Velocity difference?

[SubVet]

Question? I am a long distance F Class TR shooter, both 223 and 308. In load development for the 223, I ran across something that I do not fully understand, yet .....

Tikka T3x Sporter with a 31" Bartlein heavy palma 1 in 7 twist barrel, 0.169" freebore. I am loading Berger 85.5gr long range hybrid target bullets (sorted for weight) with Lapua Match brass (new), FGGM primers and Varget powder measured very accurately to +/- .02 grains. Use expanding mandrels for neck sizing, thus neck tension. Loaded 40 rounds exactly the same, after full length resizing and mandrels, I noticed 10 of the rounds required a bit more pressure to seat the bullets (thus slightly more neck tension) than the other 30 rounds.

Went to the range to shoot 4 ten round groups at 200 yards. Accuracy was what I expected, all four groups less than 3/4". But, the velocities were interesting. The one series was the slightly higher neck tension at 2730 fps average and all of the three lower neck tension groups were at 2710 fps average. The ES average for all 10 shot groups was under 8 fps as well as the SD average being less than 4.0. LabRadar was used for velocity.

Why the difference, as far as loading the only difference was the neck tension. Thoughts? Is this normal?

 

[jelenko]

I'm not sure there is a difference. 20fps is ~ 0.7% difference. Could easily be just normal variation.

I'd want to repeat the test with the more neck tension.

 

[Sandstorm]

Your likely build just a bit more pressure with the increase in neck tension.
It makes sence to me...Id double check that load and confirm your findings Sir.
A larger sample size.

 

[Rocketvapor]

Might be the 85.5.
I was getting coal length variations and found that the batch I have can be sorted into two different BTO groups about 0.006", long and short. Will try them at the range soon.
The 85.5 also has a driving band,intentional or not, at the junction of the bearing surface and boat tail.

 

[Doom]

More neck tension will lead to slightly higher peak pressure and slightly higher velocity. What you measured is probably real.

 

[Ned Ludd]

It could be due to slightly higher pressure generated in the cases that exhibited higher neck tension/seating force. However, I'd be cautious about making that interpretation until you have a larger sample size. I've seen velocity go both directions in .223 Rem loads with heavies using Lapua brass where a few rounds required noticeably different seating force. Sometimes the rounds that were harder to seat had greater velocity, sometimes they had slightly lower velocity. Thus, I was never able to observe a strong correlation between slight differences in seating force and velocity.

If you have some additional bushings of perhaps +/- .001" diameter, you could intentionally generate case with greater or lesser neck tension/interference fit with a slightly larger sample size and see whether the correlation remains the same. I've thought about doing this experiment, but have never gotten around to it.

The other consideration with the small .223 Rem case is internal volume. The case volume variance within a single Lot# of Lapua .223 Rem brass can be sufficiently large in some Lot#s to generate velocity variance of as much as 20-30 fps. I have found over the years that Lapua .223 Rem brass exhibits a very solid linear correlation between weight and volume. Some Lot#s have had a much wider case weight/volume range than others, and thus showed much greater velocity variance if the cases weren't sorted by volume, or by weight as a surrogate for volume. You might try sorting a few cases by weight and doing a similar experiment.

Finally, if I understood your post correctly, the velocity measurements were taken with virgin (new, unfired) brass. If that interpretation is correct, you could also be seeing some other effect generated by the virgin brass, which has not yet been expanded to fit the chamber more closely as does fire-formed brass. It would be interesting to determine whether the correlation you observed between seating force and velocity is maintained once the brass has been fire-formed.

