Variation in velocity readings - Chronograph

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Walknby

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I'd appreciate some feedback from more experienced reloaders. Are the velocities listed below normal?

I'm loading 147 gn, 9mm with 3.2 gn Vahalla N310 and a COL of 1.120. Yesterday was my first day out with a chronograph (Caldwell Ballistic Precision Chronograph). Here are the readings in FPS.

1. 790
2. 845
3. 857
4. 841
5. 850

Average = 836.60
Standard Deviation is 27.73

The shooting distance was consistent at 12 feet. Specs say shooting distance should be between 10 and 15 feet. I actually stood in the same foot prints.
I'm loading with a Dillon Square Deal.
Before loading I ran ten loads and measured the total at exactly 32.0 gn. When done loading I recheck the press by running 5 totalling 16.0 gn.

I am going to double check the press by weighing several individual loads but do your velocities normally vary that much? It seems excessive but I have nothing to compare my experience to.

Thanks!
 
There is only one shot that is off, being the first one. The other ones are pretty consistent. I suspect it may have to do with a clean cold barrel.
 
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10 shots run over the chrono will lessen the high/ lows and give a better indication of how the load is running

if you have no crimp or very little, a med crimp will also help with the load
with some powder you nead a heavy crimp.... but it is harder on brass
 
pacobillie said:
There is only one shot that is off, being the first one. The other ones are pretty consistent. I suspect it may have to do with a clean cold barrel.
Click to expand...

The cold barrel theory is interesting. I checked other series I shot that day and in 8 of the 9 the first shot was noticeably lower than the others. (These were with loads from 3.10 gn to 3.25 gn.)

Test #. Total Spread / Spread with 1st shot eliminated
1. 51 / 8
2. 67 / 16
3. 27 / 20
4. 53 / 42
5. 57 / 25
7. 32 / 9
8. 54 / 22
9. 26 / 26 (On this one it was the 4th of 5 that was noticeably lower 820, 823, 829, 803, 823).
 
When I was working up Cowboy loads in my 38/357 I found that sometimes the brand of primer makes a difference. I also tested with 5 shots powder forward and 5 shots powder against the primer, to check for position sensitivity. Sometimes that is all a certain combo will give you. I strive for single digit Sd, or a bit over.

My load in 38 Spl brass works best with a CCI 500. When I use the same bullet, and a wee bit more of the same powder in 357 brass, a Winchester SP works best. A change in primer can make a big difference in velocity, app 30 fps in some instances.

It's pretty easy to go through a bunch of powder, primers and bullets in search of a good load.

I have about 1/2 # of AA 452 left, and then I'll start the quest for another load. Not much AA 452 left around, I think it was discontinued before Y2K. Pity, not a good powder for max loads, but the starting and mid loads works well. I hear it also worked for trap/skeet in the 12 ga.
 
You don't say what you are shooting, but since it's 9mm I assume a mag-fed auto pistol. If that's the case, another possibility is first round out of the magazine is different. If you've ever seen ammo testing procedures, they specify how each round is to be lifted from the tray and loaded into the chamber in an effort to keep the powder consistently positioned against the primer.

Easy enough to test for, just load two mags of five rather than one mag of ten.
 
Assuming a semi, how are you letting the slide forward to seat first round? Back in my competition days, a few of the top shooters would pull the slide to the rear instead of thumbing off the catch. Theory was that it replicated the action of the slide during recoil, seating the round more closely to subsequent shots. Maybe didn't really make a difference but can't argue with their shooting.
 
just having some fun said:
can explain 80% of velocity difference .. the other 20% is chance
Click to expand...

That is something else I hadn't considered. Thanks for the info. It will be interesting to repeat the loads with matching cases. I've bumped the load to 3.30 gns (N310) and hope to get out in the next few days. I'll try to do up some matching cases to try as well.
 
yonderin said:
Assuming a semi, how are you letting the slide forward to seat first round? Back in my competition days, a few of the top shooters would pull the slide to the rear instead of thumbing off the catch. Theory was that it replicated the action of the slide during recoil, seating the round more closely to subsequent shots. Maybe didn't really make a difference but can't argue with their shooting.
Click to expand...

I pull the slide to the rear and release.

And I neglected to say earlier it is a semi (M&P 9)
 
I found the first round in each string for my Shadow is also lower than the others, almost every single time. I mentioned this in a different thread and Sean from Freedom Ventures commented that he also found the same thing.

So for reloading consistency, I go by all the values except the first. lol!
 
That works to get an idea of a load consistentcy but in real use we still have to contend with the first shot. Maybe it's one of those things we just need to live with.
 
jethunter said:
That works to get an idea of a load consistentcy but in real use we still have to contend with the first shot. Maybe it's one of those things we just need to live with.
Click to expand...
Yup. I accept the low fps first shot, and use that with a short string to get an idea of how my rounds will chrono at an IPSC match. The testing procedure in the rules seems quite fair, but I make sure my PF is high enough to not have any surprises.
 
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