Follow-up to Overpressure signs

[Ike666]

This is a follow up to the thread “Overpressure signs” after another day at the range.

I loaded up some intermediate loads of 4064 at 40.5 and 41.0 grains. These loads were still using a COL of 2.80. The cases are LC Match (mean for 125 cases was 178.5 grains). I also loaded one string at 41.5 grains again, but this time following Loader9’s suggestions set the COL at 2.775.

All loads are topped with a Hornady 168 gn HPBT Match bullet, lit off with Federal 210 Match primers, and fired out of a Rem 700 with a 26” heavy barrel.

The original issue was with primers backing out at the start load (40.0) and flattening at a load of 41.5 gns. I did measure the primers backing out and they were right at 0.010” which Unclenick indicated was WNL.

I got lots of good suggestions, some if which I could try and some which will have to wait until I get the necessary tools (chamber length gauge and maybe a set of headspace gauges).

The following picture shows the three loads (40.5, 41.0, & 41.5 – front to back) fired today. There is some flattening of primers in all three loads and there was some very slight backing out in three cases with the 40.5 gn load.




The case heads were all measured in the .469 to .472 range after firing, and the paperclip test indicated no faults in any of the case walls. I don’t think I’m really looking at an overpressure situation here. I may need to further evaluate the headspace issue. All these cases were FL-sized. After I re-prep the brass I’ll load up some with a collet neck-sizing only to see if that improves the flattening. I may also try some harder primers. The groups are below.

[velocette]

Ike;
Your case weights for LC match mine nicely. Nice to know that my scale matches others very well.
My experience has shown that Fed primers are softer than most (all) others. They are easier to press in, easier to fire and will show flattening when others will not. Your loads are well within normal ranges.
Have you had your rifles headspace checked? If you have a bit too much headspace, when the round fires, the primer can back out & then be flattened as the case moves back with the pressure. (consider how much effort it takes to seat the primer, & then the same effort will move it back a bit with some room for it to move.) Have you had the same problem with other primers and / or other cases?

Roger

[RWNielsen]

good shooting by the way.....

[Unclenick]

Ike666.

The 0.010" is the maximum for a chamber at the FIELD REJECT gauge limit, but if and only if the cases start at 0.002" below minimum. In that case, an average chamber would be about 0.005". So your chamber may be long, as Velocette suggests. When you get a gauge you'll learn where the cases start and where they finish? It can be useful to buy a good quality GO gauge to set your gauge to zero against (this would be minimum headspace).

If it turns out that chamber is long and you don't want to mess with rechambering (if it is otherwise OK) then you might look at the Redding competition shell holders which have added thickness on top so your sizing die sets up to leave the cases longer. You don't usually want or need to push the shoulder back more than about 0.002" for magazine feed from a bolt gun. Those shell holders make it easier.

[Ike666]

Nick: The cases were trimmed to 2.005". It looks like I'll need to spend the $70 or so at Midway to get the GO, NO-GO, and FIELD gauges. I just got a new Winchester M70 Stealth and I can use them for there too.

I haven't had these things happen with mixed commercial brass or the Lapua, so I'm hoping the chamber is not too far off. It is a stock, out-of-the-box, Rem 700 PSS. I traded a Springfield M1A for it in 1999. I've enjoyed the Rem but always felt that was not the smartest trade I've ever made.

I probably won't re-chamber it anytime soon since it really does shoot better than I can.

[JR_Roosa]

Don't bother with the chamber gauges yet. Also don't bother with the field gauge even if you do get a set. GO and NO GO are all you really need.

Get a headspace gauge for your cases first. Either the stoney point (now hornady) or the RCBS. If you measure a fired round it will estimate what the headspace for your rifle is. I would measure one with a flat primer. Once you know it, as long as you always shoot reloads it doesn't really matter if it's long or short because you will always be either neck sizing or pushing the shoulder back 0.002 to 0.004 or so once they get too tight for the chamber.

