Testing Neck Tension

[Blue68f100]

In Ref to this thread https://www.thehighroad.org/index.php?threads/the-things-we-do-that-effect-accuracy-or-dont.836543/ I think have come up with a way to test neck tension in how it effects accuracy.

What prompted this is I had some new Lapua brass that when I went to seat the bullet it took so much force it damaged the bullet, 4 like this. I have run into this before so I expanded the necks and resize the necks with 0.003" neck tension. So I decided to use them for foulers, instead of breaking them down. I shot these 4 and 5 of the ones I just loaded up. The damaged bullets group was 1/2 the size of the good ones at 300 yrds. I then decided to turn the necks on all the remaining brass. I did not do a full cleanup/removal but have the max delta at 0.0003" on the OD now.

So the test I have planed is to start at 0.002" neck tension, increasing by 1 bushing size to 0.005"?. May wait till after I shoot the 0.004 before loading the next group. Will load 10 of each, using Rem 7 1/2 primers, Hornady 140gr ELD-M bullets, 42.3gr H4350. The powder was dispensed using a RCBS Chargemaster 1500, then checked and adjusted using my GemPro 250 to the nearest 0.02gr. Will be test fired in my custom 6.5CM-AR that I built that has produced 1" groups at 300 yrds. I will single feed these so the bolt does not change the bullet position, OAL. I'm using a Redding S die and bushings. All will be crony, at time of test. I have recorded OD/ID during sizing. When you get to 0.004" the ID reduces more than 0.004".

I have a range in my back yard. The 300 yrd range was cut through a timber area and is very will protected from side wind. The only area in the open, is about 35yrds at the target end. All this land is flood plane so it's low and not exposed to normal wind conditions.

Looking for input on what else may be done or added.

On a side note: I had wrist surgery last Wednesday to clean up some cartilage damage, so it may be awhile before I can shoot. So I have time to get things ready.

 

[Toprudder]

Good luck with your surgery.

I will be looking forward to your test results.

 

[pert near]

Very interesting. It sounds like you have all the bases covered & I too am anxious to learn from your results.

 

[243winxb]

An auto loader may benefit from more neck tension. http://www.exteriorballistics.com/reloadbasics/gasgunreload.cfm

Read the "neck tension" part.

.002" gives about 35 pounds bullet pull. 35 is the minimum for GI ammo.

 

[Blue68f100]

When I'm testing loads I hand feed the ammo into the chamber so there is no chance of the bullet being moved. This also slows the process down so the barrel can cool off. Once I have what I want I test the loads using the magazine.

The pain in my wrist from the surgery has finally started backing off. As long as I don't do any angular movement.

Question: I'm planing on shooting these up and down the scale so barrel fouling is minimized. 1-2-3-4-4-3-2-1... etc. Is there a better way ?
I will also shoot 5 rounds to get some temp in the barrel and confirm zero before testing starts. It's summer time here in Texas, hopefully the heat mirage want be too bad. This just messes up my vision pretty good.

 

[Blue68f100]

Got a chance to shoot these test rounds this morning. Weather was a mixed bag. When I setup it was 74F with a 90% + humidity, was getting some light sprinkles. About 1/2 way though the front that brought the light shower moved through, had high winds. It also lowered the humidity, cooled off briefly to mid 70's then started heating back up.

The first attempt to run these last week got aborted due to rain. And looking at what I had on paper, did not make any sense. So aborted test. Packed up and headed back to my shop just before the bottom fell out.

Second time around. Had to load up some more ammo. Only had 50 left of Lapua new brass to use.

Setup: New Lapua brass, expanded, necks turned minimal to just take the spots that were 0.001" high down to less than 0.0005". Then used my Redding S bushing dies and started sizing the brass in groups of 10, 0.001" reduction in OD. Ended up using bushing 0.289, 0.288 (std 0.002"), 0.287 and 0.286". Used a RCBS ChargeMaster 1500 to dispense the powder, calibration done and verified with check weights. Any over shoots where dumped back in, and redone. I did not check the loads with the GP250 this time. From past experience I have found that this unit is normally within ±0.04gr. Runout was not checked this time. Used the Redding Competition Seating die to set the 140gr ELD's with the VLD seating stem. The std seating stem hits the bullet tips. Load 42.3 gr H4350, Hornady 140gr ELD's, Rem 7 1/2 primers. This load was developed using Starline brass. Water volume checks shows them to almost the same volume, < 1gr difference. The main difference is the flash hole. The Lapua used 0.059" (1.5mm) where Starline uses the std 0.062".

