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The entire time I've been reloading 556/223, I've been getting random, extreme spreads from around 7 fps up to 108 fps. Of course, the 7 fps is no problem, but that 108 fps certainly is. And while that high of a spread is rare, I regularly get spreads above 50 fps, which isn't great. For example, here's a table that shows the ES of 52, five shot charges. Most are 5 charges with 5 shots each but there are a couple with 6 charges. And, the table is going to look a bit odd, because my first 5x5 set was 0.2 gr higher than the subsequent ones. After shooting the first set, the top charge looked a little hot even though it was well under the recommended max.



As you can see, one, that's a lot of shooting and chronoing, two, there is randomness in the velocity spreads, and three, the velocity spreads are way too high. The ES's over 50 fps are highlighted in red. Those over 50 fps make up a whopping 38.5% of the whole - that's way too much and way to significant.

I have tried everything; talked to everybody that reloaded; and finally by picking the brains on Defensive Carry, I got a direction to pursue, and I thank them profusely for their patience, suggestions, and support.

Because of the random nature in the data, it just did not seem to me like this could be a powder brand problem, a charge weight variation problem, a bullet shape or weight variation problem, a primer problem, and because it has happened across various ARs and bolt guns, it isn't a gun problem.

For the longest time, I believed it was something I was doing wrong or not doing when reloading. However, my reloading was precise. It didn't make sense to me that I could reload so precisely and still get ES's of 50+ fps. Just for the record, here's how I reload.

Starting with clean cases:
  • Size the cases
  • measure overall length, trim if over 1.755", discard if under 1.750"
  • prime: if pocket is loose, decap and discard case; replace the case
  • clean the lube off all cases
  • chamfer inside and out
  • charge the cases: dispense on Intellidropper, confirm on beam balance, adjust until BB zeros - every charge!
  • seat the bullets: seat, then measure the CBTO of every round, adjust until within +/-0.001"!
  • for ARs: crimp
  • for bolt guns - no crimp
I have literally loaded thousands of rounds this way and chrono'd every one of them. I've tried different powders, different cases, different bullets, different primers, and still the high ES's persist.

Then in a thread on Defensive Carry we were discussing the problem and the idea of neck tension variation came up with regard to crimping and that started the gears turning. It would turn out that neck tension variation was not the problem, but it would lead to identifying the problem.

Needless to say, it would be quite difficult and expensive to be able to measure neck tension, maybe very impractical also. So for lack of a better approach, I turned to QuickLoad. Of course QL is theoretical but it has proven to be pretty reliable and usefully accurate. I tested the effect of neck tension by changing what QL labels Shot Start (initiation) Pressure. That's probably not quite the same as neck tension and crimp pressure, but it should be close. The default value is 3626 psi and with 24.2 gr of A2520 the predicted velocity is 2678 fps. If I double the Shot Start Pressure to 7252 psi, the velocity changes to 2728 fps. That's a change in velocity of 58 fps.

To me, it seems reasonable that neck tension can and does vary. Of course we have no way to know if it doubles, but if one case had a slightly thinner wall, and was a little more supple, and another case had a slightly thicker wall and was stiffer maybe the doubled neck tension would be reasonable. Even if it doesn't double, lesser variations coupled with other issues could have an adding/subtracting effect and generate wide variances. Sounds good anyway! :tongue:

Another thing that makes neck tension suspect is the random nature of the spreads. Notice in the table above, the spreads greater than 50 fps are all over the place. And they are unpredictable, i.e. random in nature. I would think varying neck properties would show up as random, i.e. there is certainly no reason to believe the neck tensions would be in a predictable order.

I began to look at neck tension variations coupled with case volume variations. As a starting point I doubled the neck tension (using QL) and found that a 50% change in neck tension can cause a 58 fps change in velocity. Next, I changed the case volumes. I needed a starting point and since I had previously weighed 100 Hornady cases, I used the results of that. The weight varied from 89.7 to 99.1, that's 9.4 grains, or a percentage variation of 10.4%. While I think that's awful, I have seen 223 Lapua cases vary by similar amounts. So based on 10% weight variation, I changed the case capacity by 10% and found this:

case vol.......vel
30.0 gr.......2738 fps

and 10% more volume
33.0 gr......2605 fps

That's a change in velocity of 133 fps. The highest velocity variation I've seen with my reloads is 108 fps.

So now let's combine the neck tension and case volume and see what possibilities we get. I'll let the neck tension vary by 20% and the case volume change by 2 gr. That's about a 3% change in case volume. We could have velocity variations as shown in the table below - and everything in between! Here's the numbers:



Notice this accounts for the randomness and the max spread is 109 fps, remember me saying I had seen velocity variation as high as 108 fps? The low spread is 11 fps.

If we look at the entries in the table where one parameter changes and the other doesn't, for example row 1 and 2, we see there's a 98 fps change due to case volume change alone.

If we look at rows 1 and 3 where the case volume remains the same but the neck tension changes by 20%, we only get a change of 11 fps.

So a case volume change of 6.7% generates a velocity change of 3.7%. OTOH, a neck tension change of 20% only causes a velocity change of 0.4% (less than 1%).

I'm not sure any of this will hold up in court, but unless you've got a better idea that explains the velocity variation and randomness, this is my story and I'm stickin' to it! :tongue:
 
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