004"
Fast forward to last week, I have 3K rds on the barrel, erosion is at 0.015". I thought, hey I need to check seating depth again.

What did the test show me? The original seating depth, now 0.020" off the lands was the best (0.3" prone with sling). Any variation below or below the group would open. This particular barrel apparently isn't driven by jump, but by the cartridge package. I have pontificated before with fellow shooters that I think the preferred load of a barrel is more about the combustion package you are building in the cartridge vs the pressure you are tweaking with distance from the lands.

Good topic, thanks for posting. Curious what others have found.

 

[Ned Ludd]

Given that a typical Lot# of match bullets might show as much as .015" to .020" OAL variance (or more), COAL doesn't necessarily mean that much when used as a measure to define the relationship of the bullet ogive to the lands. CBTO is the better measure of actual bullet seating depth, because it describes as closely as we can easily measure the relationship between the bullet and the lands.

When we change seating depth, we will also change 1) the effective case volume, and 2) the amount of bullet bearing surface gripped by the neck. In my hands, these two extra variables are not typically as noticeable in terms of precision as actual seating depth (CBTO) within the ranges of seating I typically test (i.e. from .003" off to around .030" for mist of the bullets I use). To this end, I start charge weight testing at .012" or .015" off, because that is close to the center of the seating depth range I will usually test in the next phase of load development. That way, I am only moving the bullet half the total seating depth test range in either direction after I have settled on a charge weight. I do not typically find that moving a jumped bullet in/out by .010" to .015" has much of an effect on velocity/pressure. In fact, the effect is usually not discernible in a statistically meaningful way given the relatively small sample numbers we typically fire. Jammed bullets are likely a whole different animal.

Certainly, if you have to alter seating depth of a jumped bullet over a range of ~.050", or even more, you will start to observe a noticeable effect on pressure/velocity due to the change in effective case volume. Nonetheless, it is not uncommon during a seating depth test to find one or more test increments (I typically use .003") where the groups are mediocre, but a single additional increment tightens them up to basically one ragged hole. In other words, the beneficial effect of seating depth in terms of optimizing precision can often be very dramatic. My feeling is that slight changes in the effective case volume or the amount of bullet bearing surface gripped by the neck when changing the seating depth of jumped bullets within the range of about .010" to .015" or so is not likely to cause nearly as dramatic an effect as the seating depth change itself. In other words, such very subtle changes will often be lost in the noise.

FWIW - I sort bullets by OAL, primarily for the purpose of pointing them to increase BC, but it also has the added effect of making COAL measurements more consistent with the CBTO measurements, as well as improving load consistency. I generally always load for a match from within a single length group of bullets (.0015" total length variance per group). Using this approach, the variance between CBTO and COAL measurements observed with bullets that are not length-sorted largely disappears.

 

[NorCalMikie]

When "chasing the lands", you're not changing the powder charge, you're just moving the bullet closer to the lands. You ARE changing the OAL but not the powder charge.
All my chambers were cut with ZERO freebore so I can stay close to the lands.
And when changing anything, you only want to make "one change" at a time or you end up chasing your tail.

 

[mikecr]

I don't care about youtube videos or books. What I know is that best initial CBTO (as fully tested) holds through accurate barrel life -unless you're relying on the high starting pressure of ITL seating for your load.
So for a hunting capacity cartridge, you're better off to find a best OTL CBTO.
This, because you don't need high starting pressures with anything but a small underbore cartridge.

 

[Straightshooter1]

I keep reading things like, my load works best at .010 off the lands, or .015 or .020 off the lands, or whatever. . . and I find that has absolutely no meaning to me since such statements typically don't state anything about the COAL or freebore that's involved. Fore any particular cartridge, .010 off the lands in a freebore chamber measuring .150 is very different than .010 off the lands in a chamber with a freebore of .230. It seems better to simply know the COAL even though COAL's can have substantial differences and better yet, it'd be better to know the CBTO. But there's always this emphasis on jump . . ????

 

[Ned Ludd]

I keep reading things like, my load works best at .010 off the lands, or .015 or .020 off the lands, or whatever. . . and I find that has absolutely not meaning since such statements don't typically don't state anything about the COAL or freebore that's involved. Fore any particular cartridge, .010 off the lands in a freebore chamber measuring .150 is very different than .010 off the lands in a chamber with a freebore of .230. It seems better to simply know the COAL even though COAL's can have substantial differences and better yet, it'd be better to know the CBTO. But there's always this emphasis on jump . . ????

