Shoulder postion and accuracy?

[lazyeiger]

I made a mistake when pushing back the shoulder on my last lot of reloads, (308 F/TR) I measured wrong and actually did not push the shoulder back at all, hence the bolt closing was tight. I shot them anyway and the acuracy was terrible, I was having trouble holding 1 MOA at 200 yards.

How does the shoulder pushback effect accuracy? Say I have the shoulder pushed back 0.001" or 0.003" or even 0.005" will this have an effect on the accuracy?

Thanks

Jon

 

[Heavies]

I am interested in this too. I had found that in my loads if I bump between .001-.002" then the POI is the same.

Once I bumped .005" on a few rounds by mistake. The two shots grouped but at a different POI.

I would like to see what others have found.

 

[357Mag]

Lazy -

Howdy !

Dependent on your type of bullet seater, the issue may not be so much that the case' shoulders
have been shoved back to where they don't contact the chamber' shoulder......

It may be that for bullet seaters that work off the case shoulder being in-contact with a shoulder on the sliding case "sleeve", seating bullets on cases' with a fore-shortened base-to-shoulder dimension..... will throw-off bullet' seated position in the case; a commensurate amount.

Had that happen to me, using a Hornady "universal"-style bullet seater ( in this instance,
working w/ .35 Remington ).

Another thing to wonder about is whether bullets crammed into the rifling; are doing more of the "head spacing" than "bumped" case are ?

This then brings up neck tension, and whether bullets are really being jammed as-much-as is supposed ?

Didn't mean to high-jack the post or anything !

With regards,
357Mag

 

[fguffey]

Lazyeiger, time, time is a factor, to control time I use the companion tool to the press, the feeler gage, the most humblest of tools, it is a standard, it is a transfer, I change the effect time has on a round when fired with a feeler gage..

F. Guffey

 

[lazyeiger]

Lazyeiger, time, time is a factor, to control time I use the companion tool to the press, the feeler gage, the most humblest of tools, it is a standard, it is a transfer, I change the effect time has on a round when fired with a feeler gage..

F. Guffey

I don't understand what you are saying here? Where is the time changing? the amount of time you keep the press loaded? How do you use a feeler gage?

Thanks

Jon

 

[Redhighpower]

Hello. When you loaded the cases without pushing the shoulder back and had to force the bolt to close ,you were using the bolt and chamber to size the case. I think you may have noticed a sharper recoil. I think bumping the shoulder back .001 to .003 is the right idea as the bolt will close without force. That is what most of my shooting buddies use. I size my .308 semi-autos .002 to .003 and my bolt rifles .001. Larry

 

[Shynloco]

Jon,
To answer your question about the effect on accuracy, the answer is "YES" shoulder angle does affect accuracy. That is why the serious and competiting Benchrest Shooters Full Length Size their brass each and every time they reload after shooting. Part of the reason is that they want the casing and bullet therein to be set the same in the chamber each and every time they shoot, hence ultimate accuracy. Their headspace is carefully measured and if the casings shoulders are off, it changes the locations (even though perhaps minutely) of the casing and the bullet when it is seated into the lands and grooves. Even being off by .001 to .003 is unacceptible to those aforementioned shooters. SO if you Full Length size you casing each and every time, in theory the shoulders are returned to their trued position on each and every casing. Any variance thereof is unacceptible period. Now I don't compete mostly because of my dimibished eyesight over the years and frankly, ain't got the bucks for a Custom made Benchrest rifle. But I do have a factory made Benchrest Long Gun with which I measure the shoulders on each and every piece of brass of reload. The same holds true for factory made general production rifles of all calibers. There is a shoulder setting that affects their accuracy as well. NowI have and use RCBA Precision Mics for each caliber that I reload. That includes my .308 that I shoot using either 168 grs or 175 gr Sierras. Using the RCBS Mics which are"indexed" for each caliber, the position and measurements of the shoulders where the bolt close with the least resistance, is the best shoulder location for that particular rifle to attain the best accuracy since all I shoot anymore is Target Shooting. SO yes shoulders do affect accuracy because accuracy depends mostly upon finding what each particular rifle likes the best loads in all facets of reloading. Shoulder angles are but just one of those facets of accuracy. Hope that helps.

 

[mytwo60]

I was going to post this very same question. My rifles for some reason like a bump of .004-.005 vs. .002. I get better groupings for sure. I have no idea why this is the case other than a different spike in pressure.

Be nice to hear other explanations. btw, I measure each casing before and after sizing. In fact, I measure each case at least twice just to make sure.

 

[Laurie]

+ 1 on what Shynloco says - by having either a crush case to chamber fit or an over-loose fit with too much headspace, you inject all sort of new variables into the ammunition and rifle performance. Too much headspace has the further disadvantage of firing pin energy being used to push the mass of the cartridge fully forward before it does its proper job of detonating the primer potentially giving inconsistent ignition.

Some rifles seem to group fine with a crush cartridge fit, others don't like it all. I suspect this is at least partly down to how stiff the action is and/or bolt lug lock-up. A crush fit has the effect of pushing the bolt back hard against the receiver locking surfaces which sounds a good idea, but it may inject a load of new stresses into the action and/or affect the bolt's closed position marginally. In either event, you've injected a change from the action's behaviour that would apply with the normal case to chamber clearance and for which you worked your load up. Probably too, although all rounds seem to give equally hard bolt closure, some will inject more stress onto the bolt lugs and through them the action body than others increasing the variable element. A small clearance of 0.001-0.002" is ideal. If you measure the case-head to shoulder datum line using a case gauge and callipers, you usually find that there is some variation after sizing too due to variations in brass hardness, degree of lubrication, speed the press handle was operated, press linkage play, and/or the Hand of God! However, with good tools and good brass that doesn't need re-annealing, the variations are small. If you are going for 0.001" clearance, it does mean you have to measure a large sample after sizing though to ensure that the example that gave that clearance on setting the die up wasn't at the large end of the sample clearance-wise (or short end of the case headspace sample is you see what I mean).