Gun used is my AR-6.5CM with a 8-32x Nightforce Benchrest scope. This has a SS 24" heavy barrel (E. Arthur Brown Premium Long range barrel) that I flutted to aid in cooling and to lighten. Used a bob-sled to feed the bullets to not change the OAL when cambering. I shot from a Lead Sled, 25# shot added, with a base that hooks the front of my shooting bench. The legs of the table are 1' in the ground, no cement.



My range 300 yrd range... Photo shows my cooling fan attached.



Now the interesting part .... the data.... I shot 5 fouling shots before the test. This barrel shoots good once it gets some heat into it.
Crony data fps, ES, SD on side. Looking at the data points, the 4th shot group is when the shooting conditions started to change. Temp dropped, wind picked up, started humidity dropping (90%+ - 60%). Start of test 74F, ended at 84F. The velocity increased, when this started, POI raised. The starting velocity was slower than I normally see by as much as 50fps, POI low. I'm assuming the denseness of the air caused this, since they were close to normal when I stopped. The nice thing about this range is that it's mostly protected from the wind, and is shaded most of the time. There is 40 yrds exposed to E and SE winds at the target.



Shot left to right 10 shots over a 2.5 hr period. I attached (taped) a thermocouple to the barrel, just in front of the gas block. This readout (in the photo) was acting strange, jumping all over the place. It would read 143+ (1st photo) and I could hold my hand on it, so it was not hot at all. Half way through I made a trip back to the house to pick up my larger unit. This unit has a rapid scan rate and reads to 0.1F. It allowed me to see when the heat started to drop vs rising soak time. I did not start using the cooling fan till the barrel made it to 90F. Except the last 3 shots, rain was starting. Then it normally only needed to run a few minutes to bring it back down. This blower is powerful enough you can feel it blowing on you 10 away.

Target info: Heavy lines are 1" square boxes with smaller 1/4" spacing.
Top left 0.289" bushing, 0.001" neck tension
Top Right 0.288" bushing, 0.002" neck tension
Middle Left 0.287" bushing, 0.003"+ neck tension
Middle Right 0.086" bushing 0.005"+ neck tension
Once you got to 0.003" reduced size bushing the ID moved more than 0.003" This was checked with pin gauges and calipers.

When I was counting bullet holes I was missing some on the Top Left. Closer examination showed the 2nd hole down is 3 rounds.

Conclusion:
Need to do some more testing using the Starline brass once fired, annealed, prep routine.
Fine tune the loads for the Lapua brass, smaller flash hole.
Either go real light or heavy with the crimp. The heaviest neck tension produced the best group lower ES, SD. I've seen this before.

Try to do the testing in a more stable environment. Not during rain and frontal passing.

 

[Walkalong]

My range 300 yrd range...

Very nice, wish I could stretch it out that far.

 

[Nature Boy]

The heaviest neck tension produced the best group lower ES, SD. I've seen this before.

I’ve read several other people who say the same thing. I don’t have any bushing dies so there’s not much I could do about it in my process.

Looking forward to seeing more test results

Very nice range too. I’m jealous

 

[Walkalong]

Light neck tension is easier to be consistent with. The bullet slightly into the lands can get the good consistent start pressure you need.

 

[243winxb]

When you get to 0.004" the ID reduces more than 0.004".

Once you got to 0.003" reduced size bushing the ID moved more than 0.003"

What is the fired brass outside neck diameter? The Redding website said

It has come to our attention through customer calls and our own use of the bushing style sizing dies that in certain instances, a given neck sizing bushing will produce a case neck diameter that can be several thousandths of an inch smaller than the actual diameter of the bushing. This idiosyncrasy occurs when the neck diameter of the fired case is a great deal larger than the diameter of the neck sizing bushing, such as occurs when factory chambers are on the large side of the tolerance range and the brass is on the thin side. Typically, we have not noticed any problems until the case neck is reduced more than 0.008-0.010".

i reduce about .008" , but dont have pin gauges to check ID.

Hope you have better conditions next time. Nice setup.

 

[Blue68f100]

Light neck tension is easier to be consistent with. The bullet slightly into the lands can get the good consistent start pressure you need.

Very true. The reason most shooting in competition set them long and let the bullet make contact with the rifling. But this being an AR I need neck tension to hold the bullet.

 

[Blue68f100]

What is the fired brass outside neck diameter? The Redding website said i reduce about .008" , but dont have pin gauges to check ID.

Hope you have better conditions next time. Nice setup.

I don't recall what the fired OD is right now. This barrel has a min speck chamber so it not on the loose side. I will measure them when I go back out to my shop and let you know. Remember these were NEW LAPUA brass that I used.

