velocity consistency

sam menard

sam menard

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I've been reloading for a couple of years now. I bought a Hornady Lock 'n Load kit and didn't trust the scale so I bought a Lyman Accu-Touch 2000 scale. I also have an F1 Shooting Chrony. The load that I've settled on is as follows: 180 gr. Nosler Partiton(PPT), Winchester 760 powder and Remington large rifle primers. The load that I`ve worked up consists of 55.5 grains of powder. Using the chrony at the range, my velocities can range between 2750 - 2800 fps. Is the variation normal? I try to be super careful when I`m weighing out the charge, often pulling bullets and re-weighing the powder. Consistency has improved since I purchased the new scale, but I reckon that velocies should be tighter. What kind of velocity consistency is considered normal? Thanks.
 
That's not a bad deviation. Many little thing's will factor in. Neck tension, internal variance's, bullet imperfection's all add up
 
Two things to try:

Try a Winchester Large Rifle primer, and try a Magnum primer. Although the Rem primer is hot, maybe a different primer would help.

And when using the Chrony, aim at something on the far side, so the path of each bullet across the screens is identical. Different paths will give you a 15 fps difference. The clock speed of the Chrony is very high, so tiny variations will register.
 
I think the answer is "depends", depends on how big of sample you took. 2750-2800 is the Extreme Spread, but more meaningful would be the Standard Deviation. do 5 or 10-shot group as Ganderite suggested and get the SD calculated(there are online calculators), the lower the SD, the better.

sam menard said:
I've been reloading for a couple of years now. I bought a Hornady Lock 'n Load kit and didn't trust the scale so I bought a Lyman Accu-Touch 2000 scale. I also have an F1 Shooting Chrony. The load that I've settled on is as follows: 180 gr. Nosler Partiton(PPT), Winchester 760 powder and Remington large rifle primers. The load that I`ve worked up consists of 55.5 grains of powder. Using the chrony at the range, my velocities can range between 2750 - 2800 fps. Is the variation normal? I try to be super careful when I`m weighing out the charge, often pulling bullets and re-weighing the powder. Consistency has improved since I purchased the new scale, but I reckon that velocies should be tighter. What kind of velocity consistency is considered normal? Thanks.
Click to expand...
 
The lower the SD the better, so far as the ammo is concerned.

HOWEVER, the reason you are looking at SD is probably related to accuracy. That is rifle dependent. The low SD ammo might be great in one rifle and terrible in another.

The barrel vibrates. It is important that the bullet exits while the muzzle is flipping up (known as positive compensation). Since the rounds come out and different velocities, the slower ones tend to print lower, making a vertical string. If you have positive compensation, the slower shots exit with the muzzle aimed slightly higher, offsetting the effect of the slower velocity.

If the barrel has negative compensation, the velocity differences are exaggerated.

Your barrel might be "positive" around 2700 fps, and "negative" around 2800 fps. So as you experiment with more or less powder, you may be improving the SD, but moving into the range of "negative" compensation.

As so many have posted here, the proof of the ammo is shooting it at the appropriate range. 100 yard groups and a Chrony don't mean much. the 2700 load with the SD of 11 might shoot much worse than the 2750 load with an SD of 19.

I tested 308 ammo with a rail gun at 525 yards and off sandbags with a 24X scope at 1000 yards. velocity and SD were just notes that had little to do with load choices.

525 yard test group (20 shots)

RAILGUN308AT525.jpg


About 1 3/4" vertical
 
Under most circumstances, the chrony is only good for a rough idea of your FPS, and that's fine. If you're trying to get single digits, not the unit for it. I now tend to think its better to just spend the $ on better reloading equipment anyhow and see what it says on paper.
 
50fps, if it's standard deviation then it's a lot. If it's difference between extremes (e.g.most ammos around 2750 but 2 extremes at 2750 and 2800) then it's pretty good. Keep in mind the chrony itself isn't perfect, and if you were to place 2 chronies of the same brand/model one behind the other, they would register a slightly different speed, so you have a measurement imprecision right there.

If you want to know what's "good" and what's "bad", get 2 boxes of ammos, one the cheapest you can find, and one match grade, and chrony them. Normally, the cheap one should have less accuracy and higher std dev on velocity.
 
Good read below, found it at Accurate Shooter then read the complete test at targetshooter.co.uk.

I also agree with Ganderite on trying a magnum primer with your double base powder that has more deterrent coatings than single base powders.


LR Primer Types Tested for Velocity, ES/SD, Group Size and More!
http://bulletin.accurateshooter.com/2015/06/lr-primer-types-tested-for-velocity-essd-group-size-and-more/

Large Rifle Primer Performance by Laurie Holland
http://www.targetshooter.co.uk/?p=1471
 
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