What are your experiences using optimum barrel time (OBT)? If you're not familiar, it is a calculation based strictly on barrel length that provides several harmonic nodes for the bullet travel time through the barrel. Then you can use a program such as Quick Load to calculate the barrel time, which for a given bullet can be tuned based on powder type and amount (ie pressure and velocity). I found that this calculation agreed with ladder tests for my 308 and 6BR, it accurately predicted the flat spot for shot dispersion. For my new 223 I looked at a narrow range of powder near the OBT and while the vertical dispersion was not entirely flat it was narrow, and loads at the OBT give excellent groups. The Chris Long OBT is based on the calculation of a pressure wave that travels back and forth through the barrel, changing its diameter in the process; not exactly the same thing as the barrel "whipping". Going to the range this afternoon to try a few new loads using RL15 for the 308 to see if this short-cut process works out again. Share your results, or if you have ladder test data I will be happy to run them through Quick Load to see if it fits. An interesting approach that needs more validation.
Interesting indeed. I've been shooting and hand loading for most of my life and this is the first I've heard of it. Usually I just shoot a ladder test and go from there. I'll keep watching this thread for more input and results, you can never learn too much!
Precision Shooting magazine did an article years ago on barrel length, harmonics and accuracy called "The Houston Warehouse Experiment" Most interesting was apparently they decided that 21.75" was the ideal barrel length.
http://www.angelfire.com/ma3/max357/houston.html
Several issues later a scientist type hooked different rifle barrels up to sensors that produced readouts of barrel vibrations and harmonics. Bottom line was apparently rifle accuracy is all about OBT in conjunction with barrel harmonics. The way I figure it barrels are like Buddhists...."Nothing is real. Everything is harmonious vibrations." So I guess barrels that go Hummmm are the ones that really shoot.
Something to keep in mind about that 21.75" datum... that was specifically for a 6 PPC. Not necessarily a constant for everything - though its pretty darned close.
Chris's OBT theory, in conjunction with OCW testing and software like QuickLoad can provide some impressive results. The only problem is... using QL correctly is almost an art unto itself. Certain parameters need to be tweaked to match the actual results you're getting at the range in terms of velocity, or else the results that QL predicts will be skewed. Case in point: the QL data for N150 and RE-17 in the last version I had bore almost no resemblance to reality, and had to be heavily massaged to get it to spit out anything approaching what I was actually getting for MV. At the time I didn't have a laptop that would run QL... and being able to tweak it on the range while testing over a chrono helps a lot (IIRC, thats how Chris used it). But by the time you get done screwing around with all of that... you could just about have a load worked up conventionally anyways, and guess what? You'll probably be pretty close to an OBT/OCW load anyways. It's still fun to play with, but lets just say some of the glamour has worn off for me.
I applied the timing to a workup I was doing with 85gr 243. I had worked up a load that seemed to work good enough using 414. HPBT Sierras would group right at 3/4" and when I plugged in some Nosler Partitions they grouped a little over an inch. I thought little of that though, always heard they weren't necessarily the best grouping. Could get the timing to work quite right for OBT with 414 so switched to H4831sc which I also had worked up to 48gr. At that load it gave reasonable grouping but about 100fps(3060fps) less than 414 (~3200), that was my reason for working up the 414. Load worked out to be 47.7 grs according to Quickload with the OBT table. That put the bullet at the muzzle about .01ms ahead of the node. Sierra and Nosler clocked pretty close to the same this time, just under 3200, 3195fps. The Sierra group was one ragged hole 1/4" with one "flyer" increasing it to 1/2". The Nosler group decreased to just over 1/2" with no "flyers" :-)........ It may all be coincidence but seemed to work for this load. I've started looking at some of my "pet" loads and they seem as well to follow pretty closely to the OBT timing. I'd give it a good chance if I had refined those loads to PBT timing perhaps they would be even better.
It looks like it's key to get the predicted velocity to match the actual velocity by tweaking certain QL parameters. Then you can start checking powder weights and COALs to see if they hit an OBT node.
Ladder test on R15 from 44.0-45.2 gr gave a node at 44.7, so this approach helped to limit the testing range. Running a comparison of Varget vs R15 in Quickload, the calculated velocities are faster for R15 at the same pressure. Interestingly the barrel time correlates perfectly with pressure, NOT velocity. Next will work on seating depth, which will factor into Quickload pressure and velocity.
This is the most miss quoated part of the entire article. The 21 3/4 inch length was what they found what works on a 22 PPC. This doesn't mean that it is a generic barrel length to be applied to all calibers. As far as the Houston warehouse I have talked to two shooters that shot at the Houston warehouse(s), with one of them being very close to Virgil King and was at every session they fired. The both have told me that "What worked inside the warehouse doesn't work in real world, outside." I been told that "Unless you plan on having matches in a wearhouse, then take everything written in that article and throw it out the window." Why people take this article as gospile, I will never know.Originally Posted by wbm
Last edited by 82boy; 02-25-2013 at 10:59 AM.
Nobody else trying the OBT approach? Since I'm semi-retired maybe I have too much time on my hands!
Not sure how the velocity, pressure, and barrel time could vary. I thought velocity was tied to pressure and barrel time was tied to velocity.
The pressure that is typically reported is the max pressure. According to Quick Load, R15 has a slower burning rate than Varget and the max pressure occurs later in the barrel as well(QL gives pressure and velocity profile along the entire barrel length). So while the max pressures are important for safety considerations, it is the complete pressure profile that explains velocity. I can't explain why the max pressure determines the barrel time.
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