results with a cronograph

[Kevin Stenberg]

Yesterday i did a ladder test on a 223 rifle. I was testing Varget and some 62 gr. jacketed bullets. All charges were hand weight. Loading was done in 1 sitting, in sequence, and all loaded the same way (to the best of my ability)
I had 2 test groups. 1 loaded to feed from my AR magazine. Their loal was 2.188, which was in the middle of the canalure. The other group was loaded to chamber length for my Handi 2.283. All loads were shot in my H&R Handi.
My test was not to test group 1 against group 2. But to see the speed increase of each powder increase within the group.
Group 1 powder weight 21.7 to 24.9 in .2 grain incramants. Group 2 powder weight was 23.5 to 24.9 also in the .2 gr. incramants.
The question i had was concerning an uneven rise in velocity. It happened 3 times in group 1. An not at all in group 2. After thinking about it for a while. I am thinking the velocity fluctuation was simply Standard Deviation rearing its ugly head.
An example is shot 1 at 21.7 grains powder gave velocity of 2428 fps. Shot 2 at 21.9 g.p. was 2383 fps. Shot 3 at 22.1 g.p. was 2415 fps. Shot 4 was 22.3 g.p. with a velocity of 2470 fps.
Would this lot of 4 shots be a node?

 

[Rick]

How many shots with each charge weight? A rule of thumb is a 50 fps velocity increase for each 0.5 gr charge increase in mid capacity cases. That's not set in cement, it could be 40 fps or 75 fps but overall 50 is a decent average. When working up loads with a given powder I stop increasing the charge with the first group that doesn't increase the velocity by that average 50 fps. That's where that powder/bullet/cartridge has reached and or passed where it will work best. Most times not at max pressure but at the practical limit of that combination.

When working up accuracy loads I never chrono less than 10 shots of each load and that is only a reasonable idea of the uniformity of that ammo.

The standard deviation is the number which describes uniformity. The smaller the number, the more uniform the velocity. A standard deviation of zero means every velocity was the same. A standard deviation of 32 means that 2/3 of the individual velocities should be within 32 fps of the average. Ken Ohler

I don't know what you mean by "Standard Deviation rearing its ugly head". All groups have a Standard Deviation even if it's zero.

 

[Brad]

For ladder testing I don’t look as much at velocity as I do point of impact at a longer range. I want to find a powder where a .2-.4 gr change doesn’t give any elevation change.
This is best done at 300 yards or so if
possible.

 

[waco]

We have been having excellent results with the Scott Satterle ladder test.
No shots on paper. Just chrony work.
It just plain works.
Google it.

 

[Kevin Stenberg]

Rick Each powder load had only 1 round of ammunition made for it.
On the 4 shots i had the question about. Am i wrong in my definition of SD. It is the difference in speed of a group of shots with the same quantity of powder in each shot. The difference is calculated from the highest to the lowest. which gives you the SD.
I was thinking the 4 shots had the same affect on the bullets but displayed the definition of SD
Side note the same 4 shots on paper gave the largest group size of all the rounds shot.

 

[Rick]

Rick Each powder load had only 1 round of ammunition made for it.
On the 4 shots i had the question about. Am i wrong in my definition of SD. It is the difference in speed of a group of shots with the same quantity of powder in each shot. The difference is calculated from the highest to the lowest. which gives you the SD.
I was thinking the 4 shots had the same affect on the bullets but displayed the definition of SD
Side note the same 4 shots on paper gave the largest group size of all the rounds shot.

The standard deviation is the number which describes uniformity. The smaller the number, the more uniform the velocity. A standard deviation of zero means every velocity was the same. A standard deviation of 32 means that 2/3 of the individual velocities should be within 32 fps of the average. Ken Ohler

With one shot fired of each load your Standard Deviation would be zero. So if I am understanding you correctly your trying to get an SD from several different loads combined? Maybe I'm just confused about what your trying to do. A 10 shot chrono test of all the same load would give you only a fair idea of velocity uniformity, if those results are favorable that load should be re-tested several times to have a reasonable idea of uniformity. As to uniformity the only thing a one shot group will tell you is that yep, it went bang. Even velocity of one shot will not tell you with any certainty what the velocity really is within 50 fps or more.

 

[Winelover]

I was about to say the same thing. I usually shoot 10 shot groups, especially with cast. I don't worry too much about velocities, until I get a decent group, I can live with. With semi-auto's, if it wont cycle and feed it's pretty useless. IMO. So most starting loads are questionable.......I'll start somewhere in the middle and then go up or down, from there.

 

[Winelover]

Don't get too hung up on SD. I have had SD's in the single digits, only to have shotgun pattern groups. There are a lot of variables to contend with.................lube choice, COAL, crimp tension, primer selection, brass manufacture and prep, ambient temperature, alloy selection, GC vs PB. Probably, missed a few.