I did a minimal neck turn so I would not reduce them near as much. It would take another 0.0005" to have them all true. But stopping short they will even out on there own after a few annealing/firing cycles.

 

[Blue68f100]

Got a chance to measure the OD of the fired cases. They were running 0.295" as the norm. I have bushing from 0.292-0.284". If I'm needing something I don't have I just turn one on my lathe.

 

[cw308]

I'm shooting a bolt action Rem 700 308 , Benchrest shooting only , Jewell 10 ounces trigger . We charge exactly the same way , RCBS ChargeMaster 1500 then dump into the pan on the GemPro 250 , nice little accurate scale .

I went down that road with the Redding S Type bushing dies to play with neck tension but my runout was poor , I don't neck turn so I use the expander ball , found that going back to the standard RCBS standard Full size die , sizing .0015 - .002 case headspace using Redding Competition shellholder set of 5 , my runout averaged .001

I wet tumble with SS Pins to clean my brass an lightly dry lube the inside of the necks with lmperimp dry lube on a cleaning mop works for me . Your AR is a very nice set up , 6.5CM is a great round. Keep up the good work , or should I say fun not work. Be well.

Chris

 

[Walkalong]

I went down that road with the Redding S Type bushing dies to play with neck tension but my runout was poor , I don't neck turn so I use the expander ball ,

Using the expander ball defeats half of the purpose of using the bushings, poor runout. Bushing with unturned necks is better than an expander ball with unturned necks. You do not have to neck turn to use bushing dies without the expander, regardless of what some folks on the net say.

 

[cw308]

If you size without the expander and you not neck turning your brass the die will center the neck in the die but if your brass thickness isn't the same around the case neck when you seat the bullet is when your runout will go off . The expander trues the inside diameter of the cast instead of the seated bullet acts as the expander . If your necks are true the yes I would skip the expander ball . For me the expander with the standard F/L die my runout is like I said .001

lm not shooting long range , the ranges in my area are max. 200yards , is the distance I only shoot , and at that range runout doesn't effect much . I tried the bushing dies to see if changing neck tension would shoot any better , from .001 on up , my standard die gives me .003 , as long as I size with .0015 to .002 no more or less with a .002 jump , my groups are good for me . Now only concentrate on my form . Makes shooting much more enjoyable the playing with loads.

 

[Blue68f100]

Round 2 with Starline Brass...

Starline brass that the necks were turned minimal to just take the high spots off that were 0.001", now all in the <0.0005". Brass is once fired, wet SS cleaning, annealed, then sized with Redding S bushing dies and started sizing the brass in groups of 10, 0.001" reduction in OD. Ended up using bushing 0.289, 0.288 (std 0.002"), 0.287 and 0.286". Used a RCBS ChargeMaster 1500 to dispense the powder, calibration done and verified with check weights. I did not like the crony number last time so I checked and adjusted the load using my GP250 as final. So these loads are within the GP ±0.02gr accuracy.

The weather was better this time around, but was heating up toward the end. At start of test 79°F, 2 hrs later 96°F. It started heating up toward the end, the last 8 shots the temp rose 6°F. Normally it was only going up 1°F per 4 shot group.

The load is the same as last time 42.3gr H4350, Rem 7 1/2 Primer, Hornady 140gr ELD-M. At the very end I shot 10 rounds using CCI #41 primers, just to see if there was a difference. These were sized with the 0.287" Bushing.

Fired 3 shots to confirm scope settings.


I monitored the barrel temp and started cooling it after 4 shots. By then the barrel temp was up to 99°F. 3 min run time had the temp down to 94°F. I used this cooling cycle for most of the test, increasing time as the ambient temp rose. The very last 4 rounds it had heated up so much that it was going to take near 7-10+ min to get down to 94. I elected to run the fan for 5 min and record the temp. In all cases the barrel temp was < 99°F. I should note that the wind was swirling through the trees. Wind 5-10 mph coming from 9:30-10:00 direction. When the wind would kick-up you could see the barrel temp drop 2-3°F in a very short time. Soon as it laid, it would start creeping up. I shot these in alternating order to try to even up on barrel fouling, 1-4, 4-1, 1-4, 4-1........ After finishing I shot the CCI #41 primers, group 5, giving it 5 min to cool.



Crony at 15' as before.

