OzFclass

I recently loaded of rounds in two groups of significantly different neck tensions and therefore differing bullet seating force. The purpose was to determine if that would have any effect on muzzle velocity, fall of shot and accuracy.

This exercise was prompted by curiosity as to why I could not see any correlation in MV and fall of shot when I previously measured the speed. (when the barrel was relatively new)

Here is some data that may be relevant:
Barrel is a 30” Maddco 8 twist in 223R with 2300 round count.
Load was 25.5 grains of 2208 behind 80.5 Bergers. I used an RCBS 10/10 scale to measure the charge.
Cases were body sized then necked with a Wilson die in two groups of 12 each. The 1st group was necked with my usual 0.248 bushing and the 2nd group with the smaller 0.247 bushing.
The bullets were seated with a Wilson seater, and a K&M arbor press with force gauge to an approximate 3 thou jump.
I did not adjust the scope after the sighters and kept the same point of aim.
Range was 400 meters.
Each group was fired separately. The 1st group was with a relatively cool barrel. The 2nd was started about 20 minutes after the 1st group finished.
Magnetospeed V3 was use to measure speed.
Shot 7 in the second group was entirely my fault and should be excluded from any considerations.
ES back when the barrel was new was 35 fps. As you will see it is now 35-44 fps. The MV back then was either side of 3000 so it is now slower.
Seating force is highlighted in yellow.

I still cannot see any correlation.
I believe that if the bullet is travelling faster then it drops less and should print higher on the target. I don’t see that here.
Is the correlation too small to show up against my point of aim differences in each shot? This is not something that keeps me awake at night but it does puzzle me. Can anyone shed some light?

Muzzle velocity is only one factor in POI (elevation in the case) change, some other powers to be will likely chime in here.

A faster shot will not always land higher on target, some other contributions factors that effect this are, barrel compensation profile, harmonic length (this one still is black magic to me), and launch angle.

There are several threads in here that explain these inputs, I would provide links but I'm on my mobile.

In short: a positively compensated barrel will throw slow shots high on the target while a negative throws them low.

If you use the search bar to look for williada, read all his posts, they are becoming the bible.

Looks like you have a slight positivity compensating barrel!??
The extra tension lowered your extreme spread too! Annealing your necks may lower it more. And that's another black magic thing !

Rick You seem to be getting very high velocity from 22.5 gn of 2208, must be a misprint. You possibly need to go for a greater difference in bush size to show up any real change in results. The 223 is more critical of consistent neck tension rather than actual neck tension so it would be necessary to probably neck turn and anneal cases prior to testing to get actual bullet seating pressures as close to the same for loads in each batch. Regards Malcolm.

Rick, they are 2 very nice shoots well done.

I personally think it's difficult to give accurate advice on the cause of POI differences at ranges past 200y, even a little less. Especially with great targets like that where there's only 3 shots over both targets that are inconsistent. There's too many environmental conditions that could have been at play which we weren't there to see. For example, maybe with shot 5 of T1 and shot 1 of T2, the targets may have been shaded a little more from cloud shadow. Not saying it is, but it's just an example. There are so many more possibilities.

Going off the MV's and the main group shape though, I think it looks like you could be running at either edge of a node. Especially as Graham has pointed out that some of your higher shots are lower MV and visa versa, indicating that it might be positively compensated which means that your not in the middle of the node.
I think that if your trying to reduce a little vert from that main group shape (at this distance) you should look at reducing your ES. That may come from tweaking your powder charge by up to +/- 0.2 gn or something to that effect.

I'll make a comment on my own experience with neck tension. I have no idea whether it's relative to your situation.
I've found that when seating bullets in a batch of cases where the neck O.D's are uniform, sometimes the I.D's are not uniform for various reasons and you can feel that in the seating pressure, as you know. When I measure COAL, base to ogive, The cases that felt tighter to seat bullets generally don't allow the bullet to seat as low as the others and that's where differing neck tensions can cause a difference in seating depths. I've seen this cause up to .004" difference COAL. That's enough to change group shape. I think that's the biggest concern with inconsistent neck/seating pressure.
You don't notice this unless you measure every cartridge. I've gone to the length of adjusting the seating die a little more for a second seating of the tighter ones to get them all spot on and they shoot perfectly despite obvious seating pressure differences.

My experience is based on jamming bullets which is less sensitive to neck tension difference than jumping bullets by many accounts.

Nice targets

Rick there are few general principles to consider before we tackle your groups.

Firstly, the Magnetospeed adds ½ a pond of weight to your muzzle and distorts a group if subsequent groups are fired without it. There are two key reasons: (A) It requires about 4 ounces muzzle weight to counter torque on a .223 at your velocity and twist rate with an 80 grain projectile. The Magnetospeed is double that and generally groups tend to be better with one on, because the torque is largely taken out of the equation as it effects recoil for bag handling, reflected vibrations travelling backward and forward along the barrel prior to bullet exit and barrel lift for its compensation profile. So the muzzle weight is a damper. (B) On the other hand, assuming the groups were tight, you need to consider by way of fine tuning at what angle projectiles exit the barrel in terms of their compensation profile. Dealing with this aspect may only require a couple of grams to make significant change in launch angle or a ¼ of a turn of a tuner. The point I make is the application of weight at the muzzle has three distinct roles.