The bottom line is that it is beneficial to maintain reasonable ES/SD values. Most F-TR shooters I know that are using heavies in .223 Rem typically generate ES values in the neighborhood of perhaps 20-25 fps, whereas .308 Win rounds prepared using the exact same techniques are likely to yield ES values of about 8-15 fps. A significant part of the difference probably lies in the difference in internal volume. In the smaller .223 Rem case, everything one does during brass prep and reloading potentially has almost twice the effect on velocity because the case volume is approximately half that of the larger .308 Win case. Keeping ES/SD to a minimum in .223 Rem loads usually necessitates sorting brass, accurate powder weight measurement, testing various different primers, and a number of other approaches. One has to determine which of these approaches provides the best results empirically. If you determine that a difference in seating force is a significant factor in causing velocity variance in your hands, I would suggest segregating and using the rounds that required higher seating force as foulers/sighters, or perhaps shooting a single string/relay with them if there are enough. Alternatively, turning necks may be another approach that can help minimize velocity variance as caused by variance in neck tension.

 

[SubVet]

First, thanks to all for responding. Some very good discussion, confirmed what I initially thought - that increased neck tension will exhibit slightly increased velocities. But, food for thought on further testing …..

The use of virgin versus once fired brass, need larger sample size, use of various size mandrels for different neck tension to see if there is actually a correlation, since I also shoot in 308 do some testing with other caliber. Lots more range time, yea …..

Again, thanks and much appreciated.

 

[rwj]

Went to the range to shoot 4 ten round groups at 200 yards. Accuracy was what I expected, all four groups less than 3/4"

Very nice for 40 shots!

 

[SubVet]

Thanks ..... the Bartlein barrel, extensive load development and a March 10 by 60 High Master scope really helps the efforts for group size. Also, no wind at all .....

 

[Dave M.]

I would say something isn’t right because your ES of less than 8fps, should certainly result in a SD of less than 2fps; which for a 10 shot string is excellent. Although possible to have a ten shot string with an ES of 8, and an SD 4; it certainly would be viewed as an anomaly; and definitely would not be repeated three more times in a row. Both the ES and SD you state ms would be excellent for 10 shot strings and would be considered to be world class ammo.
Dave

 

[Doom]

I would say something isn’t right because your ES of less than 8fps, should certainly result in a SD of less than 2fps; which for a 10 shot string is excellent. Although possible to have a ten shot string with an ES of 8, and an SD 4; it certainly would be viewed as an anomaly; and definitely would not be repeated three more times in a row. Both the ES and SD you state ms would be excellent for 10 shot strings and would be considered to be world class ammo.
Dave

This is what can happen when the precision of the item being tested is on the same order as the accuracy of the instrument doing the testing. In this case the Labradar accuracy is +/- 2.7 fps, Typically the instrument should have an accuracy of ~4x the expected variable being measured to have negligible effect on the reading.

 

[Dave M.]

This is what can happen when the precision of the item being tested is on the same order as the accuracy of the instrument doing the testing. In this case the Labradar accuracy is +/- 2.7 fps, Typically the instrument should have an accuracy of ~4x the expected variable being measured to have negligible effect on the reading.

True. But even so, it’s nearly impossible to shoot 10 shots over any chrono device and get an ES of 8 and also still have an SD of 4. I’ve included a sample of 10 shots that would yield both an ES of 8 and an SD of 4, and the 10 shots are not very realistic or indicative of normal 10 shot strings.

 

[Doom]

are not very realistic or indicative of normal 10 shot strings.

I totally agree that that type of result is unusual, and questionable. It would take not only extreme care in the preparation of brass and assembly but a lot of luck. Doing so over 40 rounds and in 223? I don't use a Labradar so I don't know what issue might cause a error in reading velocity, but I would love to see the raw data.

 

[SubVet]

The ES (<8) and SD (<4) numbers I quoted were the average of the four groups. Given the brass was new and my loading is very focused on load as well as neck tension consistency the ES and SD are not unusual. The load development was extensive as to "waterline" (minimum vertical) and seating depth for groups. This rifle is exceptional at 600 yards.

 

[RegionRat]

Have you had a chance to chew on Ned Ludd's comments as far as virgin case volume versus once fired?

Any more insight as to what caused your perception of bullet seating force variations?