Check the headspace on fired and resized brass. I'll bet that you'll find that you a currently resizing your brass exactly as much as the primers are backing out. I also bet that if you measure commercial brass it falls right between your fired brass and your sized brass.

By the way, your "flattened" primers are backing out too, just they are getting squooshed by the stretching case. Once you figure out the proper adjustment of your sizing die, your primers will have nowhere to back out to. They will stop looking quite so flat because the squooshing process makes the primer look lile a little mushroom with a ring where it fills in the little bevel on the primer pocket. If they can't back out and if the pressure is in the normal range, the primers won't flow into that little bevel.

If you find that your fired brass is way longer than commercial loads, then drop the money on a set of chamber gauges, or just have your gunsmith check it, because he'll need to adjust the headspace if it's wrong anyway and he already has all the right gauges.

-J.

[44 AMP]

I haven't had these things happen with mixed commercial brass or the Lapua,

Just curious, does this mean you have fired mixed commercial, etc in the Rem 700 that is now backing out primers?

Have you fired your load in any other rifles? IF so, does it do the same thing in them?

A quick look at an old Hornady book says your load of 41.5gr 4064 well below max, but that was in Winchester brass. Is it possible the GI brass is pushing up the pressure too much for this particular rifle? Could it be that simple?

[F. Guffey]

I want to go over that one more time, seems I can never get it sorted out because events do not happen one at a time (in my mind), on the Internet anything is possible.

The firing pin hits the primer, then the whole sum of all the weight, bullet, powder, case and primer outruns the firing pin strike?? The firing pin being mechanical outruns the exploding primer, then the shoulder of the case hits the shoulder of the chamber and then in time the primer is pushed back?

Then I chamber an 8mm57 in an 8mm06 chamber with .127 thousands head space, pull the trigger, then eject an 8mm06 case with a very short neck and no protruding primer that has been flattened for what ever reason, .127 thousands is enough travel to unseat a primer, if it was possible for the cartridge to out run the firing pin strike there would be a light strike on the primer and no explosion, and with all the after market mislabeled tools that check head space? no one can measure the the ability of the firing pin strike to shorten a case.

When I pull the trigger my firing pins crush the primer FIRST, THEN the primer (bang) drives the case forward and in doing so separates itself from the case (or tries to). That is the first rule in wreck, stove-in, change. Even without a chamber and when an auto is setting at a red light is hit from the rear by another auto driven by a distracted driver the rear of the auto starts to absorb the impact, the rear bumper is destroyed before the front bumper starts to move, except on the Internet??? NO, NOT IN MY CHAMBERS, the firing pin strikes the primer, before anything moves the primers ignites and blows the case forward THEN etc., and somewhere in there the primer absorbs the impact of the firing pin strike before the front bumper starts to move, except on the Internet.

Then the military case, backed out primer and high pressure? On the Internet, yes, but not in my chambers, high pressure is high pressure and everything is relative, the flash hole increases in diameter, the primer pocket increases in diameter, the diameter of the case head increases (when it is measured before and after) and the case forms to the chamber or when removed is a mirror copy of the chamber (except for the spring backs), so there is a conflict, the explanations do not explain the protruding primer.

I would suggest sizing the cases using the same methods and or techniques, install primers (no powder, no bullet) chamber the cases and then pull the trigger, the cases ejected should have protruding primers, the primer drove the case forward and itself back, to make the test more effective, try setting a bullet to increase the effect.

I fire form when it is not possible to form a case before firing, I form once, I do not use a filler with reduced loads,

Primers backed out .010 thousands: Full length sized cases are minimum length, the chamber is go-gage length, that is a difference of .005 in the world of averages, a chamber with head space issues will go to .009 thousands (+/- a little), then there is the field (reject) length chamber, that one goes to .014 thousands, If head space is compared to the full length sized, minimum length case ( over the counter commercial ammo)). Compensating for the chamber that is not perfect, does not seem fair, my press and dies have threads and are adjustable, some purchase custom stuff (tools), I custom size cases with nothing more than tools I use for other purposes.

