9mm - Large variances in FPS

[CZinSC]

For the last month or so, I’m getting large variances in velocity. Anywhere from 1065 down to 979. The majority of the rounds if I shoot 30, are OK, it's only a handful of the rounds that are way off. Overall the Standard Deviations are averaging around 15-17 which isn’t so bad, but the low end numbers are what has me banging my head against the reloading bench. I’ve never had this issue before. I’ve always had very consistent loads with all of the same exact components and load. These loads are for shooting Minor PF, so the lowest acceptable is 1008 fps, but I like to have them average in the 1030-1040 range. So 980 is nowhere near acceptable.

• CZ SP01 – 25K rounds through it ( the gun isn't the issue though, i have confirmed the same results in two other guns )
  
• 4.32 gns of N320
  
• MG 124 FMJ, 1.145” OAL
  
• WSP, Mixed Brass
  
• Was using the 2X Powder Hopper and Baffle from Uniquetek, plus Micrometer bar. I have since gone back to the stock powder hopper to see if that was the issue. Still using the micrometer.
  

I have done the following trying to fix the issue:

• Disassemble and cleaned all of the dies
  
• Polished the powder hopper
  
• Changed powder, ( three separate lots of N320 )
  
• Changed primers, both WSP and CCI get same result
  
• Changed batteries on the Chronograph…a couple of times
  
• Used chronograph in Sun and Shade
  
• Called Dillon, they were no help, had no ideas.
  

The only other thing I can think of is the temperature. I’ve been shooting at around 50 degrees, and the reloading is all done in the garage where the temp is routinely in the 40’s. However, I’ve always heard that N320 is not temp sensitive.

I am at my wits end. I'm shooting the FL Open next month and I can’t get my loads to chrono to the point where I am comfortable going to a major match with them.

Anything I haven’t done that you can think of???

Help! Please.

[Sarge]

Stop using mixed brass! It really does make a difference. Load 50 or so of the same headstamps and try again.

Black sharpie on those shiny golden bullets.

All the work you put into the press, do you know for a fact the powder charges are off? I know it's tedious but run about a hundred charges and weigh every one of them. If they are within a 10th then start looking at the chrono end of things. Try shooting smaller groups to keep the barrel cool. As the barrel heats up in cold weather things might be changing? Keep track of when the readings change. is it always the first few, last few?

[DWFAN]

Try a buddies chrono as well. Fresh batteries in your chrono maybe?

Sent from Tapatalk, pardon the errors

[GrumpyOne]

Mixed brass.... No doubt... It doesn't matter what temp it was when you loaded them, or really what temp it was when you fired them (unless a few were kept in the freezer before you shot them). Bump up your load a couple of tenths if you are that worried. You won't feel the difference from your present load to a few tenths more....

[polizei1]

Most likely it's either because of the mixed brass, OAL variation, or both. Since I'm reloading mixed brass and my OAL varied, I was getting a SD up in the 50's. I know, absolutely terrible, but that's the price you pay. What I did was load more powder and jump the velocity up to where I was getting 135pf.

Just a suggestion.

[Action Pistolero]

Mixed brass is more than likely the problem. Case capacities vary from man. to man. Overall length of those cases varies also. This changes the amount of crimp on each round which changes pressure and therefore velocity.

Throw ten charges with the powder measure and weigh them all togerther. They should be close to 43.2 grains. Since you have the micrometer charge bar, it is easy to get the powder charge damn near perfect.

[CZinSC]

Well, loaded another batch. All Win Cases. Was very careful, very smooth strokes on the press. Ran the rounds over my buddy's chrono. Same shit. Velocities from 950 to 1060. That rules out my Chrono being the problem, and the mixed brass.

Going to go to my buddy's house this week and load on his 650 with my components. Should be a good test on whether or not it is my press/dies or not causing the problems.

[Hi-Power Jack]

I'd bump up the grains of N320 - your lowest loads are 121.5 PF - just bump them

up so they are 125.5, and you're set to go.

How's the accuracy of your current loads?

[Guy Neill]

To check for the probability of OAL variations, load up, say 10, rounds and number them. Measure the OAL of each and record the results.

Then, at the range, shoot a round such that the first numbered round is fed into the chamber. Remove it and re-measure the overall length. Do this with with each of the numbered, measured rounds.

If you see notable shortening of the rounds from chambering, it is likely a major portion of your problem.

If you do see variations as a result of the chambering, check both your sizing die and expander. If the cases have thin walls, they may not be being sized sufficiently. Also, a smaller expander ball (assuming the sizer is adequate) may need to be smaller to secure the bullets. Normally we expect the expander ball to be about 0.002" smaller than the bullet diameter (you may want to check a number of the bullets for diameter as well, to see if they are varying).

Changes in OAL with the 9mm have tremendous changes in the operating pressure because of the small case volume.

Keep us posted.

Guy

[GrumpyOne]

Don't discount the brass either. How many times had the Win brass been fired that you tested the second time? Brass that has only been fired once will chrono different than brass that has been fired 10 times.

[captray]

As I remember my statistics ....

+ or - 1 SD = 67%

+ or - 2 SD = 95%

+ or - 3 SD = 99.5%

So in order to be 99.5% sure you pass the chrono stage you need a median fps of 1056 (1008 + 3 x the SD of 16).

Bump up the charge for the FL Open and try to lower your SD in the future. Although, I agree 16 ain't bad.

Ray