There was an article in Precision Shooting many years back by a frustrated M14 service rifle handloader on this issue as he got huge variations - and the M14 (it was alleged) is a design where big headspace variations are a BAD THING - too much and you get terrible groups; too little and you risk a breech explosion as the design can let the striker drop without 100% bolt lock-up. (Can't comment on this as no M1A / M14 experience, but I've read of cases where faulty factory ammo caused blow-ups in tightly breeched competition rifles.)

 

[tazzman]

Hi everyone,

Just a few thoughts (It's late here now) considering this problem.

I think that it is most important to keep the projectile in line with the centerline of the bore.
So let's look at this from this point of view.
If your rifle likes the bullet jammed into the lands, then we should consider the bullet lined with the bore.
If, however, your rifle likes to jump the bullet, let's say .020, then, if the shoulder is bumped .002,.003 back, neck clearance to the reamed chamber, .002??, then I could consider that the round is no longer lined up with the C/L of the bore.
What happens to the brass at the moment of ignition when the round is lying at the bottom of the reamed chamber?
Irregular brass expansion. Bullet is not launched inline with the bore. So what's next?
Am I splitting hairs?
Now, as lazyeiger mentions, his shoulder set-back was not done in accordance with what he thought.
I would think that, in his particular situation, pressure felt by the brass/bullet was increased, hence change in accuracy as compared to previous experience.
My take on this is: if you like bullets jammed into the lands, then your bullet is inline with the C/L of the bore.
If you like jumping the bullets, then a slight tension when chambering the round (hopefully felt by the shoulder aiding in centering the round) will probably ensure alignment of the projectile to the bore.

Any more and you risk losing accuracy. In this, I mean that shooters shooting for score or group will notice this difference, but hunters will probably not (1/4 min of deer)

Just my .02 CDN.

Phil aka tazzman

 

[Leaddog]

FL size each time with a bump back of .001 works well in my two F Class .308's. Each rifle has it's own dedicated die set (Redding S Dies).

 

[Shynloco]

Jon,
Lots of good advice from some experienced guys. Now my comments referenced "Benchrest" shooters because their rifles are usually the most accurate and produce the best and tightest groups. And even folks like Charlie Watson's F Class rifles produce some incredible groups. But thats in part because their rifles and equipment are fine tuned, hence "dedicated dies." But whether your rifle likes their bullets to jump or whether they like to be jammed .010, gotta get those shoulder right for best accuracy and each rifle will dictate what it likes the best. Good luck and have fun.

Alex

 

[lazyeiger]

Lots of great information here, thank you everyone!

Now down to actualy putting this all into practice! I have a redding full length sizer/ bushing die and a Sinclair head space caliper attachment. I popped the primers out of some fired cased and measured the base to shoulder, the first problem is this is just not an accurate measurement with the calipers and Sinclair widget. I can vary the length by about 0.003" just by how I do this ( is the RCBS tool more consistant?).

Now I set up the die and resized/bumped back the shoulder 0.002" or so I thought! I did 50 cases and the measurement varies by 0.005"!

What am I doing wrong! Maybe the RCBS shellholder set with different thicknesses would help the consistancy of the press operation?

Help!

Jon

 

[Keith Glasscock]

I've run into a few things with inconsistent sizing; you might check these:

Make certain that the decapping rod isn't hitting in the web of the case at the flash hole.

Check to see that the butt end handle of the press isn't touching something at the full-down position (or any other part of the press for that matter).

Make certain that all of the cases you are working are the same brand with the same number of firings.

Lube, lube, lube consistent lube makes a difference!

Expander ball - I assume you aren't using one, but if it runs tight, it'll pull the headspace longer on the up stroke of the handle

Timing - Push the handle smoothly, all the way down and count to two before raising it again - sometimes we think we hit bottom when we just bounce the handle down and back up.

Die - make ceratin it isn't moving around - seen it, done it...

Caseholders will allow you to change the amount of sizing, but will not change the variation

Hope this helps,
Keith

 

[LongRanger]

Lazyeiger said: "Now I set up the die and resized/bumped back the shoulder 0.002" or so I thought! I did 50 cases and the measurement varies by 0.005"!

Don’t consider the FL sizing/bumping operation complete with just one pass into the die. The amount of variation you are getting can often be reduced by pumping your press handle more than once when resizing/bumping. I ALWAYS rotate my cases about 120 degrees after each stroke (3 times into the die). Just rotating the case once by 180 degrees can show a measurement difference vs the first stroke, and even just leaving the case in place and stroking it a second time can show a difference so it’s the repetition that counts. (I rotate when FL sizing under assumption the case will be straighter, but it is mostly a “feel better” action that takes very little time).

Should you find that doing the above works for you, the “final” amount of bump will probably be more than you intended, so adjust your die accordingly.

You’ll find some cases will still be stubborn and resist being beaten into shape. They need to be annealed and if still unwilling to "submit", should be tossed into your “fouler” pile if you are a competition shooter.

Frank B.