standard deviation

[TestPilot]

Hummm, great discussion but I noticed no one mentioned OBT/OCW in any of the posts...If you're not shooting near/on one of those nodes the results are going to be significantly influenced by small load/performance variations...If you are shooting on/near a node, then those small variances don't cause you to fall off the edge and shoot a flyer...

 

[camac]

True - I sup[pose that is why I like using the chrony to get me into the Optimum charge/ tension, seating depth zone - I actually think it is more an Optimum velocity node than charge weight. The chrony helpsfind the boudaries so I can plant myself right in the middle. Variations to seating depth, bullet tension, charge weight, powder batches etc all can then be looked back at chrony to see if you have fallen out of the optimum velocity range (OVR??). Ideally a low velocity spread with low SD right in the middle of an OVR.

 

[Steve Blair]

I believe that OBT and OCW have some validity regarding barrel vibration and burning/pressure characteristics, but they are not hard rules that hold in every case. If they were, barrel tuners would not work and the BR and rimfire folks that use them would have wasted their time and money. Tuners clearly do work so the answer is more complex than just barrel excursion time or charge weight. A chronograph is a good tool to characterize loads and can reveal ignition problems (as in my .223 788), pressure swings and load problems of various types. Your chrono will not sight the gun or pull the trigger and it cannot put all the information you gather into a coherent pattern. But, it is a valuable load development tool.

Ideally, we want loads and firearms that produce small groups with small extreme spreads and do it consistently. It is sometimes difficult to achieve. Tools like QuickLOAD, a good chronograph, your own and other shooters' experience, quality reloading and measuring equipment, a well-made and correctly functioning firearm, and a full understanding of the process from components to target can reduce the effort significantly.

 

[TestPilot]

Hey Steve...no doubt accuracy is a complex bowl of Jello that's hard to get your hand around...I don't think OBT is about harmonic barrel vibration at all as I understand it...It simply describes the times (nodes) when the pressure shock wave generated by the burning powder has made a round trip from the chamber and back and hence is also the time when the muzzle end of the barrel is at it's most quiescent state with respect to this shock wave. This effect will happen independently of any barrel tuner. I believe barrel tuners address the harmonic vibrations of a barrel...

 

[Steve Blair]

TestPilot,

You are correct and I made the point badly. I was trying to say that if OBT is the overriding characteristic that the theory says it is, a tuner should have little or no effect. Although I do not use them, I have witnessed a shooter tune a so-so load down to nearly a bullet-diameter group. The ammo was all identical. It is pretty clear to me that at least several factors are involved.

When I was working on the Ultra-Lightweight Howitzer program, the engineers at Picatinny used internal ballistics characterizations from BRL because it was just too complex to model accurately from first principles.

 

[TestPilot]

Very kewl Steve...I did 5 yrs on the Army FCS program...never got into the hardware side too much, just software. And for sure, it's about understanding not about right/wrong...Appreciate the dialog.