how did this happen? i lowered my charge by one grain and the average velocity increased. the velocities listed are for each shot in my strings. these were taken on different days. temperature was the same, or very close to it, on both days. both days were cloudy. chronograph is a Oehler model 35 set up at 15 feet from the muzzle. notice that the extreme spread and standard deviation dropped way down with the 45 grain charge, but the velocity went up. any ideas what caused this velocity increase??

Cartridge - 8x50R Lebel
powder - IMR-3031
bullet - 200 grain Speer

charge = 46 grains, MV = 2482, ES = 36, Sd = 11

2503
2492
2490
2472
2473
2486
2467
2490
2469
2482


charge = 45 grains, MV = 2487, ES = 20, Sd = 7

2479
2482
2496
2492
2489
2481
2483
2496
2476
2496

Bill the lighter load was more consistent, with different barrel and chamber vibrations. A combination that gave slightly more velocity in that particular gun.Next gun
although same caliber and make, might be opposite...Ed

Did you put the powder in the case the same way both days?

Ed: that's an interesting thought. i'd never considered that. thanks.

Bill: yes i did put the powder in the same each day. since this is a small load, as compared to the case volume, i saw no need for a drop tube. i put the cases in a loading block and set the funnel directly on top of each one. plenty of air space in each case.

Bill first of all I notice that the highest velosity obtained was with the largest charge even though the aveage vel. with the largest charge was lower. The difference was very small as was the sample size.However there are a few possibilities that come to mind. [list]
1.Different primers.
2.Different seating depth.(how far down do u shove that 200gr. bullet?)
3.Powder compression.
4.One group fired more rapidly than the other.
5.Small difference in screen placement.
6.New cases in one goup and used cases in second group.
7.Neck sizing on one vs. FL sizing on other.
There are a plethera of possibilities when you are talking small differences such as this. I hope someone can be of greater help. roger

Roger: thanks for the help. i've been at this sport for some time. i think i can discount all the usual causes of variation. you have a good checklist there, but i think i handled all of these items pretty well. while there may have been some small variation in the time interval for one group versus the other and there may have been some slight difference in the placement of the chrongraph, i always strive to make these things as uniform as possible. like i say, i have been at this activity for many years. that's what makes this so remarkable to me. i have never seen a velocity go up when the powder charge is reduced. and to me a reduction of a full grain represents a large reduction in this instance. this just doesn't make any sense to me. pretty puzzling..

anyway, thanks for the input. i appreciate your help.

Neither the mean or the standard deviation of the two samples are different enough to tell that there is any difference between the two samples. As far as can be determined, the two samples are drawn from the same universe.

There are two explanations that seem likely to me:

1. You have exceeded the highest practical load for your rifle. There comes a point when adding more powder adds to pressure but not to muzzle velocity.

2. It could be a difference in the temperature of the rifle. Chamber temperature has a strong influence on pressure and muzzle velocity. Could that lighter load have been shot in a hotter gun?

Given that the difference between the mean velocity of the two ensembles was 5 FPS, and that the standard deviations of the two ten event ensembles were 11 and 7 FPS respectively, this variation of the mean is statistically insignificant. In a normally distributed ensemble (implied and/or assumed by the way the standard deviation is calculated), about 64% of the events within the ensemble will be less than one standard deviation from the mean. You would have to have ensembles an order of magnitude larger to even start to be able to accurately (accurate? statistics? sorry ...) detect a shift in the mean of the process by this small amount.

In short, I would definitely call this the same load performance. It is a very good performing load, by the way, from a velocity consistency standpoint. If it shoots good, you are done!

Denton: the samples were both shot in the same rifle, under very nearly identical conditions. like i said earlier, the ambient air temperature was close enough that i could not tell a difference. likewise, i shot the samples as close as i could to the same speed. i use this rifle as a single shot, so this also helps to keep the firing rate down.

the first point you make is one i have heard before. i have never seen it happen, but i have heard this thought before. perhaps that is what is actually happening here.

thanks for your insight.

techshhoter: i'd love to respond to your first paragraph, but i'll be damned if i understood it..... i thought an ensemble had something to do with a lady's clothing.... god, but i love bein an Okie.....

however, your second paragrpah even i can grasp. you're right. that 45 grain load is the one for me. i was shooting off of a fence post at a little under 50 yards with the military sights and it grouped just over 2 inches. for a guy like me, that's a pretty good place to stop. there's no tellin what a real marksman could do.

thanks for the help.

Bill, if the numbers are representative, I have a beer that says you can eliminate the problem by seating deeper or crimping the bullet.

I have a 35 Whelen that does the same thing with 3031. My theory is that the powder starts moving too early, and once it is in the barrel, it never gets lit. When I increased the seating depth to a full caliber, the ES went way down. Like down by 2/3rd. A more vigorous primer (a WLR or 210, for example) might be helpful, but you have to increase neck tension. HTH, Dutch.

When I work up a load, I like to prepare two cartridges at each of several pressures, and fire them in ascending order. I track pressure and muzzle velocity vs. powder charge.



With my 308, and AA2520, I started the process, and about halfway through, SMACK!, my MV hit a ceiling and refused to budge upward as powder charge increased.



Since you are carefully holding everything constant except powder charge, it sounds like the same thing may have happened to you. Dutch may very well be right on his solution. I haven't experimented with that, but it makes sense that one or both ideas might work.



To elaborate slightly on the stats thing: The 10 shots you fire are a sample that represents the huge number of rounds you might fire through the rifle. Since it is only a sample, it's going to be a little off, one way or the other. If you shot another 10, you'd get a slightly different answer, but it would be close. When two samples are as close to each other as yours, you can't tell whether there is a real difference, or if it was just "luck of the draw". When the difference between the two samples gets bigger, you can say that it is much more likely a real difference than random chance. Neither your means nor your SD's can be distinguished from each other, i.e., no detectable difference.