Shoulder Set-back surprising results

[Williamb]

I normally fireform my brass before working up a load. However I purchased quite a bit of once fired 7.62 LC brass from Scharch. Really beautiful brass fully resized, cleaned and trimmed. With so much brass I thought I would work up a load which I did while fireforming it. In my REPR is shoots in the .5s.

Now after fireforming I have set back the shoulder about .002. On the brass received from Scharch the shoulder was set back .01. Big difference .008. Without thinking much about the change in case capacity I used my same load of IMR4895 as I developed when using the brass directly from Scharch with the shoulder setback .01.

Well in the brass with the shoulder only setback .002 the groups opened up to about 2.5 to 3 inches. I never realized that the shoulder set-back had such an influence on case capacity. No biggie I will just work up a new load. Probably need a slightly higher weight of powder to attain the same pressure.

I'm just wondering if anyone has any comments on this and does my observation make sense.

 

[Tangodown911]

Re: Shoulder Set-back surprising results

Any change in velocity? That will tell you alot.

 

[Williamb]

Re: Shoulder Set-back surprising results

Good question. I didn't have my chronograph with me but I suspect velocity is different otherwise it should group the same with everything else constant.

 

[LoneWolfUSMC]

Re: Shoulder Set-back surprising results

I think you will find that there is something else at work here.

 

[Grump]

Re: Shoulder Set-back surprising results

Just run the math and apply some brain cells to the problem.

.008 further back on a column that's 0.454 inches OUTSIDE diameter and you have a pretty slim possible change in case capacity.

Plus, case capacity is usually overshadowed by chamber dimensions, which are the same regardless of how far you do or don't resize the case.

Methinks you are fixated upon the least important of the possible variables here.

 

[BigMahi]

Re: Shoulder Set-back surprising results

Hows the brass from Scharch - how many reloads can you get from them? What is the general headstamp mix that you received? I'm thinking about ordering from them and would appreciate some info thanks.

 

[johndoe]

Re: Shoulder Set-back surprising results

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Williamb</div><div class="ubbcode-body">I normally fireform my brass before working up a load. However I purchased quite a bit of once fired 7.62 LC brass from Scharch. Really beautiful brass fully resized, cleaned and trimmed. With so much brass I thought I would work up a load which I did while fireforming it. In my REPR is shoots in the .5s.

Now after fireforming I have set back the shoulder about .002. On the brass received from Scharch the shoulder was set back .01. Big difference .008. Without thinking much about the change in case capacity I used my same load of IMR4895 as I developed when using the brass directly from Scharch with the shoulder setback .01.


I take you are shooting at 100 yds? That is a Hugh difference, have you checked
the runout on the loaded rounds?
Well in the brass with the shoulder only setback .002 the groups opened up to about 2.5 to 3 inches. I never realized that the shoulder set-back had such an influence on case capacity. No biggie I will just work up a new load. Probably need a slightly higher weight of powder to attain the same pressure.

I'm just wondering if anyone has any comments on this and does my observation make sense. </div></div>

 

[vman]

Re: Shoulder Set-back surprising results

Groups opened up to 2.5 to 3" !!!

At what distance was this? And in what orientation was the spread of the group? Was it scattered, was the spread in a horizontal plane, or was it in a vertical plane?

seems a bit odd to me

 

[squirrelsniper]

Re: Shoulder Set-back surprising results

I've had this occur a couple times (on factory bolt guns), but I never found the cause to be a powder charge issue. The chamber being slightly off the centerline of the bore was what I found to be the culprit on both rifles. The "looser" fit allowed the cartridge to sort of align itself during firing, while the tightly fireformed cases held the bullet off the bore's centerline and caused the bullet to enter the rifling at a slight angle which resulted in a lot of yaw (though it did seem to work itself out somewhere between 200 and 250yds). Yes, it was annoying, but sacrificing a bit of case life and FL sizing the cases back to "standard" dimensions resulted in both rifles producing very good accuracy.