Secondly, it is the barrel length in terms of it harmonics like the slat on an xylophone that if it was to give the sweetest note it will give the tightest nodal group then the barrel length has to be right, and for the node to operate effectively it is velocity dependent. That means using your best reloads to keep spreads down to match that barrel length.

Let’s hone in on the neck tension. It’s one of the last things I do and it is only remedial to subtly change the group shape to make groups less wind sensitive after I have sorted the charge and seating depth and assuming I have other key tune factors sorted as some people have been following. It is the other key tune factors which have more impact on the velocity, hence bore-time and launch angle than seating tension.

Greater seating tension may improve burn efficiency if you are not jammed into the lands and you require a free flight to modify exit timing. But too much and you can distort the alignment in the case and a bullet started off centre will continue off centre because the projectile is malleable and will increase its external yaw as a consequence of its internal yaw.

What many do, is anneal their cases to sort friction variations or turn or trim their cases and have the correct neck clearance so carbon does not build up. Get that right and best neck tension is easier to detect. You need to try three different neck tensions for better comparison.

The key to testing is to get the order of magnitude of each right, then alter one variable at a time to refine things. For statistical significance the test should have 30 minimum samples if used in isolation if a true correlation is to be made.

There is a general assumption that a case should have a high powder density to burn efficiently. So the selection of the right powder is critical as is a matched primer. Typical ignition problems result in vertical groups. If that is right a good starting point is about .001” neck tension for jammed projectiles and .002” neck tension for free-flighted projectiles.

If we look at your groups below Rick, I think your groups in the first instance require more powder before you can see the subtleties of tune. I’m running 25.4 of 2208 in an old seasoned .223 barrel having carefully built it up now that its colder. Your loads should be seasonal.

Your groups are demonstrating a changed group shape due to better ignition but the vertical trend is still there nonetheless. This requires more charge before we even consider an appropriate compensation profile which is illusory due to Magnetospeed weight. If you are going to use a Magnetospeed, make a false weight to compensate for it when the Magneto speed is off your barrel. I like Malcolm's observation. Hope this helps Rick. David.



PS Thanks Jason, hope its more a help than a hindrance to fast track a few and I never stop learning either.

Thanks for the replies, they raised several interesting factors that I had not considered and one or two that I had.

Yes it was a misprint, it should have read 25.5 grains. I doubt I would ever have seen it, thanks Malcomb, would have been quite clever to get those velocities with such little powder.

I realise now that I should have explained the testing when the barrel was fairly new. Had I done so, your comments may have been different.
I fired 5 shots each at 24, 24.5, 25, 25.5 grains. Best group was 25.5. I then fired 5 each at 25.3, 25.5, 25.7 grains. I was advised not to exceed 25.7. Best group was again at 25.5. Interestingly a friend with the same action, barrel, etc found 25 to be best.

I then tested 5 shots each at 5 thou jam, on lands, 5 and 10 jump. Close to lands was best. I then tested each side and found that as best as I could measure that 2 thou jump produced the best result.

I will try 25.7 when the weather gets a bit colder but as you may have seen the target temp was 30C in the pics above.

I will also buy a smaller bushing and see if that will clarify matters.

I have been very pleased with the accuracy of the 223 and the whole package (action, stock etc.), supplied by Matt P by the way. I believe that most inaccuracies are my fault, for example Gp2 shot 7 where I screwed my breathing and shot anyway. I should also have mentioned that some of the vertical could have been caused by the westerly running up the valley and across the Gosford range. That condition can really play with your mind.

My main purpose for the post was to find why MV and FOS did not correlate. I now have some idea and after read the replies again and again I might be even better informed. Thanks a lot.

OK Rick do some new testing with the Magnetospeed off the barrel. Test a few shots later if you want to know what they are running at. Try a few 10 shot groups with different primers with the load you feel performed well as you are pretty close to mine on a full case. Do pay attention to the pressure signs. I was getting better results with would you believe a magnum primer in the 223 to light it up. It was not until I tested the primers did I see vertical/ignition disappear.

Next, I would be inclined to make several rows of aiming marks with 3 columns for a combined charge and seating depth test on A4 paper and test shoot at 140 yards. Because the .223 is such a small case you will notice .1 grain increments. If you can't read your scales use a magnifying glass at these small increments. On each row you will test a different seating depth. Column 1 you shoot a lower load, column 2 your base load and column 3 your upper load. Make three loads of each charge to shoot on different rows representing different seating depths.

May I suggest loads of 25.4, 25.5 and 25.6 to be tested at seating depths of 5 thou jam 3 thou off and 5 thou off on three separate rows. Prior to testing run 4 warm up shots through. So on row 1 you shoot with a delay of 45 seconds between shots an incremental load round robin starting on the aiming mark with 25.4, the second aiming mark 25.5, and the third aiming mark 25.6 at the jam seat. Repeat on the second row with 3 thou off and so on. I do this testing at 140 yards. Just to recap, there should be 3 shots fired in total of the same load on each aiming mark when finished the round robin at the same seating depth. Each completed row represents the same seating depth with different charges. Each column represents the same charge at different seating depths. You can extend these rows by .005" graduations for more results if you wish. I test out to about .020" with a .223.

Then if you want, send me a scan of the group sheet by PM.

David.

Thanks David, I will be in touch.