Setting the shoulder back

[cdoc42]

For years I've set my dies so that a resized case sets the shoulder back 0.002" from the fired size, the idea being it prevents excessive case stretch while improving accuracy.

Tonight, I'm perplexed. Cases fired in my .223 AR-15 measure 1.460" using a Stoney Point Headspace gauge on my caliper. Resized, they measured 1.451" so I proceeded to reset the die.

I eventually set it so the resized case measured 1.458" but when I dropped the case into the chamber and closed the bolt, the extracted case measured 1.455"!!

It did this repeatedly. What explains this? Should I resize the cases to 1.455" even though upon firing it expands to 1.460"?

[Jim Watson]

I think you are seeing what is known as "drive in."
The bolt is slamming the round into the chamber hard enough and far enough to set the shoulder back.
I think you are ok at 1.458" even though the gun will knock it back to 1.455".

[steve4102]

When a round is fired in a bolt action the brass case expands to fit the chamber, then it springs back just a little to allow for easy extraction. This fired case is now an almost exact copy of the chamber it was fired in.

Things are a little different in a semi-auto. When the round is fired in a semi-auto the brass case expands to fit the chamber, but when the round is ejected from the chamber it can still be under a slight amount of pressure. This pressure can cause the case to continue to grow after it has left the chamber. A fired case from a semi-auto may be longer or fatter or both than the chamber it was fired it.

Page 3.

http://www.zediker.com/downloads/14_loading.pdf

[cdoc42]

Steve, thanks for the link.

Jim, here's a portion of that link - although the article refers to the M-14 in .308, does it apply equally to the AR-15 in .223?

You suggested a resized 1.458" case is ok even if the chamber knocks it back to 1.455 - but my understanding from the excerpt below is now is there no room for expansion?


"Any hitch in feeding or lock up, especially such as
might be caused by an over-blown shoulder, is an
invitation to an open bolt detonation at worst
(more about this “quirk” in an associated installment)
and inconsistent target performance at the
least."

"…it’s tough to know with
certainty that the fired case reflects chamber
dimension.…"

{Edit: Please read board policy on posting copyrighted materials. A couple of sentences that are directly to the point are OK.}

Because of this, I wonder if I should resize to 1.453"? (Or, actually, I should have left it at 1.451" where it was when I started....)

[steve4102]

What I do with my AR's is, size the case with little or no shoulder bump. Place the sized case in the chamber and ease the action closed, do not let it slam shut. It will stop short of closing completely. I push lightly on the forward assist, if it snap shut with very little effort I am GTG.

If it takes effort or will not close, I adjust the sizing die for about .003 more bump and repeat the chambering process. I keep doing this until the case chambers with very little effort on the forward assist. Then I take my measurements with my Stoney Point Gauge and record.

[Jim243]

There are easier ways to insure proper fit in the chamber of your AR.

1. First re-size your cases with a X-Small Base Resizing die.

2. To insure proper head space and shoulder, use a L.E. Wilson gauge or another manufactures gauge to insure your die is set properly.

Then you are good to go.

Jim

[steve4102]

Yup, those case gauges work too, but I prefer to set my dies up for my Chamber and not for a gauge. After all I am shooting my ammo from the chamber not the gauge and they may indeed be slightly different.

[higgite]

I might be overly cautious, but having a manual bolt closure resize my brass would make me nervous. Wonder how much it's resizing when it auto feeds a round?

A Forster .223 Rem go gauge (1.4636") measures 1.450" on my Hornady/Stoney Point gauge. That's equal to the minimum dimension of a SAAMI .223 Rem chamber. I FL resize my AR-15 fodder to 1.450-1.451" (by Hornady gauge) for reliable feeding and a little insurance against slam fires. I like to give the firing pin a little room to roam. ymmv

Bolt guns are a different story.

[Bart B.]

The firing pin hitting primers usually sets shoulders back from its impact when the round is fired. If peak pressure is too low, it won't expand the case enough to push the back of the case rearward until the case head stops against the bolt face. Case headspace will be shorter after firing than before.

[Jim243]

Case headspace will be shorter after firing than before.

?????? I don't think I agree with this observation. For a bolt action rifle, this can be true, but we are talking (I think) about semi-auto rifles and they extract the case before it has a time to cool and regain it's shape.

It would be nice if this happened, but I have way too many 223 cases that have to be trimmed and shoulder bumped back into spec. (about 60% of them) It will depend on how soft or hard the brass is.

Jim

[Bart B.]

Jim, how long after firing does a case take to cool and regain its shape?

Doesn't the fired case cool down the same way regardless of the rifle they come out of?

Note also my mention of reduced loads which causes this problem in any bottleneck chamber with cases headspacing on their shoulders. With near or full power max loads, this won't happen. When powder charges are about 10% or more below max, it happens frequently.

[steve4102]

Doesn't the fired case cool down the same way regardless of the rifle they come out of?

No, Not at all the same.

Fire a round from a bolt action. Immediately open the bolt and remove the brass, not hot at all and you can immediately inspect it without burning your fingers.