 

[Williamb]

Re: Shoulder Set-back surprising results

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigMahi</div><div class="ubbcode-body">Hows the brass from Scharch - how many reloads can you get from them? What is the general headstamp mix that you received? I'm thinking about ordering from them and would appreciate some info thanks. </div></div>

I'm pretty pleased with the brass. I bought 2000 pieces and most of it 95%+ was LC. After that Winchester. I'm on my 4th reload on some of the brass and it looks fine. Thinking about annealing to extend the life of the brass but the break-even on the cost of the machine versus buying new brass suggest that is not the reason to buy an annealing machine.

 

[Williamb]

Re: Shoulder Set-back surprising results

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: squirrelsniper</div><div class="ubbcode-body">I've had this occur a couple times (on factory bolt guns), but I never found the cause to be a powder charge issue. The chamber being slightly off the centerline of the bore was what I found to be the culprit on both rifles. The "looser" fit allowed the cartridge to sort of align itself during firing, while the tightly fireformed cases held the bullet off the bore's centerline and caused the bullet to enter the rifling at a slight angle which resulted in a lot of yaw (though it did seem to work itself out somewhere between 200 and 250yds). Yes, it was annoying, but sacrificing a bit of case life and FL sizing the cases back to "standard" dimensions resulted in both rifles producing very good accuracy. </div></div>

I think your observations are what I am experiencing. I am although shooting a LWCR REPR semi. Full length resizing back to SAAMI specs restored the accuracy and tightened the groups back to .5s. since I experienced the groups opening up.

By the way I am shooting at 100 yards. The groups when they opened up for the most part were +/- circular. I did have a few groups which were absolutely vertical but I believe I allowed the rifle to bounce off the bags. I initially thought since I was using a 20 round magazine in a semi that the bullets were being set back in the case from the recoil; however, as hard as I tried I was not able to press any bullets into the case when manually pressing them against the bench. I measured them and they were the same COAL. At the end of the day I went back and shot some rounds that had the shoulder fully set-back .01 and the groups were .5s. All my loads are individually weighed. I am also using a Redding competition seating die. As far as I can figure by setting back the shoulder minimally as opposed to .01 it has effected the accuracy. It defies logic.

I am going to work up a new load with the brass set back .002 - .003 and see what happens.

 

[Furtaker_.223]

Re: Shoulder Set-back surprising results

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Williamb</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigMahi</div><div class="ubbcode-body">Hows the brass from Scharch - how many reloads can you get from them? What is the general headstamp mix that you received? I'm thinking about ordering from them and would appreciate some info thanks. </div></div>

I'm pretty pleased with the brass. I bought 2000 pieces and most of it 95%+ was LC. After that Winchester. I'm on my 4th reload on some of the brass and it looks fine. Thinking about annealing to extend the life of the brass but the break-even on the cost of the machine versus buying new brass suggest that is not the reason to buy an annealing machine.</div></div>

I have bought 3000 rounds from them and of the 3000, 2000 I bought primmed. I have not had any trouble with it. Out of 2000 pieces, maybee 10 were dented too bad for my liking. But all in all I will give it a 8+ out of 10

 

[former naval person]

Re: Shoulder Set-back surprising results

Williamb...I'd look for something else in the reloading area besides shoulder set back. Do you have a way of measuring run out of the neck? Can you indicate the base and body and see if the base is square to the body? I have seen instances where the bolt face isn't square to the chamber causing the base of the fired cartridge to wind up off square to the body...there is no way when reloading to correct this condition. When you reload that brass the effect is to cock the ammo at different positions within the chamber, opening the groups. Not saying that this situation pertains to you...but it is something to consider. If the purchased brass is square, and your bolt face to chamber isn't then when you first fire the brass you'll be accurate. When you reload you are producing an out of concentric condition to your ammunition. Shoulder set-back has nothing to do with it. Happened to me with a rifle rebarreling once upon a time. JMHO

 

[Williamb]

Re: Shoulder Set-back surprising results

Just came back from the range. Today groups ran the same with shoulder set back .01 as received or with my FL resizing and set back to .002-.003. I really can't explain why the groups had opened up before but right now all is fine. I used both today and both shot just fine. I use a Redding T7 press so there's no messing with the dies once set-up. I weigh each charge so I'm really at a loss as to what may have occured.

 

[former naval person]

Re: Shoulder Set-back surprising results

Shoulder mirage.

 

[Williamb]

Re: Shoulder Set-back surprising results

you may be correct.