To a certain extent, that is correct. The distance from touching or "into" the lands is a relative value dependent on where you think the lands are located from measurements made using a Hornady OAL gauge, stripped bolt method, or some other approach for finding the lands. There are potential sources of error in all those measurements. Nonetheless, we pick a value and use that as our reference for use with a specific bullet and load. Until land erosion changes this value, it is simply a reference to allow us to define a CBTO or COAL associated with a particular load and seating depth. As long as loaded rounds were measured after taking the chamber measurement and groups were fired on a target over a range of seating depths, the approach of using a reference value is valid because it is very easy to measure the CBTO of loaded rounds with very good precision and accuracy. Once you know which CBTO value provided the optimal recision on the target, it is also very easy to consistently reproduce that exact CBTO in loaded rounds. COAL is a little harder to reproduce with the same precision unless using length-sorted bullets because of nose length variance in a typical Lot# of bullets.

The decision of whether to jump or jam a bullet is largely defined by the intrinsic properties of a bullet itself. Some clearly prefer one or the other, and only by testing is it possible to know which. Most of the bullets I routinely use prefer to be jumped. The point you made regarding CBTO and/or COAL having little or no relevance to freebore length is correct, except that freebore length is important for how far out in the neck a given length bullet can be seated. Other than that, it's back to simply generating a good reference number for a given bullet in a given freebore chamber. Making dummy rounds or significant experience loading a particular bullet may make it easier to cross-reference specific information against what others report, but such measurements still have to be taken with a grain of salt. Again, a reference value is primarily useful only for that particular rifle, or rifles with very similar freebore length. One way in which this effect is observed from time to time is when someone reports a load for a rifle with a very generous freebore length. Those that may not know that particular rifle has a long freebore, or that are accustomed to using loads with the same powder/bullet for a rifle with a much shorter freebore (i.e. loads with a much shorter CBTO) will often comment about such a load being very "hot". In reality, it may or may not be, dependent on how long the actual freebore is. I recently worked up a couple loads for a .308 Win with about .040" longer freebore than what most F-TR shooters are using for the Berger 200.20X bullet. Accordingly, the CBTO values for the new load with Varget came in at 0.8 gr higher charge weight than before. Had I not done the testing myself and known about the longer freebore, the new charge weight would have seemed ridiculously high, possibly even unsafe, had I seen someone report it in an online shooting forum. Nonetheless, pressure in the new load is still predicted to be well under SAAMI MAX, largely because the CBTO/COAL measurements are almost .040" longer

When I first started reloading, it took a very long time to wrap my head around these "relative" measurements, and using them as reference values. My background as a scientist made me want to have a set, definable value for everything. Unfortunately, we can't always do that easily in the reloading process, so certain reference values are inherent, unless someone is willing to go to rather extraordinary lengths. Most people just accept this and move on, but I still personally have mental reservations about relative measurements and reference points. I take them, I use them, but they are still a source of minor mental irritation to this day. The important point, as you noted, is that the reference values used are critical to a specific load/situation, but may largely have little meaning for a different load/rifle setup. I always try to keep that notion in mind when looking over other's reported values, but it also means on occasion that someone else's reported load information isn't of great use to me if the rifle they loaded it for is a lot different than my setup.

 

[NZ_Fclass]

By coincidence, I was watching a "Johnny's Reloading bench video on You Tube while eating lunch.
He was testing some sub-sonic loads in 300BLK with 1680 and 296.
Wow - the difference between the 50 thou OAL's was a lesson and would certainly have needed to add .1 or .2 to keep velocity between loads.

 

[wkdickinson]

Given that a typical Lot# of match bullets might show as much as .015" to .020" OAL variance (or more), COAL doesn't necessarily mean that much when used as a measure to define the relationship of the bullet ogive to the lands. CBTO is the better measure of actual bullet seating depth, because it describes as closely as we can easily measure the relationship between the bullet and the lands.

When we change seating depth, we will also change 1) the effective case volume, and 2) the amount of bullet bearing surface gripped by the neck. In my hands, these two extra variables are not typically as noticeable in terms of precision as actual seating depth (CBTO) within the ranges of seating I typically test (i.e. from .003" off to around .030" for mist of the bullets I use). To this end, I start charge weight testing at .012" or .015" off, because that is close to the center of the seating depth range I will usually test in the next phase of load development. That way, I am only moving the bullet half the total seating depth test range in either direction after I have settled on a charge weight. I do not typically find that moving a jumped bullet in/out by .010" to .015" has much of an effect on velocity/pressure. In fact, the effect is usually not discernible in a statistically meaningful way given the relatively small sample numbers we typically fire. Jammed bullets are likely a whole different animal.

