Factory ammo "headspace" dimension question

[Brians356]

I've been measuring fired 6mm Rem cases using the Hornady .375" bushing. Cases fired in my R-700 come out about 1.774" which means they are very close to the SAAMI minimum (Max Cartridge 1.7807-.0070 = 1.7737).

I happen to have some unfired Federal "6mm Rem 100-gr SP" ammo, and measuring these they are 1.765" or ~.009" shorter than SAAMI minimum (if I am reading the SAAMI diagram correctly.)

I'm surprised how short these Federal cases are at the shoulder. Is this typical for factory ammo?

 

[jsn]

I once measured borrowed headspace gauges in .308 and 30-06 using the proper Stoney Point bushings, and they were both .009 short of their alleged specifications. I attributed this to the radius on the measuring end of the bushing, accordingly I view the bushings as more of a relative measure than absolute.

Do you have a before firing measurement for your once fired brass, and an after firing measurement for your Federal ammo? Those numbers would be interesting to see and compare.

 

[Brians356]

Do you have a before firing measurement for your once fired brass, and an after firing measurement for your Federal ammo? Those numbers would be interesting to see and compare.

Not yet, but I soon will! Some fired Federal cases (7x57mm Mauser) I do have have more consistent neck thickness compared to typical Remchester.

I take your point about bushing aperture and absolute datum line measurement. But still, the other fired brass I have measured indicates the chamber (assuming about .001" rebound for brass in general) is close to .009" longer than the Federal cases.

 

[243winxb]

I'm surprised how short these Federal cases are at the shoulder. Is this typical for factory ammo?

Yes, they have to fit all firearms. The measurement taken from fired brass out of your rifle may not give a true reading from head to datum. It may take 3 firing of neck sized brass/loads to fully expand to the chamber. The tools used to measure are comparators for checking fired & resized brass. Many will not give readings close to SAAMI standards. Some fired brass may come out of the chamber smaller (head to datum) than it went in.

 

[savagedasher]

I just took 10 new loaded un fired and I found .003 was the norm shorter then the fired. The 10 fired cases all were .0000 to.0005 difference. Not one was shorter. I assume the .003 shorter was a average of new cases. And the fire formed had .0000 to .0005 difference. But as much as .003 long. Larry

 

['Freak]

Sinclair inserts are machined with a taper same as the shoulder angle the case bumps against when it’s chambered, not to a particular orifice size as with the Hornady/SP inserts.

 

[Brians356]

[Note: Some original errors corrected below]

Sinclair inserts are machined with a taper same as the shoulder angle the case bumps against when it’s chambered, not to a particular orifice size as with the Hornady/SP inserts.

Good to know. But the Sinclairs, however they choose to shape them, would need to report the same length as a theoretical orifice with an abrupt corner (obviously an impractical design since the sharp edge creates a couple of problems.)

I have a set of precision pin gauges ("go" or minus .0002") and the Hornady "375" bushing will accept a .374 easy slip fit, but rejects a .375. So it is slightly undersize, but has a slightly radiused edge which if designed properly could produce a tangent point right on the shoulder datum line intersection at .375" diameter. Emphasis on "could." I doubt they are that tightly controlled dimensionally. And Hornady does not tout them for precision measurement, only for revealing relative changes after sizing/firing, to the accuracy of a typical caliper (what, plus/minus .001"?)

Another clue as to the tolerances is that their "330" bushing will not accept the .329 gauge, but does accept the .328 easily, so here we have an additional ~.001" undersize orifice compared to the "375" bushing.

Still, back to my 6mm Rem cases: Given what I wrote above about the "375" bushing, my hunch is it's within .001" of reading actual case length at the headspace intersection. And the Federal ammo is .009" shorter than some misc. cases which have been fired in the chamber many times without body resizing. All signs point to the ammo being at least .008" shorter than SAAMI minimum case spec.

 

[bigedp51]

The SAAMI specifications you are looking at are "guidelines" and not written in stone.

If you buy a actual chamber headspace gauge to compare the newly made unfired cases you may find more cases shorter than the SAAMI min dimensions.

One of my very first posting here was asking about .243 cases that were over .009 shorter than SAAMI minimums and having .012 head clearance when chambered

What bothers me is the empty cases you buy to reload are frequently shorter than factory loaded ammunition in headspace length. And it has nothing to do with the type gauge you are using to measure the case and everything to do with the quality control standards and stock dividends.

 

[Brians356]

What bothers me is the empty cases you buy to reload are frequently shorter than factory loaded ammunition in headspace length. And it has nothing to do with the type gauge you are using to measure the case and everything to do with the quality control standards and stock dividends.

Very good points. FWIW I had searched here, and I did stumble on an old post of yours from years ago wherein you said you had encountered unfired cases 0.011" shorter than SAAMI minimum (if memory serves).

In reading the story of the mythical "Houston warehouse" I seem to recall the chap who owned it (and developed unbelievably accurate loads in it) said he fire-formed all his reformed Sako 220 Russian cases w/ inert filler before doing any tuning on the necks (which is where a lot of secret sauce was applied.) Seems like that might be good practice for the rest of us.

 

['Freak]

Don’t really care what the number long as my notes reflect how and what was used in arriving at that number from a case fully fire formed then ideally sized to get the kind of fit I’m looking to repeat for use in that chamber. Since I’m lacking instruments and dealing with fully assembled rifles it’s all got to be by touchy-feely and trial and error in adjusting the die.

