OBT chart for QuickLOAD users

[Steve Blair]

I made the attached OBT chart and graph that gives some insight into the relationships of the various nodes as barrel length changes. The "BL" column is barrel length and corresponds to the graph X axis. The nodes are represented by the color keys at right and by the Y axis. I find it useful to highlight a row of barrel times with Acrobat Reader's highlight tool, for easy reading. If you want to delete the highlighting, click on it and hit the Delete key. I hope some folks find this useful.

 

[MrMajestic]

Thanks Steve!

 

[Nomad47]

Thanks, Steve. Chris Long had sent me a graph some time ago that he had created for his Dasher. Seems to me he only had a couple lines and velocity was included. I'll see if I can find it.

I have used QL and OBT to decide what barrel length to buy. On occasion, a slightly shorter barrel will have an easier to hit node. On another occasion, I had bought a 22-250 Competitor pistol with a 18" barrel and a 9 twist. It turned out that I could shoot bullets heavier than 64 grains but I had to sacrifice so much velocity to hit a node that the 64 Bergers gave me the best combination of accuracy, OBT node and velocity.

I created an excel chart that gives me all the nodes when I enter a barrel length. You probably already have something similar, but if you don't, I would be happy to share.

On a slightly different note, I have screw on Harrel's brakes on my four barrels. I think a combination of OBT to end of brake and QL barrel time to muzzle yields the most accurate results for me. But Chris says the brake has no affect on OBT. In other words, he says the 'swell' travels to the end of the muzzle, then back - not to the end of the brake. and back. Your thoughts?

 

[Steve Blair]

Chris is correct. The steel must be solid with no discontinuities. Be sure to include the barrel threads in your length measurement. They are still part of the barrel. I chronograph at 15' and determine how many fps are lost in that distance (~6-10) and add that to the instrumental velocity. Then, I enter all data in QL and fiddle the Ba factor to arrive at the correct velocity. I modify the cartridge data file with additional entries that correspond to my actual capacity measurements. Take a look at the attached image. The entry in "Selected Cartridge" is for my "Blue Box" 6BR brass. It has a capacity of 38.90, more than the 37.80 of the generic entry. The Ba of .520 corresponds to actual velocity for my charge of H4895 @ 70°. The pressure is Ok (22 loadings proves that!) and the barrel time is sitting .015 ms from Node 4. This load has an ES typically between 8 & 9 and shoots under 1/4 MOA from my Viper SS. [br]
I didn't buy into OBT until I reviewed a lot of past data and found my best loads all corresponded with OBT nodes. It may be that some other factor is driving but Chris' theory is the best I've seen.

 

[Nomad47]

I see your shot start pressure is the default. Do you ever vary this to get your loads to track? And do you know by what percentage you have had to adjust Ba to get your loads to track? IIRC, I had to adjust Ba for H4831sc by 8% to get it to track in my 284 with 180 Hybrids.

 

[Steve Blair]

Bill, [br]
I usually leave the shot start alone unless touching or jamming. My results still don't show much effect in the smaller calibers. The Ba often requires adjustment, the same powder varying with rifle and cartridge. In this example, H4895 is slowed about 6.7%. My Shehane is dead on with H4831 and 180 Bergers. Go figure.