Home Forums Hunting & Shooting Ask The Gunwriters Losing velocity, it's not the chrono...any ideas???

So after a shooting session with my 260 rem, I thought I had a chrono problem because velocities on known loads were suddently coming back 60-80 fps slower, and many folks here, and especially the folks at Oehler, were super helpful. Without rehashing all the details, it's not the chrono.

After Oehler worked over my 35p, I went out and shot again today and I probably have MORE problems than less, but not because of the Chrono.

I shot probably 50-75 rounds of 22 over it in strings of 5, 10 and 15, and it seems to match what I would expect according to the box. CCI minimags, going mid 1200's. There was considerable variation in a few of the velocities, but I'd expect that from 22's and the average was probably 1250, IIRC.

In addition, I shot the 260 again. And again, it was about 80 fps slower than previous shooting sessions had established, just as it was at the beginning of this week.

But this time, I had found an additional box of loads that I had loaded up last summer when I established load data. It was from the same batch I had chrono'd before and set my expected velocities. I brought those along and shot those along side the more recent loads. Lo and behold, right where they usto be. Perfect velocities from the stuff from last summer. So two, supposedly, identical loads, consistently 60-80fps off, shot at the same time, same gun, back and forth over the same chrono.

So I reserved some of both batches and took them home and pulled the bullets and weighed the powders against each other. The weights were the same. Some were dead on, some a little low, which I probably lost a little or something in the pulling, but the distribution/average was identical between both batches. So it's not a powder measure mistake or problem.

Checked overall lengths. Check.

The only things I could find are the following:
1) The faster (older) stuff was a LOT harder to pull the bullets. I actually cracked my front door threshold hammering them out. Neck tension must be higher, a lot higher. How this is possible, I don't know as the same seat die was used (Forster Ultra), and no adjustment was made. But maybe neck tension comes from the sizing die...I can't remember...but that's important because...
2) All the brass is Lapua from the same box, but the faster stuff was previously fired in my rifle and barely partially resized to just push the shoulder back 15 thousandths on a forster benchrest FL die.
The slower stuff, I had not used previously. I did not size it, as it came sized and chambered just fine, I just primed, powdered and seated.
3) It is possible that maybe I used a different lot of powder. I thought that both sets came from the same can, but maybe I ran out earlier than I remember. But there is no way I can verify this and is speculative.

So now I am on to a new problem. Next step will be to load up the cases that I pulled the bullets from and see if the new 'sized by lapua' stuff vs the old 'sized by forster' stuff ends up being the answer.

I wonder if I need to resize them? Or should I just powder them seat them and send them?

Any other thoughts on what could cause this problem?

Thanks!

Fired brass chronographs faster.

The first time you fire it some of the energy is absorbed ironing the crinkles out of it.

That's true, but the difference wouldn't be 80 fps.

I suspect neck tension is the main problem. Even if new brass will hold bullets, I run the necks over an expander ball of the correct diameter to even out neck tension.

That variation could be caused by:

- Cold weld between the bullet and case neck increases pressure/velocity slightly
- Formed brass
- Different lots of powder

Pretty serious STRESS goin' down here ( particularly fro the Oheler outfit, it would seem).

Did the POI shift appreciably ?

Custom barrel / match chamber ?

....or Rems notoriously capacious hole in front of the bolt for this particular cartridge ?



...no way THAT might affect anything,....

Okay, so looks like neck tension/new brass is the likely culprit. I can test that, no problem.

So what is the best way to do that. Can I just take the cases I just pulled the bullets from and repowder and reseat? Or will the seating, pulling and reseating have screwed up the neck tension so that I need to resize entirely?

I'd like to just powder and reseat so that very little is changed from cases that I know worked in the past, but if neck tension is the issue, I don't know if pulling the bullets ruins that.

Neck tension is set by the sizing die, not the seater. Neck tension was way higher on the old loads because of a cold weld forming between the bullet and case neck.

Neck tension isn't "set" by the seater. It depends on how much the neck is sized, and how soft/hard the neck is.

What about outside temp? Extreme variances in weather temps when you shot over your chrono may cause this. Just a thought. Some powders are more temp sensitive than others.

Thanks for all the advice! Responses are coming in faster than I can read them, so I had to go back and edit.

So 1, what is a cold weld?

2, John, how would you go about testing this to get to the bottom of it quickly?

Thanks!

They were tested on the same day, time, going back and forth, so temp etc. is all the same.

If you are losing powder when you pull bullets, you are doing something wrong right there.

I had not planned to speculate in this thread since WE already discussed the O 35 and Cold Weld in the "Sticky Bullet" thread.

However this statement from crossfire tells me a lot.



Sorry crossfire - but lot/lot variation CERTAINLY can produce 60-80 fps variation. Sometimes even MORE.

I see your 'whistle' icon so maybe you're being sarcastic in a friendly way ?

Yes - IF you changed lots of powder that 'could'be the cause. Over the yrs I've seen a whole lot of variation from Lot to Lot. Some is almost insignificant, some extreme.

Others have already mentioned neck tension AND/OR cold weld. Both are worth checking.

catorres1 - I know you have been occupied and so IF you haven't read the "Sticky Bullet" thread in this forum, it might be worth your time.

Jerry

We NEED a "friendly Sarcasm" icon / smiley,...whatever ya' call them.

I'm more than hip to lot to lot variations, Sir, and was joshing for sure.

GTC

Since I didn't know for sure, I thot you were AND

I was hoping you were.

Jerry

You are right, I have not read the sticky bullet thread. I'll check that out, thanks for the tip!

John,

would something similar to the Lee Factory Crimp Die help to even out the release of the bullet from the neck on firing?

Just a thought, as those already seated bullets of both batches could be run through a FC die, no?

Geno

As a point of reference I have some 223 brass. Nickle plated Remington brass. Once fired I get 3050fps, using everything the same in New brass I get 2950fps.

The fact that the older ammo clocks the same now as it did last summer sort of eliminates seasonal variations.

I don't think we;d see a cold weld situation in just a few months,given normal ammo storage would we?

My first hunch is different powder lot, but the OP has not told us unless I missed it.....Are the new and old loads from the same powder lot, or not?

It would be no trick at all to see that much velocity loss from one lot to another,depending on the powder.

Are you really pushing the shoulders back 15 thousandths?