We spend a lot of time talking about THE BEST PRESS, something like not being able to see the forest because the trees are blocking the view.

F. Guffey

[Jim243]

That 40.5 grain group is really nice. Looks like you have found the sweet spot for you rifle. You can discount that single flyer from a cold barrel, the rest of the shots look great.

Match primers are suppost to be more consistant, I don't know if they are hotter or not, but don't think so. The primmers do appear to be softer by the way your firing pin is almost punching throu, but that's only on the 41.5 grain loads.

I would suggest neck sizing those cases (already fire formed to your rifle) so there should not be any issue as to case length, shoulder or headspace. If you neck size only. Then try some CCI or Winchester primmers and see what the result come out.

Good Luck
Jim

[SQUAREKNOT]

If you are full length resizing-try backing out the die .030 and see how they chamber. I had a similar problem and my HOWA 30-06. It will shoot
almost one hole- 5/8" X 7/16" outside measure of the holes. You will have to play around with the setting but it worked for me.

[Ike666]

On the 40.5 gn target the Lapua impact (low) is just a remnant that I wanted to fire (rather than pull) and is not part of that group. The 40.5 group is a cold bore group, the first round POI was dead on the center line. The actual flyer in that group is the straggler to the left.

The POA is the center of the square and the scope is zeroed at 200 yards. POI is about 3.5 inches above POA at 100 yards. These targets are all fired at 100 yards from a Harris bi-pod.

I did full length size all this brass as it had not been previously fire formed to this chamber. This was the first time I loaded this brass. I do remember that all this LC brass required that I really honk down on the sizing stroke. For the future, for this brass, I plan to neck size with my Lee Collet die. But I'm also thinking I'll back the FL die out just a tad (0.03" you say) for use with my gas gun.

[Unclenick]

0.030" back out will only size the neck. Not good for the gas gun, which will want the should set back at least 0.002" from its chamber size.

I believe Mr. Guffey is correct that the primer actually completes driving the case forward. The sucker goes off fast, and the vent (the flash hole) is too small for the gas to escape faster than the little explosion drives the primer cup out to the rear. The primer cup thus ends up acting like a little piston, pushing equally and oppositely against the bottom of the cup and the bottom of the primer pocket. Indeed, with the still higher brissance of non-toxic primers this is enough of an issue that they have to drill the flash holes larger to prevent the primer from over-mushrooming itself before the powder pressure reseats it. It is also why magnum primers often have thicker cups.

I'm just checking numbers: The old SAAMI drawing I have puts the case datum at 1.634 +0.000"/-0.007", or 1.627"—1.634". The chamber dimensions, at the same 0.400" diameter shoulder datum by which the case should position is specified, is 1.630" minimum. That means there is an allowed interference fit of 0.004" between a maximum new case and a minimum chamber. This is made possible because there is a 0.0011" minimum chamber diameter over maximum case diameter for the case to expand into when slightly crushed by a bolt.

I know of nobody actually making new cases that long. Most new cases I measure are at about 1.628", or 0.002" below the chamber minimum. (NATO 7.62×51 cases have the same spec as commercial .308 cases, but the NATO chambers are cut to a 1.635" GO gauge to eliminate that possible interference fit for self-loading feed reliability.) The Current .308 NO-GO gauge for new chambers is 1.634", like the case maximum, while FIELD NO-GO or FIELD REJECT gauges are 1.638". (The old allowance was 1.640", IIRC?) So, if you have typical new brass that is 1.628" to start, then when the primer extends 0.010" high with a low pressure load, that should put you at commercial FIELD NO-GO/REJECT maximum (1.638"-1.628"). If you have a truly minimum size new case, then 0.011" would do it. Manufacturers normally aim for 1.632" to be in the middle of the range with new chambers.

