Optimum Barrel Time and Accuracy Nodes

[LRRPF52]

Have we discussed this before? I just came across these concepts, and found them very interesting. Basic premise is that the longitudinal shock wave that resonates from chamber to muzzle, then back and forth, significantly affects accuracy if the bullet is leaving the muzzle at the time the wave is at the muzzle.

Seems to be a lot of data to support that this is the explanation for unexplained open groups with certain charge weights, while others fall into certain accuracy nodes.

Apparently, Quick Load has a function to deal with this, called Barrel Time, measured in microseconds. The links have tables with the accuracy node figures for different barrel lengths.

I've had this feeling about load development and barrel harmonics that has been nagging me, with the suspicion that there is some type of physics behind accuracy nodes, but I just couldn't put my finger on it. Apparently, these guys have proposed one theory, and testing by various participants has shown that there is reproducible merit to the Optimum Barrel Time hypothesis.

Pretty crazy stuff.

http://www.the-long-family.com/

http://optimalchargeweight.embarqspace.com/#

[Whelenon]

interesting idea, just scanned through it, will have to give it a read. Thanks

[LRRPF52]

Here's another one on barrel harmonics with dynamic diagrams/animations:

http://www.varmintal.com/amode.htm

[Drifter]

I have heard of the barrel time stuff, but can't say that I've really put much thought into it. However, I have considered that a "tuned" AR's harmonics can contribute to consistency, but my thoughts could be misguided.

I use and recommend the OCW. Though it probably relates to harmonics and barrel time, it doesn't take much thought to put it to use and clearly see the results. I don't recall if it's part of the official test, but I usually conduct the OCW in round-robin fashion. For example, if I have five different charge weights to test, with five rounds of each, the shot / target sequence changes each 5-round string. For the first string, I shoot load #1 at target #1, then the next shot is load #2 at target #2, and so on. For the second string, I start with load #2 on target #2, progressing through to finish the string with load #1 on target #1. Obviously, the final (5th) string will start with load #5 on target #5. It helps to make notes as you go so that cartridges are loaded into the mag in the correct sequence, and that targets are shot appropriately as well. I sometimes do this test twice on the same trip (and with another more skilled shooter behind the rifle when possible), just to confirm the results. On the next trip, I might run a similar test, but with smaller differences in charge weights. The goal is finding a charge weight range that not only results in tight groups, but also minimal POI shift.

After the OCW has narrowed down the charge, I might do a similar test varying bullet seating depth (instead of charge weight, since it has already been nailed down).

Combine this type of load development with a temp stable powder, and the potential benefits are obvious.

Another evaluation that I plan to try is the ladder test. Details on this link:

http://www.6mmbr.com/laddertest.html

And for an interesting read regarding accuracy, check out this link pertaining to "Secrets of the Houston Warehouse":

http://www.angelfire.com/ma3/max357/houston.html

[rasp65]

LR About 10 years ago I found the Varmint Al website. I found that he had a wealth of knowledge about reloading. shooting and hunting. Al was an engineer at the Lawrence Livermore lab and still had access to some of their computer modeling software. He has a good explanation of the effect of vibration on the rifle barrel. There has been a few articles on 6BR.com about the subject as it is of interest to benchrest shooters. You will see some shooters have adjustable barrel tuners that have fine thread pitch so they can fine tune at the range. I have found with the Grendel that when I change from one bullet to another my POI can shift. For instance I was testing a hunting load,100 TTSX over 30 gr of H335. I have a load that I use to warm up the barrel before I shoot my test loads it consists of 26gr of TAC 120gr Speer in a fire-formed case. I shoot about 20 of them in 5 shot groups. The rifle was previously sighted in with the TTSX. The 4 groups were centered about .5" high 1" left. The TTSX were .5" right .25" high. The wind was no factor as it was blowing from the direction I was shooting. My only explanation for this is harmonics.

