OBT and Quickload question

[memilanuk]

FWIW - my optimized loads do not typically fall "spot on" the OBT node chart values from Chris Long's website. Rather, they always seem to be just a very slight tick slower. For example, in .308 with a 30" barrel, the Node 4 OBT (1.3684 ms) is the most realistic one to hit with 185 gr bullets. My final optimized loads have typically been in the 1.370-1.372 ms range, just a bit slower than the value in the chart. The same is true for almost every .223 load I've developed, as well.

Just out of curiosity... have you measured the actual distance from crown to bolt face? Chris had mentioned this to me years ago, something about using 30.5" instead of 30", etc. etc. and at the time I really didn't 'get' what he was talking about. Later it finally clicked. The barrel itself is may be 30" long, but on a lot of actions, some portion of the case head sticks out beyond into the recessed bolt face - maybe 0.2"? Anywho, if you find your OBT loads seem to be just a fuzz slow compared to the calculated times, you might try measuring that distance on your particular gun and seeing if tweaking that number in the OBT spreadsheet helps. One of my guns measures 30.2", another dead on 30", etc. Sometimes it helps, sometimes... it gets lost in the noise.

 

[Mozella]

If you actually calculate the error of using MV numbers 10 feet from the muzzle, you will find that it is off only by 1-2 fps. I doubt any chronograph most of us use is that accurate.

What you're talking about is not really an error, it's a difference. And, in most instances it's larger than 1-2 fps.

I position my chronograph 11 feet from the muzzle and, according to the automatically calculated data, the difference between the measured velocity and the velocity at the muzzle is right around 7 fps, at MVs in the 2880 fps range. The accuracy is guaranteed accurate to 7 fps in that range, but that's not the same as resolution, which is 0.2fps. The fact that the accuracy spread is about the same as the difference in question is not a valid reason to ignore it.

For example, if I'm using a 1/2 MOA rifle to shoot in F-Class where the X is 1/2 MOA, I don't wave it around and pull the trigger anytime the cross hairs happen to be moving around somewhere inside the X ring. That's a recipe to come in last place. I aim (assuming no wind) precisely for the center of the X. In other words, the resolution of my aim point is much more precise than the accuracy of my rifle.

If you use a modern chronograph, they are indeed able to measure velocity to impressive resolutions and pretty darn good accuracy as well. Electronics advance at a very rapid pace these days and a very expensive chronograph designed and built a number of years ago is not going to be as good as a modern chronograph even though the new one might be significantly less expensive.

So yeah, a good chronograph will not only calculate the difference between MV and what you measure at the chronograph some distance away, it is able to do it quite well.

 

[savagedasher]

Monte - I actually have measured the muzzle to boltface lengths, and use those in the QL. Fortunately, I don't have any yet that aren't within 0.1" of the advertised length, which is likely close to the accuracy to which I measured them using a marked cleaning rod. Reading through Chris' work carefully (several times, there is a LOT of information there), my guess is that my observation of optimized loads falling ever so slightly behind the OBTs listed in the Table may simply be due to using 0.1 gr as my minimum charge increment (i.e. one tenth grain more powder outs the barrel time slightly faster the the OBT table value). I have generally chosen the lower (0.1 gr) charge weight during load development, putting the barrel time slightly on the slow side, but close enough that 0.1 gr more would end up faster. Alternatively, it may have something to due with the way QL calculates barrel time. In any event, it is very consistent in my hands, so that's where I almost always end up.

With regard to back-calculating true MV when using a chrono placed some distance in front of the muzzle, depending on the caliber/cartridge, a 5-10 fps difference can be enough to make a 0.1 gr difference in the predicted OBT charge weight. That may not seem like a lot, but it's 0.1 gr closer to being out of the window, so I would recommend trying to estimate actual MV as close as possible. The simple approach I mentioned above using JBM took less than one minute, so it's not like it's any great time expenditure. Many of the small things we do (or don't do as the case may be) in reloading can have cumulative effects on the final product. For that reason, I tend not to overlook or ignore doing the small things, especially those that require very little effort. Like all else, YMMV.

I have 4 dashers 3 different barrel company's 28 29 30 31 inch barrels shooting the same powder and amount same bullets and the same seating depth.
Barrel time 1.23 1.260 1.287 1.301 change powder and adjust barrel time and it don't shoot. My 1.314 Bt changing powder and adjusting the speed the 1.314 was junk . it liked 1.357. Larry

 

[Marksman63]

I have 4 dashers 3 different barrel company's 28 29 30 31 inch barrels shooting the same powder and amount same bullets and the same seating depth.
Barrel time 1.23 1.260 1.287 1.301 change powder and adjust barrel time and it don't shoot. My 1.314 Bt changing powder and adjusting the speed the 1.314 was junk . it liked 1.357. Larry

Probably yours is a singular case, as it is against any internal ballistic concept, apart OBT theory.