Don't let the maximums worry you too much. NATO chambers can go to 1.6455" before rejection, and the ammo still works. The trick is to resize cases for your gun if the chamber is long. The penalty for failure to do so is decreased case life and the need to carry a broken case extractor round with you, just in case you get a head separation.

[Ike666]

Let me see if I've got this now. The distance the primer extends beyond the case head is an estimate of how much too short for the chamber headspace the cases are. The firing pin drives the case and primer into the chamber as it is detonating the primer. This leaves a small gap between the boltface and the casehead into which the primer initially expands. With the lighter loads the case expansion during firing never pushes the primer back into the pocket.

This is likely because the shoulder setback on the cases is too far from the full length sizing operation. If I adjust the die (not as much as 0.03) to set the shoulder a little farther up the case then the loaded rounds should headspace on this chamber better.

This brass is now fire formed to this chamber. If I just neck size it, the shoulder should now be in the correct position?

I've got a Dillon case headspace gauge and I checked each case before loading. This operation suggested that these cases were all sized to the correct headspace - what am I missing?

Also, if the case head is held to the boltface by the extractor am I actually headspacing on the extractor? Is there that much "slop" in the extractor hold? This is more of a concern with my new rifle, a Model 70 with the CRF bolt.

[F. Guffey]

No, it means the case when fired did not fill the chamber, psi is equal in all directions, if the head of the case does not move back when the case body is locked onto the chamber wall, the pressure required to stretch the case is too low.

Forming? I do not fire form, I form first then fire, after firing the formed case I have once fired cases. The question? I do not know but it seems to me there is a mental block with reloaders, seems they can move the shoulder back with the press, die and shell holder but can not move the shoulder forward without firing and that is the reason the only answer is 'FIRE FORM'!

Keep it simple, I have not heard that one in a while, I keep it simple by forming cases to fit the chamber first before firing, all I have to do if find a case with a shoulder that is tooooo far forward to chamber, then start sizing without the 'guesstimate of a turn' I adjust the die and control the length of the case from the head of the case to it's shoulder, and it helps to have forming dies, a very functional and necessary die that goes beyond 'nice to have' as opposed to specialty dies, most of my dies are in the category of 'versatile dies' the versatile die can be used to reduce the length of the case from the head of the case to it's shoulder by .017 thousands under a go gage length chamber to infinity, it can be used to off set head space, in thousands, from .005 under a go-gage chamber to infinity AND it can neck size, all for one price.

No you are not head spacing on the extractor, firing an 8mm57 in an 8/06 chamber creates a head space of .127 +/- a few, after firing the 8mm57 case is ejected as a short neck 8mm/06, if one wonders if the case head spaced on the (claw) extractor they should purchase a feeler gage to check space between the back of the extractor and receiver, on receivers that are control feed, there is not a lot of room, on control feed receivers that allow a case to be chambered and the bolt to closed there is a lot of room, if the person doing the measuring can figure out how to move the case back and forth with the bolt closed, after all, when the bolt closes, the lights go out and the chamber gets dark. And I believe my computer would run out of ink before I could convince someone it can be done.

F. Guffey

[Unclenick]

Ike666,

Your fired cases are now a close fit in your chamber. When they were fired they expanded to fill the chamber and didn't spring back more than around a thousandth. The Dillon gauge should give the OK only to a case that is sized to fit any and all properly chambered .308's. The fire-formed cases, however, are only known fit yours. And because they fit it so well, if you size only the necks and shoot those same light loads, there should no longer be any gap at the breech for your primer to back up into. Since the primers no longer back up, when you use the higher loads that gave you very flat primers before, they now should be less flat because there is no longer a protruding portion of the primer to swell without constraint. That's the experiment that needs to be tried. Does the load that produced such flat primers before still flatten them as much when the case is only resized at the neck?

[Ike666]

Thanks Nick - that's what I was thinking. I'll repeat both the 4064 and the Varget loads with neck-sized brass and see what happens.