[CTone03]

With OCW tests, if you follow Dan Newberry's method to the letter, you're actually using the OBT method when you start to adjust the seating depth. The OCW/round robin finds the node for your rifle, and the seating depth times the bullet's exit from the muzzle. I've been very successful using the OCW method to find a load, and then using Quick Load to help time the bullet, though it's tougher with newer cartridges like the Grendel and new powders like XBR. With older cartridges and powders - say, 308 Winchester and Varget - you can almost pick a perfectly timed load right in the middle of a known node from the start; but I would still advise working up for safety's sake. To give you an idea of how I found a load in the past:

Pick bullet/powder that I want and work up in charge with a round robin/OCW test loading to magazine length, recording all the data that I can. I do another OCW to narrow down a potential node, and combine that data with the one from the first test (following Newberry's method to the letter). Using Quick Load, I'll enter the data from the tests, and compare the figure it spits out in the lower right-hand box for the "Barrel Time, 10% PMax to muzzle" with the figure for my barrel length from Chris Long's OBT chart, which for me has always always always matched the results from my targets. Always. I can then adjust the seating depth figure in Quick Load to bring the "Barrel Time, 10% PMax to muzzle" figure to one that matches the OBT chart for my barrel lenght, and I'm done. I've always got real results from going this route.

[LRRPF52]

CTone03,

When you say "real results", what type of groups are we talking about? Have you noticed that you can make a mass-produced button-rifled pipe shoot to 1/2 MOA or better, or has it always been necessary to use a match barrel?

I really think there is merit to this OBT concept. I'm trying to decide what I will use for both my Grendel and .260 Rem. I would like to get both consistently under the 1/2 MOA area, which I have already done with the .260 Rem.

Drifter/Rasp65,

I read that entire Houston Warehouse article, which was very interesting. Biggest things I took away were the brass prep and 21 3/4" barrel length, but I'm no benchrest guy. I have suspected that neck tension plays a big part in consistency, but I won't be chasing any zero groups any time soon. I'll be happy with .25 MOA if I can get that consistently, which is hard for most people to exploit from a practical shooting position.

[LightningII]

does Browning still use the BOSS on their rifle barrels?
I always wondered if it really worked.

[CTone03]

My results have been for both match barrels in my guns (cut and button rifled), and for bone stock Remmy 700s that belong to friends. With bolt guns, getting consistent five shot groups under an inch is easy. I say consistently under an inch because just about any rifle manufactured these days will shoot a five shot group under an inch **every now and again**, and between 1 1/2" and 1" consistently, with good quality or match ammo. To give you an example for gas guns, I had a 308 AR with a 20" Fulton Armory Criterion barrel that would shoot bona fide 1 MOA 10-shot groups at 100 yards on demand both summer and winter, and that's with handloads that I made using the OCW method and tweaking the seating depth using the OBT method. It would very often shoot 1/2" five shot groups, but I could count on keeping ten shots on a one inch dot from a bench in any weather here in Virginia. That's consistency. I have a DPMS AR - a AP4 I think is the model - with a stock, hammer forged, lite profile 16" barrel that will consistenly shoot 5/8" five-shot groups at 100, and have kept MOA five shot groups at 710 yards. This with an OCW and OBT developed load using new Winchester cases, 69 grain SMK, and 24.5 grains of Varget. I forget the seating depth.

I've had outstanding success recently with a MK12 clone that I built with an 18" Les Baer barrel chambered in .264 LBC using the same method. Components are x1 AA brass, 123 grain AMax, 28.3 grains of XBR at 2.265" - which from reading this site is very close to a known favorite load, but I developed the load all on its own by starting low and working up, then doing the OBT/seating depth testing using Quick Load, and finally shooting it at 652 yards. I'm now developing a load using 123 grain Nosler Custom Competions and Benchmark, but I'm stalled in that one at the moment.

To take this even further, I had been using the OCW method for a while with that 308 gas gun, but when I read about using the OBT method and comparing it to the Barrel Time field in Quick Load, I went back and compared every target that I kept from load development; litterally, about fifty or more targets from OCW tests (I'm a handloading addict). These targets were from loads made using Varget, Benchmark, H380, RL15, RL17, and 8208 XBR, and bullets from Nosler, Sierra, and Hornady in weights of 155, 165, 168, 175, 208, and 210 grains, and at every point where I worked up in charge weight, where the groups were tightest corresponded exactly to the OBT chart for my barrel length. Every time. Using that fact, I was able to help some friends find a load for stock bolt guns that on their first try would outshoot any factory or handloaded ammo that they had. I am firmly convinced that Dan Newberry and Chris Long have figured out how to make the most accurate ammo.