If you are getting good waterline, I would shoot more and worry less... you are doing more right than wrong.
Keep it up! YMMV

 

[SubVet]

At to Ned Ludds advisory comments, my next emperical testing will include once fired brass, weighing the brass (correlation between weight and volume), several different mandrels to test neck tension more throughly, 308 caliber testing and lastly more rounds fired (larger sample size). Also, I already use the tighter neck tension rounds as sighters or foulers.

Thus, I will definitely get more range time ..... yes!

I get a good waterline at 200 yards, at 600 and beyond is a different story. The wind. There is an "art" in reading the wind amd I am not a Van Gogh ..... lol. But, in time will get better.

I specifically use my 223 with Berger 80 grain (velocity 2850 fps) or 85.5 grain (velocity 2710 fps) for 200 to 600 yard events and the 308 with Berger 185 grain (velocity 2690 fps) or 200.2 grain (velocity 2570 fps) for 700 to 1000 yard events. Time spent on load development and seating depth testing has definitely been worth it as I am positive the rifles / loads are adequate for the task. Now, it is just me.

 

[RegionRat]

I get a good waterline at 200 yards, at 600 and beyond is a different story.

On a windy day we can see horizontal spreads in switchy conditions or gusty conditions that will spread the shots along a horizontal pattern, however....

Waterline isn't something we talk about at short range. These are soft terms without formal definitions, but waterline is really a Mid Range to Long Range discussion, out at a place in the trajectory where velocity and harmonics might cause vertical issues, in other words load tuning matters at all distances but there are velocity issues at distance.

At the trajectory of a high power cartridge within 200 yards, it is very difficult to detect the vertical dispersion of a tuning problem that would be from inconsistent velocity. Once you get where the average speed has decayed and the impact on the target shows a vertical spread, then we have a different view.

 

[Dave M.]

The ES (<8) and SD (<4) numbers I quoted were the average of the four groups. Given the brass was new and my loading is very focused on load as well as neck tension consistency the ES and SD are not unusual. The load development was extensive as to "waterline" (minimum vertical) and seating depth for groups. This rifle is exceptional at 600 yards.

Trust me when I say your 600 yard rifle is not exceptional. There are guys on this forum that can shoot the tits off a fly at 600 yards. As for your reloading attention to detail and care, I’m not doubting that you took great care to make your loads. But if you understood how impressive a 40 shot ES of 8 and SD of 4fps was, then you wouldn’t be posting it here and asking if the data was “normal”. Data that matched the numbers you posted would be exceptional. Please attach the raw data for the four groups so that we can see how you managed to get such a low ES and, relatively speaking, high SD. It’s damn near statistically impossible.
Dave

 

[Dave M.]

At to Ned Ludds advisory comments, my next emperical testing will include once fired brass, weighing the brass (correlation between weight and volume), several different mandrels to test neck tension more throughly, 308 caliber testing and lastly more rounds fired (larger sample size). Also, I already use the tighter neck tension rounds as sighters or foulers.

Thus, I will definitely get more range time ..... yes!

I get a good waterline at 200 yards, at 600 and beyond is a different story. The wind. There is an "art" in reading the wind amd I am not a Van Gogh ..... lol. But, in time will get better.

I specifically use my 223 with Berger 80 grain (velocity 2850 fps) or 85.5 grain (velocity 2710 fps) for 200 to 600 yard events and the 308 with Berger 185 grain (velocity 2690 fps) or 200.2 grain (velocity 2570 fps) for 700 to 1000 yard events. Time spent on load development and seating depth testing has definitely been worth it as I am positive the rifles / loads are adequate for the task. Now, it is just me.

Let us when you get it dialed in to an ES of 0fps.

 

[SubVet]

Well, "tits off a fly", very good description, sounds like an ES of 0 fps. WTF!

You (Dave M) and (Doom) seem to have an unnatural obsession with ES and SD. I started this thread talking about neck tension and slight velocity differences with generally informative answers with some good recomendations. Not ES and SD. Typical of any internet forum, you run across the normal and abnormal responses.

Thus, simply fuckoff if you can"t be civil and hopefully help folks who are learning.