Fire a round from a semi-auto and fetch it off the ground immediately. Pick it up and feel the heat. After you burn your fingers, what a few minutes then you can pick it back up off the ground and inspect.

about semi-auto rifles and they extract the case before it has a time to cool and regain it's shape.

It would be nice if this happened, but I have way too many 223 cases that have to be trimmed and shoulder bumped back into spec. (about 60% of them) It will depend on how soft or hard the brass is.

IME, it also depends of the burn rate of the powder. The slower the powder the more the case seems to grow. I don't know if this is due to the burning of the powder in the chamber or the Increased Port pressure causing a faster more violent cycling of the action.

[Bart B.]

Steve, I was asking about them cooling down to ambient temperature, not how long it took to do it.

Every max load fired case I've measured, from M1's and M14's as well as different bolt guns, have a shorter case length after firing than before. They all grew in length after full length sizing; typically about .001" longer each time. They also had longer case headspace by .001" to .003" after firing that was reduced about .001" to .002" after being full length sized, depending on how the die was set in the press.

[cdoc42]

Bart B., are you measuring the OAL of the case or the shoulder to base? I'm far from an expert but my thought is the case may get smaller OAL, but the expansion to meet chamber dimension also expands the shoulder and body of the case. When you resize you squeeze all that down and there's no where to go but laterally, so the OAL increases. ??? That apparently doesn't happen in a standard bolt rifle because, as pointed out, the case doesn't get as hot and therefore doesn't continue to expand outside the rifle chamber as would happen with a semiauto.

[stubbicatt]

I don't know guys. For self loading rifles, I just full length resize per instructions and discard after 4 loadings. I realize this is sort of anathema, nevertheless, it's what I do.

[F. Guffey]

jim243, to give the case a chance the gage must not be stood on its end, the case can protrude through the gage and be supported by the neck giving the user the illusion the case body is too long, the case must be supported on the datum inside the gage.

F Guffey

[F. Guffey]

cdoc42
Senior Member


Join Date: March 13, 2005
Location: Pennsylvania
Posts: 346 Setting the shoulder back

--------------------------------------------------------------------------------

For years I've set my dies so that a resized case sets the shoulder back 0.002" from the fired size, the idea being it prevents excessive case stretch while improving accuracy.

Tonight, I'm perplexed. Cases fired in my .223 AR-15 measure 1.460" using a Stoney Point Headspace gauge on my caliper. Resized, they measured 1.451" so I proceeded to reset the die.

I eventually set it so the resized case measured 1.458" but when I dropped the case into the chamber and closed the bolt, the extracted case measured 1.455"!!

It did this repeatedly. What explains this? Should I resize the cases to 1.455" even though upon firing it expands to 1.460"?
cdoc42 is offline Report Post
----------------------------------------------------end of the quote

odoc42, you need a plan, try this, slam the bolt close on a case you have measured for length between the shoulder/datum and case head, THEN! take another case with the same measurement and gently in a case friendly manner allow the bolt to close slowly and with control, then! measure the case length of both cases from the usual places as in from the datum/shoulder to the head of the case.

There are a lot of quotes about 'someone said' most stories start with "Hatcher said etc..", my opinion, always boring stories. Anyhow, I am the fan of measuring before and again after, if you have an M1 Garand, measure the length of cases to be fired, slam the bolt close then measure the case length again, then compare.

I do that, I no longer make it a practice but I have chambered cases in a chamber "THEN! drove the cases into the chamber with a pin punch about the same diameter of the case head, never failed, the case got shorter and filled the chamber and I never failed to lube the case, not a good practice in a crowd, even the strongest get dizzy.

F, Guffey

[F. Guffey]

“Setting the shoulder back”

Setting the shoulder back and ‘BUMP!’ as in bump the shoulder back, again, I have not found a way to bump the shoulder, again, I have bump presses, I do not have a RCBS press that bumps, I have at least 3 Rock Chuckers, all of them lock-up, jam-up or cram-up, they do not bump.

Except as described when the bolt slams into the case when chambering a round, in that situation the shoulder is being set back ‘BUT! that is without case body support.

F. Guffey

[springer99]

ed

[dmazur]

Originally Posted by springer99

Using a good press and die set that's adjusted properly, how can you "set the shoulder back" on bottleneck cases.

I believe die manufacturers are concerned about this and, ideally, a die set up to contact the shell holder will not over-resize a bottleneck case (create an "induced excess headspace" condition).

However, the ability to resize to the correct dimension is a desired feature, so they can't go too far in that direction or the die won't work...

Variations in lube and the elasticity of the press can make a difference of a couple of thousandths. And the difference between min / max acceptable case dimensions to datum can be on the order of 0.006", so we're not talking about a big number here.

[F. Guffey]

You ask,

How can you drive the case into the die deeper and cause the shoulder to be pushed back?