[txgunner00]

Interesting reading. It's been a while since I read that warehouse article.


[QUOTE=rasp65;24182]LR .... I have found with the Grendel that when I change from one bullet to another my POI can shift. .../QUOTE]

I've observed this in pretty much all my rifles. I shot this a few days while function testing some hastily thrown together loads for an upcoming hunt. I cleaned out my component drawer so I had a mix of 100 gr hornady SP, 120 gr speer SP, 123 Amax and 140 gr Hornady SP. No load development of any kind. I just picked a charge were I felt it would function safely and shot to confirm. I took off my 4.5-14 and mounted a GRSC 1-4x combat scope with a horseshoe-dot reticule. These were shot at 100yds, quickly and after dark (target was dimly lit with a work light) so the groups aren't all that great but they clearly show point of impact shifts for each bullet type. I was expecting elevation changes with bullet weight but was surprised how much windage shift the 140s had.

IMAG2616.jpg

[CTone03]

To give more information on how to use the OBT method with Quick Load, I took some pictures for you. This is the workup I did last year with IMR 8208 XBR and 123 grain Amax:

Round robin OCW with a 2.265" seating depth and charges starting at 27.9 grains up to 28.5 - Picture.

The lone round was a sighter, and I adjusted my point of aim based on that one shot. Now, entering the 28.5 grain results from the target with QL, including the bullet (no 123 Amax in QL, so I used Lapua) seating depth and charge weight (don't forget to enter your water weight before you start), the velocity didn't line up - my actual velocity was slower - so I adjusted the powder temperature until the velocity lined up. Compare the Barrel Time of .984ms given with the OBT chart for an 18" barrel like mine, which shows the #6 node of .988ms being ideal, and you can see why the 28.5 grain charge works out -- Picture.

I thought the 27.9 through 28.5 grains looked like a node based on the vertical impact trend, despite the group size, as is recommended in Dan Newberry's method. Considering that, I narrowed my focus from 28.3 grains to 28.6, and you can see that they all share a common verticle point of impact, with the 28.3 and 28.4 grain groups being the smallest -- Picture.

Entering the results from the 28.4 grain group into QL and adjusting the temp to match my velocity gives a Barrel Time of .985ms, which again corresponds with the OBT chart's recommendation of .988ms for the #6 node -- Picture.

Next up was a seating depth test using a powder charge of 28.3 grains, going from 2.265" down to 2.245" OAL. I already knew that the 2.265" depth was the best before I started, but I wanted to confirm. As you can see, I have a nasty 1st round flyer problem with this rifle.


Cherry picking the best data from that target - the 2.265" depth - and putting it into QL, it again confirms the timing of the bullet with a Barrel Time figure of .990ms, which is very close.


For more confirmation I tried increasing the seating depth, and it still matched best with the 2.265" load, and still shows that I have a 1st round flyer problem. I didn't get to record velocities with this test.


Lastly, here's the best group from a shoot out to 650 yards with the finished load of 28.3 grains of XBR at 2.265" OAL. The circled round isn't from an earlier group. You can still see that flyer!


Here's the rifle.


These results are the latest that I have, and I used them to show why so many here on the site recommend 28.5 grains of XBR under a 123 grain Amax at 2.265". It works well with a new Lapua case. There are some loads I've come across with different cartridges though where the node and the Barrel Time didn't match up where it would be useful. A 308 Winchester that I had didn't like the well known 45 grains of Varget in a new Winchester case under a 175 grain SMK, and Quick Load told me that the timing was all wrong and it showed in my groups. In the end I went with 43.3 grains of RL15 with the same bullet, and it shot lights out. If you have QL, give this a try; you won't have to fiddle with the powder temperature tool with older cartridges and powders like you do with a Grendel and XBR. It's usually spot on.

