Home Forums Hunting & Shooting Ask The Gunwriters Very strange morning shooting, re: Loads, chrono velocities. Opinions please.

Load testing my Jarrett 280 AI this morning with Nosler 150 gr Long Range Accubonds and got some crazy results. Earlier I had tried RL-22, H4831SC and 7828 to gauge its preferences. H4831SC gave me the best results. Now, on to load development, and here's where things got crazy.

All things were kept consistent for comparison sakes: same lot of Nosler custom brass, Fed210M primers, all seated to same length, etc. The only variable was powder charge, and here are my chronographed results:

Charge Velocity Ext. Spread Std Dev. Group Size
61.0 2923 39 19 1"
61.2 2902 13 0.8 0.23
61.5 2898 34 69 1.25
61.7 2898 50 27 1.25

Hoping to get about 3000 fps out of the H4831SC and was very encouraged by the 61.2 grain loading but the velocity readings left me scratching my head. I've never seen this in my 57 years.

I have heard LRAB's are somewhat sensitive to seating depth....have no idea if a different seating depth would impact things in a more linear manner.

Your thoughts??? Y'all have a great 4th.

I'd run 61.2 in that COAL and not give a damn about any of the rest.

How many shots fired with each powder charge, and what kind of chronograph?

5 shot groups. Pro Chrono chronograph...had it about 4 years and it's never given me a problem.

I sure can't explain it. I'll leave this interpretation to the experts.

I have an idea what the problem might be. If you will post or send me your raw MV data vs. charge, I'll check it for you.

Please do. I've experienced velocity plateauing at or a little bit above max charge (book, taking listed/actual velocity into account) and then increasing. Best accuracy was at or approaching the beginning of the plateau which was a safe if hot charge so I stopped there. Actually load development was for a safe and effective hunting round with emphasis on safe, a little more velocity wasn't important. Didn't investigate further for a lack of time and instrumentation, had a theory but it escapes me now.

Or maybe what may be Mule Deer's thought, insufficient sample size. Sure looked like a trend repeating over two or more sessions but I may be fooling myself.

Denton if possible it would be interesting to hear your thoughts.

I'm no statistician and I didn't stay in a Holiday Inn Express last night . It seems to me that with an average extreme spread of about 34 fps with a charge variation of only 0.7 grain at the 60+ grain level, the lower velocities at the higher charge weights might be within the expected variation.

That would also be my guess.

While ProChronos are good units for the money, the screen spacing is only a foot, which means the speed of each shot isn't exact. Plus, five rounds of each powder charge ain't much of a sample.

Exactly.

With the raw data, it would only take a minute to run ANOVA and find out if there is convincing evidence of real change. I'm skeptical that there is.

Thanks so much for your input folks...much appreciated.

Too little difference in powder weight and too few rounds fired. Kinda like sloppily throwing a powder charge from a powder thrower with the charge weights weights having + or - a half grain of powder either way. Much like cronographing factory loads. For all intensive purposes they may as well be mixed together and called the same load.

For all intents and purposes, I would think for all intensive purposes you would want more uniformity than that...

Let me guess,

You have a short barreled rifle?

It wouldn't have to be very short. Nosler lists a maximum velocity for 150's and 60.5 H4831SC of 2994 fps--in a 26" barrel. The 150 ABLR has a shorter bearing surface than any other Nosler 150, so would produce less pressure and velocity.

Once again I'll comment that 5 shots per powder charge is too little to come to any conclusions. I wouldn't even bother figuring standard deviation with only 5 rounds, especially in a short chronograph at an open-air range. SD only starts to have some validity after at least 10 rounds under very controlled conditions.

Have also seen way too many single "first groups." whether 3-shot or 5-shot, that couldn't be reproduced after firing several groups.

John, I've seen this same phenoneme before with H4831, 130gr bullets and short barreled .270 Win. After a point, it just didn't matter how much powder was poured to it, the velocity would not rise, but fall of slightly. In the longer barrel rifles velocity continued to rise.

Interesting. I’ve had a couple short-barreled .270’s (20” and 21”), and in both the velocity kept rising as I added more H4831. Easily got 3000+ with 13- Hornady Interlocks in the 20” barrel with 60.0 grains.

One thing Phil Sharpe recorded when doing velocity experiments while shortening barrels and chronographing the same load in each one was sometimes the velocity was HIGHER when he shortened the barrel an inch. He shot long strings so it wasn’t due to too few shots. His guess was the inch he’d just cut off included a loose spot in the bore.

John, I've seen shorter barrels out perform longer barrel at specific charge weights as well.

In one example we had .270's with 22, 24, and 26: barrels.

In the 24" with 150gr bullet, if we backed the loads off, the 22" would give higher velocities, but at max, the 24" once again delivered higher velocities.

Usually these test you refer to are run at a fixed powder charge, so the experimenter failed to notice some of these dynamics.

Interesting you mentioned H4831. I was using H4831SC in a 6mm Rem. with 100 grain Sierras, 24 inch barrel. Wish I still had all the data. Still, there wasn't enough for more than a tantalizing tease, could have been an illusion.

I just recently had some similar data results with large spreads while shooting thru a Chrony with light shields in the mid morning. I believe the issue was with the shields not staying secured and sun light coming thru some trees. Did not have enough ammo to do any experimenting with the chrony.

I have seen loads run out of gas with 4831 as the charge went up also.

As far as I am concerned, 5 data readings are adequate to tell you something. Statistics are a good tool, but going to the nth degree never was meaningful to me when I ran manufacturing or with chronographs at the range. You don't have to go to three decimal places to know what is going on.

What does the data from the original post show for ave vel when you throw out the high and low of each 5 shot string????

What happens when the experiment is repeated????

If your equipment is not that repeatable/exact (scale, chrony,) then I personally would have little use for more data.

Just me, and I am just a hobbyist/hunter shooter.
Tim