I said chamber, not die, still I can drive cases into a die and size them (without a press), then it could be argued, the case head was driven forward, in any event the case shorten as in between the case head and shoulder because the chamber did not support the case body until the case expanded. I also have used arbor presses to force cases into chambers to get mirror images of the chamber, again, if someone chooses to try this method/technique at home do not forget to lube your case.

Back to bump when sizing, the case has a neck, the case has a shoulder, the case has a case body, by your description, by design, the last part of the case to contact the die is the shoulder, YET! it is referred to as being bump the shoulder, even though the neck came in contact with the die first, then the case body came in contact with the die and finally after everything is touching, the shoulder of the case makes contact with the die.

Reloaders have the illusion the shoulder moves, more times than not my shoulders do not move, my shoulders are erased, the shoulder I get when sizing is a new shoulder, complicated? That is OK, Hatcher did not scribe the case body/shoulder juncture on his test cases when fired them in a chamber that was reamed forward by .080”, he just knew his cases would have case head separation because he knew the cases would not stretch .080” without separating, his cases did not stretch, again, do not try this a home without supervision, All of Hatchers fans thought Hatcher knew the 06 Springfield rifle.

F. Guffey

[Bart B.]

Regarding all this shoulder and setback and die setting, consider these observations and conversations I've had regarding it.

I asked RCBS decades ago about their full length sizing die's dimensions and just how much they would resize cases. The RCBS man says all their full length sizing dies for bottleneck cases headspacing on their shoulders typically do the following.... Fired cases are sized back to virtual minimum new case dimensions. That's done as RCBS does not know what chamber cases sized in their dies will be used in; they have to fit the smallest, tightest, shortest, tiniest allowed by SAAMI specs of diameters and lengths. Makes sense to me. Got the same response from Redding, Pacific, Lyman and a couple other reloading die makers

I've got 6 or 7 RCBS full length sizing dies for the .308 Win. case. Each one set to have the shell holder stop hard against the die's bottom will resize fired cases so their case headspace (head to shoulder reference/datum) is at SAAMI minimum case spec; 1.627" +/- .001". RCBS did a good job chambering those dies. My .308 Win. barrels all have chamber headspace at 1.630" +.001".

When I fire a round in one of those barrels, the case length (CL) shortens about .002". Full length sizing that case with the die set to move the fired case shoulder move/relocate/migrate/transfer/shuffle (whatever term other than the reloading industry' generally accepted terms "bump" or "set back" pleases you) closer to the case head makes it grow in CL about .003"; it's now about .001" longer than when a loaded round before. So, the shoot-size-shoot cycle causes CL to go -.002" size to +.003" fire again -.002" size to +.003"..... and so on and the case OAL grows about .001" every cycle. That's fine by me. I trim 'em back to 2.000" when they reach 2.010" long.

If you set a .308 Win. full length sizing die such that the shell holder stops against it with the ram camming over at the top of its stroke, those resized cases will have a headspace at about SAAMI minimum specs. If they're loaded and put in a rifle with SAAMI standard maximum chamber headspace of about 1.634" (SAAMI standard limit; NO GO headspace gauge length), you'll have head clearance of about .007" between the bolt face and case head when the round fires (note the case head is not against the bolt face when the primer detonates). If your rifle's headspace is at the absolute field maximum of 1.640" (FIELD headspace gauge length), you'll have .013" head clearance when the round fires.

Repeating the full length sizing of a case the above way in either instance will soon cause incipient head separation; the case head stretches back while the front 2/3 of the case is hard pressed against the chamber walls every time its fired. After a few firings, especially with an over length chamber, the case about 1/4th inch forward from the head thins a bit more each cycle of fire-resize/reload-fire and it eventually cracks, gives way and sometimes blows completely off the back end of the case.

If one sets their full length sizing die such that the fired case shoulder position is about .001" back after sizing, minimal case stretching at the back end will happen and the case can be reloaded a few to several dozen times if it's diameters don't change much each cycle. Head clearance for each round's about .002" and that allows easy chambering as well as excellent accuracy.

Redding sells a set of five shell holders with incremental heights from .125" to .133" in .002" steps. You can use the one that when it touches the die bottom as the press cams over, the full length sized case headspace is between .001" and .002" less than its fired dimension. Use an RCBS Precision Mic or Hornady LNL case gauge to measure the case headspace before and after sizing.

[F. Guffey]

“the full length sized case headspace is between .001" and .002" less than its fired dimension”

Case head space is....?


http://www.saami.org/PubResources/CC...Winchester.pdf

Again, the case does not have ‘head space’. the case has case length, the chamber has head space, head space is the length of the chamber from the datum to the bolt face.

Then there is that part about ‘camming’ over, if I am using a Rock Chucker I am out of luck for cam over, my Rock Chuckers locks-up, crams-up or jams-up, they does not cam over, I have cam over presses, not a problem but a RCBS is not one of them.

F. Guffey

[mehavey]

If the OP says 'Chamber headspace dimension' I know what he means.
If he says 'Case headspace dimension' I know what he means as well.

Subtract one number from the other and I'll tell him if he has a problem.

[Bart B.]

Well said, mehavey!!!!