[LRRPF52]

CTone03,

Nice report. Is your barrel a LW by any chance? I've seen that before with them in a 6mm heavy barreled gun that would just shoot these insane little groups, with one obscene flyer, like Bill Clinton showing up to your picnic and flirting with the girls. That's still a great group for 652 yds, impressive group actually. I bet you would like to iron out that flyer. Did you clean between those groups?

So for my 16", I will have a different OBT. I'm wondering how that load would turn out with QL, since I don't have it. I just picked up some XBR for the Grendel too. I would like to milk the accuracy out of my economy pipe, just to put the OBT theory to the test for myself. I've achieved 1 MOA with it using 123gr Scenars. I have almost every 6.5mm Target bullet to choose from on my bench, so maybe the 123gr AMAX is in for some testing.

[CTone03]

like Bill Clinton showing up to your picnic and flirting with the girls.

Ha! Yeah, that's about what it feels like. I had the same problem with the 308 AR as well, but not my little DPMS. Weird. The barrel in the MK12 is a Les Baer, and it started out as a heavy 20" profile. I sent it to ADCO to get turned down for a Mod 0 profile and to be cut to 18". I originally tried to have Satern Barrels make me a Mod 0 profile in 6.5 Grendel, but after almost 2 months of emailing and phone calls I couldn't get them to give me an answer. A couple of weeks later they dropped the Grendel, so maybe that was it.

I normally clean between tests, but not in between groups. The OCW method negates the need for cleaning between shots. Because of the 1st round flyers, now when I do a test I load the magazine with all of the rounds stacked in sequence and fire a sighter as the first shot. Of all the research on gunsmithing forums, I can't find any definitive explaination why it happens. It just does, it seems.

For your gun, I would pick a known winning load/bullet combination from this forum with the 123 Scenar. The favorite powders and charge weights of AA2520, XBR, and such are a trend because they are on a node and are timed pretty close. If you worked up carefully to some of these loads and then tried some seating depth combinations to find your barrel's sweet spot, it won't take you long to dial it in.

[bobke]

I just want to pull up a chair and sit back and learn-this is some of the most useful info I've seen on load development in a good long while. I don't inhabit the precision forums, and could probably benefit from it, but this is user friendly and accessible. And since I'm new to loading Grendel, a perfect opportunity to employ what's been shared here for future reference. Need to do some more research on OBT, OCW, but this is a great start. This ought to be a 'sticky' for reloading portal. Thanks, guys.

[LRRPF52]

Ha! Yeah, that's about what it feels like. I had the same problem with the 308 AR as well, but not my little DPMS. Weird. The barrel in the MK12 is a Les Baer, and it started out as a heavy 20" profile. I sent it to ADCO to get turned down for a Mod 0 profile and to be cut to 18". I originally tried to have Satern Barrels make me a Mod 0 profile in 6.5 Grendel, but after almost 2 months of emailing and phone calls I couldn't get them to give me an answer. A couple of weeks later they dropped the Grendel, so maybe that was it.

I normally clean between tests, but not in between groups. The OCW method negates the need for cleaning between shots. Because of the 1st round flyers, now when I do a test I load the magazine with all of the rounds stacked in sequence and fire a sighter as the first shot. Of all the research on gunsmithing forums, I can't find any definitive explaination why it happens. It just does, it seems.

For your gun, I would pick a known winning load/bullet combination from this forum with the 123 Scenar. The favorite powders and charge weights of AA2520, XBR, and such are a trend because they are on a node and are timed pretty close. If you worked up carefully to some of these loads and then tried some seating depth combinations to find your barrel's sweet spot, it won't take you long to dial it in.

I bet ADCO cut it so fast that it induced some residual stresses into the barrel. A guy up in Ogden said that you have to take off a very small amount of material off at a time, which is what he does with his flutes. He claims that Krieger and other high-end barrel makers will still back their barrels if he does the fluting on them, while they won't with a lot of other smiths who do more aggressive removal of steel from a heat-treated barrel.