Losing velocity...it's not the chrono..any ideas

[catorres1]

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!

 

[gunners]

Try not to change to many variables at once and get yourself a notebook so you can keep track of what's going on and when. I'd resize only first then once all the cases are exactly the same you can then draw a fair comparison.

 

[Russbouv]

Try not to change to many variables at once and get yourself a notebook so you can keep track of what's going on and when. I'd resize only first then once all the cases are exactly the same you can then draw a fair comparison.

I agree with gunner. Seems to me you have a few variables there. Also another thing that I picked up on is your bumping the shoulder back 15 thousands. That's a lot !! Are the new unsized brass bumped back the same ???? I believe you only need to bump the shoulder back 2-3 thousands from a measured fire formed case.

 

[HARPERC]

How many reloads on the brass? Are you annealing necks? I think you're on the right track with bullet pull.

 

[Pdvdh]

1) Resize all cases using the same technique/dies, prior to seating any bullets;
2) Lube the insides of all of the case necks with Imperial Sizing Wax on a Q-Tip, and then remove the excess with a dry Q-Tip prior to powder charging and seating the bullets. I recently pulled close to 1000 bullets from some .223 Remington handloaded cartridges. Some of the case necks were lubed with Imperial wax and some weren't. The variance in the force required to pull the bullets from the waxed case necks was MUCH less than the variance in pulling the bullets from the un-waxed case necks. Increased case neck tension (or anything else that increases the force required to cause initial bullet movement in the chamber/bore) will increase pressure and muzzle velocity, all other factors held constant. During this process I also experienced a HUGE sensed difference in case neck tension between cases with case necks/shoulders that had been annealed prior to seating the bullets, and case necks/shoulders that had not been annealed prior to seating the bullets. Lapua brass is annealed from the factory, so this isn't likely to be the source of your high ES / SD. Unless you've fired and resized the original use cases a number of times, and then load and compare MV to new virgin brass casings. Higher case neck tension = higher MV. Based on the description you've provided, I believe that following these first two "standard practices" will eliminate the majority of your ES / SD in muzzle velocity;
3) Make sure all bullets are seated to the same depth in the cartridge cases. Especially make certain that bullets aren't seated to variable depths such that some are touching the lands of the rifling when chambered, and some bullets are seated deeper and are not touching the lands of the rifling. The difference between these two seating depths can cause a big difference in MV. I like to keep bullets a minimum of ~0.010" off the rifling if I'm not jamming bullets into the rifling, or a minimum of ~0.010" into the rifling if I'm jamming them. I don't like to take the risk that some bullets are touching the lands, and some aren't, or are only barely jammed;
4) Make sure you've used the same primers in all of your loads. This sounds simple enough, but if you have a variety of primers in your reloading room, stuff like this has been known to happen. Don't ask how I know why Magnum primers or another brand of primers with a greater/hotter "spark" will increase MV;
5) Use the same powder, from the same Lot of powder, whenever possible. When changing Lots of powder, don't be surprised if you realize some change in muzzle velocity;
6) Weigh your cases to ensure they're approximately the same weight. This shouldn't be much of the problem if you're using Lapua brass and they're all from the same box. But it doesn't take long to do this, and it's helped me identify the source of variable MV in the past. In one extreme instance, I once measured a casing that weighed notably more than the others. After scratching some hair off my head, I finally looked inside the case and found a curly cued string of brass inside the casing, which accounted for the extra weight. Looked like a shoe string French fry - the curly type. Casing weight is an indication of the internal case volume. Higher case weight will typically have lesser internal volume, causing higher pressure and higher velocity, compared to the identical load in a lighter weight casing with greater internal volume;
7) Make sure your bore is fouled before recording MV for your records. A clean bore will almost always result in a differing MV compared to the same load in the same rifle after the bore is fouled with at least one shot. Sometimes it takes two rounds fired down the bore for MV to stabilize, but a first fouling shot eliminates the majority of the difference; [PS: This obviously means your bore should be fouled before you begin your hunt - if you're relying on fouled bore MV in your ballistics program.]
8) Make sure that when you're chronographing MV, that the temperature of your ammunition AND rifle bore is about the same, in order to rule out temperature caused variations in MV. Higher the temperature the higher the MV. If you're comparing muzzle velocity on a day when the ambient temperature is 20F higher or lower, you could expect some difference in MV - all other factors held constant. Also, as the bore heats up with additional repetitive shots fired, I have often seen MV increase/decrease compared to the first shot or two fired down the ambient temperature bore. When I record MV versus temperature for entry of MV info. into my ballistic program, I try to ensure the the bore and ammo return to ambient temperature prior to firing the next shot over the chronographs;
9) Make certain all skyscreens for your chronograph are in the same intensity of ambient light. Biggest example is do not have one skyscreen in the shade while the other two are exposed to direct sunlight.

These are the MV affecting factors that come to mind. There may be others that I've not experienced, or that I've not remembered to include at this moment in time. However based on the information you provided in your original post, these steps are the ones that came to mind.

Again, I think if you incorporate the first two items above into your reloading regime, that you'll find that the majority of your difference in MV will be resolved - based on the information you've provided. The Imperial Sizing Die Wax will help minimize the differences in the forces holding the bullet in the case neck no matter all the other items that could cause variable case neck-caused bullet release force in your reloaded ammunition. I didn't include this as a mandatory step in my reloading regiment until the experiences and knowledge gleaned form pulling 1000 bullets from differing brands of cartridge cases, with case necks prepped somewhat differently among the various reloaded cartridges.

Hope this is useful, and let us know what you eventually determine to be the source of your high difference in MVs between your prior reloads, and the "new" ones in the virgin Lapua cases. It's good to learn from others' experiences.

 

[catorres1]

I agree with gunner. Seems to me you have a few variables there. Also another thing that I picked up on is your bumping the shoulder back 15 thousands. That's a lot !! Are the new unsized brass bumped back the same ???? I believe you only need to bump the shoulder back 2-3 thousands from a measured fire formed case.

Sorry, meant ten thousandths!

 

[catorres1]

Gentlemen,

Thanks for the advice. I hope to get out there next week some time to try and resolve this. Basically, I plan the following:

Newer (slow) brass that I pulled bullets from but never fired...I'll reload those without sizing, just powder bullet shoot

Older (fast) brass that I pulled bullets from sized but not since pulling the bullets...I'll reload those without sizing, just powder bullet shoot

Newer, but now once fired brass...I'll reload these through the whole process
Older brass that was fired....I'll reload these through the whole process

That should give me a good data set.

Next up, I may try the imperial lubricant as Phorwraith suggested to see how equalizing the tension affects things.

But I figure if I go through all of the above and get the same low numbers...then it has to be powder lots. I must have used a different lots in my original loadup, and I'll have to try and compensate. That will suck.