Certainly, if you have to alter seating depth of a jumped bullet over a range of ~.050", or even more, you will start to observe a noticeable effect on pressure/velocity due to the change in effective case volume. Nonetheless, it is not uncommon during a seating depth test to find one or more test increments (I typically use .003") where the groups are mediocre, but a single additional increment tightens them up to basically one ragged hole. In other words, the beneficial effect of seating depth in terms of optimizing precision can often be very dramatic. My feeling is that slight changes in the effective case volume or the amount of bullet bearing surface gripped by the neck when changing the seating depth of jumped bullets within the range of about .010" to .015" or so is not likely to cause nearly as dramatic an effect as the seating depth change itself. In other words, such very subtle changes will often be lost in the noise.

FWIW - I sort bullets by OAL, primarily for the purpose of pointing them to increase BC, but it also has the added effect of making COAL measurements more consistent with the CBTO measurements, as well as improving load consistency. I generally always load for a match from within a single length group of bullets (.0015" total length variance per group). Using this approach, the variance between CBTO and COAL measurements observed with bullets that are not length-sorted largely disappears.

When I referenced OAL I really meant CBTO, since a seating die use the base of the case and some point on the ogive for it's length. I never pay any attention to the true OAL measurement, since it means nothing unless you are loading for a magazine.

 

[ronemus]

I find that I can maintain the tune of a load by keeping jump constant, and adding powder to maintain the original velocity. The additional case capacity obtained by increasing CBTO makes room for the added powder. I tend to shoot stiff loads, so the original load is usually 100% full or mildly compressed. If a trial with a particular powder leaves room in the case, I will try a slower powder in hopes of better velocity. I compete in NRA High Power and Long Range, so the accuracy requirements are modest. This approach lets me maintain 1/2 to 2/3 MOA (ten shot groups) for the life of the barrel. I recently had the urge to try ELR, so I will probably try to get tighter groups with that rifle, and will have to evaluate whether the approach I've outlined will work for ELR.

 

[RegionRat]

Now and then I run seating depth really wide just for the heck of it. The old mantra was that seating the bullet deeper was decreasing the internal volume and that in turn was supposed to increase the velocity.

The only thing I have ever recorded with wide seating depth sweep, is the opposite in terms of velocity was not increasing but actually decreasing. Not what is often discussed or predicted by the models, but I will show an example to illustrate the point.



Starting at light jam and seating in steps for 80 thousandths, the velocity does not increase but decrease for a fixed charge. The example above is a 6mm 105 Berger Hyrbid.

Large jumps are nothing new if we consider some of the old Weatherby Magnum chambers. Those were made with very long freebore. If we started that example above by seating into the lands, the pressure jumps several thousand PSI and we would see a slight rise in speed, but one that is easy to loose with wear.

As for why setting a seating depth for a jump instead of chasing lands seems to hold, have we considered OBT theory or bbl tuner theory? Is it possible that the seating depth keeps that harmonic in the right tune?

 

[searcher]

I have A/R's that shot 1/4" MOA when I first did load development for a particular bullet and powder that would be a load I would use exclusively from then on - hopefully for the life of the barrel. I do know this - I have loaded (all at the same time) and shot over time 1,200 rounds of a load (all loaded on brass with the same number of firings - same bullet batch, same primer batch, same powder batch, etc.) and found that at the end of the 1,200-1,500 rounds, my groups had opened up no more than 1/8", as I would test on paper before loading another 1,200 to 1,500 rounds. I never find the need to change the OAL of the loaded cartridge to chase the lands in these .223's until they get to around 6,000 rounds - at which point groups still run approximately 3/8" MOA before doing a depth change. Most often - there is very insignificant improvement even doing a depth change and ladder as well, leading me to believe that, for my .223 rifles, there was no real appreciable benefit to doing an OAL change by itself. I did tweak the powder a bit (perhaps 2/10 to 3/10 grain) to get back to 1/4" groups. If one chases the lands - the dynamics do change a lot as to pressure - which necessitates a powder tweak anyway. Two ways to tackle the issue. One thing I didn't see anyone note is that velocity changes the vibe of the barrel - affecting the shape of the pattern in the group. I believe Mr. Cortina was basically saying he found that not changing depth settings was affecting pressure (and therefore velocity) less than the ensuing pressure changes which accompany depth of seating changes - or chasing the lands. He just put it a bit differently. Still - a minor 2/10-grain increment ladder test starting 1/2 grain below the normal load, increasing to 1/2 grain above - should bring into tune most seating changes - if that is the route one wants to go. If I can routinely shoot 5,000 rounds on multiple rigs and not lose more than 1/8" without adjusting my OAL, I begin to wonder if the folks "needing" to chase the lands to regain accuracy - when perhaps there were other factors at play - and possibly over-riding. I'd like to say I did this with my bench rest rifles - but I have not - as I am too busy always doing what a lot of other folks do - fiddling with a proven or new load. If not for loading large quantities for my varmint rifles - I'd have never seen this for myself.