Vertical Spread @ 300 yrds. Avg fps @ 15' ES SD
Group 1 (0.289", 0.001") 2.25", throw out the 2 high ones 1.75" 2706 28 8
Group 2 (0.288", 0.002") 1.5" 2712 50 14
Group 3 (0.287", 0.003"+) 2.25" 2718 50 15
Group 3 CCI primers 2.25" throw out the 2 high ones 1.0" 2711 44 12
Group 4 (0.286", 0.004"+) 2.5" thrown out the low shot 1.625" 2708 39 11

Conclusion:
According to the crony number the lightest neck tension (group 1) had the best numbers. Omitting the 2 upper shots had the best grouping, too. 1.375" at 300 yrds. if you only look at the Rem 7 1/2 primers. You bolt guys have a big advantage in being able to run little to no neck tension over a simi-auto. But taking every thing as a whole Group 2 wins even though it had one of the worst crony numbers (ES). Group 3 with CCI primers wins if you throw out the 2 high shots. The CCI primers showed the best group if you loose the 2 upper at 1.0". The rise may be due to barrel heating up. Since this is a AR I may switch over after a confirmation test.

Bottom line I'm pleased with the results, though my shooting could be better. But not bad of a old man blind in one eye and can't see out of the other. I think the temp had some influence toward the end. Got to get my crony numbers better. Here, I think I'm tied due to my equipment. Since I measured everything on these rounds, all were very consistent, <0.001" run out. The spread is probably due to variation of brass volume. I still have these in order, so I may measure internal volume and compare them to the velocity and see if there is correlation. If I do I will post the data vs dry weight.

I had some more shooting to do with the 224V. I saw the ambient temp hit 100°F, but leveled off at 97°F. I had no reason to stay out in the heat, but I did. Spent more time waiting for the barrel to cool. This one is not flutted and it cooled very slow. Got a email this afternoon saying my flutting cutter is on the way back after being sharpened. I have 2 barrels to do when i get it.

 

[taliv]

why are you throwing out the two high ones?

 

[Blue68f100]

I did included them in the group size. Toward the end of the test every thing heating up (including me) all groups started opening. I just showed if I removed the 2 (odd balls, maybe fliers/maybe not) I had a much better group though.

 

[cw308]

You should see how tight my one shot groups are , carrying a #2 pencil down range my groups fantastic.

 

[Blue68f100]

Got around to measuring the water volume of the fired cases. I removed the primer using a universal deprimer die. Brass was not cleaned in any way. Being a simi-auto there are small dents on the cases, all in the same place.

Bushing 0.289"
Shot# fps H2O gr
1 2710 52.36
2 2710 52.56
3 2710 52.32
4 2699 52.50
5 2715 52.42
6 2721 52.40
7 2699 52.56
8 2699 52.46
9 2693 52.56
10 2710 52.62

Av 2706
Hi 2721
Lo 2693
ES 28
SD 8

Bushing 0.288
Shot# fps H2O gr
1 2704 52.58
2 2721 52.60
3 2743 52.50
4 2710 52.58
5 2699 52.66
6 2726 52.44
7 2710 52.58
8 2699 52.48
9 2693 52.60
10 2721 52.38

Av 2712
Hi 2743
Lo 2693
ES 50
SD 14

Bushing 0.287"
Shot# fps H2O gr
1 2693 52.56
2 2710 52.44
3 2743 52.50
4 2721 52.70
5 2704 52.60
6 2715 52.74
7 2710 52.66
8 2737 52.74
9 2715 52.44
10 2737 52.52

Av 2718
Hi 2743
Lo 2693
ES 50
SD 15

Bushing 0.286"
Shot# fps H2O gr
1 2710 52.38
2 2710 52.44
3 2710 52.56
4 2715 52.38
5 2715 52.76
6 2721 52.44
7 2732 52.54
8 2721 52.50
9 2693 52.62
10 2688 52.42

Av 2711
Hi 2732
Lo 2688
ES 44
SD 12

There is some coloration with less volume = more velocity. But there is conflicting data that shows normal low velocity too. So it's a wash.

 

[Toprudder]

It would be interesting to see how case volume relates to weight.

 

[Blue68f100]

I do not have any weights prior to loading, this is fired brass (dirty). I don't have a way to clean them without removing all the makings. I did a dry weight to volume comparison a while back on another thread with this brass and Lapua. Dry weights are not a good indicator in all cases was the conclusion. I need to see if I can find that thread.

 

[jmorris]

Temp dropped, wind picked up, started humidity dropping (90%+ - 60%). Start of test 74F, ended at 84F.

You got to get up at dark to be shooting in Texas when it’s 74F at the end of July. We’ve had worse though.

Looks like you have a pretty good natural wind block too.

Keep up the good work.

 

[Blue68f100]

We had a rare north front come through that morning I was shooting. Very unusual for that time of year when we normally start off hot and get hotter.