A trinket I am looking for would be sorta like a degree wheel, a thin disc between two jam nuts on the threads of the size die, marked in 72 divisions or every five degrees, and with a pointer fixed to the press hovering nearby. Turning a 7/8-14 die in or out 1/72 of a turn or one marking to the next would represent moving the die in or out ~ .001” (.000992”) taking much of the guestimating out of adjusting the die to get just the right amount of shoulder bump.

 

[bigedp51]

A RCBS case mastering gauge can measure case stretch and thinning in thousandths of an inch and is more sensitive than my old banged up fingers and a bent paper clip.

The case below isn't magically levitating on the end of my finger, but it is blocking the gauges rod that is inside the case.

 

[wapiti25]

I want to say there are TWO sets of Max and Min dimensions. One set is for chambers and one set is for cartridges. You are trying to measure to SAAMI standards with hand tools not precision gage's. Send them back to the mfg if you think they are really bad and don't buy them any more.

 

[Brians356]

I want to say there are TWO sets of Max and Min dimensions. One set is for chambers and one set is for cartridges. You are trying to measure to SAAMI standards with hand tools not precision gage's. Send them back to the mfg if you think they are really bad and don't buy them any more.

I am comparing to the SAAMI case specs.

But even allowing for a thousandth error either way, the Federal cases are clearly much shorter than SAAMI case minimum.

I never implied they are bad or out of spec. I am trying to learn about common practice and adjust my own thinking about "how it's supposed to work". Possibly (even probably) all 6mm Rem factory loads are pretty much that same length, and it's perfectly ok. It implies there would need to be a gross case-to-chamber headspace mismatch to be dangerous - larger than I assumed.

 

[bigedp51]

I want to say there are TWO sets of Max and Min dimensions. One set is for chambers and one set is for cartridges. You are trying to measure to SAAMI standards with hand tools not precision gage's. Send them back to the mfg if you think they are really bad and don't buy them any more.

There is a reason why shooters buy Lapua brass, "BUT" some of us poor folk just buy Remchester brass and use methods to keep it from stretching on the first firing. I have never sent any brass back to manufactures because of the cost and hassle involved and just fireform short cases.

The simplest method is to seat the bullets long and jam into the lands with a upper mid range load and let fly. Some people use a "slippery" method to fireform cases but this can cause problems with older and weaker firearms.

 

[Brians356]

I have never sent any brass back to manufactures because of the cost and hassle involved and just fireform short cases. The simplest method is to seat the bullets long and jam into the lands with a upper mid range load and let fly. Some people use a "slippery" method to fireform cases but this can cause problems with older and weaker firearms.

I read up on fireforming when I got my 250 AI, and decided to go bulletless. My chamber was cut a tad long, so the parent 250-3000 case was not a crush fit as it should have been. :-\

Someone convinced me that just seating a bullet to jam would not necessarily prevent the case from moving forward. So I went the bulletless route using a small charge of fast powder (Bullseye), a pistol primer, and a lightly lubed case. The trick was to find just the right charge of powder by trial and error to get nice sharp shoulder corners, and minimal stretching at the web area.

However, I am not convinced either method has exclusivity, so I suppose the proof is in the pudding, and if you say you got the case dimensions you wanted using a bullet, that's all I need to know.

 

[BoydAllen]

I have never had a problem that was related to first firing stretch, as long as I kept subsequent FL sizing shoulder bumps under good control. This covers quite a few years of reloading, of several calibers. One thing that I see routinely done what I believe to be incorrectly, is bumping brass from its once fired datum to head dimension. Generally, one firing does not bring this dimension to its maximum, and I believe that it is from that maximum, that one should bump. I usually load at the range, and fire a case several times, without FL sizing so that it will reach its longest dimension from shoulder to head, and use it as a gauge to set bump. If I am not able to do this, and have to work with once fired brass, I set the die so that it produces the same shoulder location as fired brass, and then check the first case in the rifle. Generally this works fine. Because cases actually get longer, shoulder to head, as a die is adjusted down, before the die's shoulder starts touching and then pushing back a case's shoulder, if you like to work from feel, and you are using a gauge so as not to bump back the shoulder, you can actually have your shoulder farther forward than as fired, if that gives you a feel that you like better. Just remember to check several cases in your rifle, because differences in how they were annealed will give different dimensions, and cases that have significantly different bolt close feels, will not group as well, when mixed in a group, as if all the cases felt the same.

 

[Brians356]

If I am not able to do this, and have to work with once fired brass, I set the die so that it produces the same shoulder location as fired brass, and then check the first case in the rifle. Generally this works fine.

Not quite sure about that one comment. Do you mean "same shoulder location as the once fired brass"?

That seems reasonable. The party line if you read the basic instructions in books is to dutifully bottom out the FL die and resize back to something presumably close to the factory ammo. Aiming to size back down no farther than a once-fired case seems like a better rule of thumb, if your main concern is smooth chambering, but the secondary concern is not overworking the brass unnecessarily.

Thanks for those detailed comments, good stuff. At my advancing age I am still gleaning new ideas, which is one of the reasons this hobby stills holds my interest.

 

[BoydAllen]

...yes, the same as the fired brass Make sure to either reseat the primer (to get it well below the head) or remove it before measuring the fired case. The usual directions that come with dies are for once a year deer hunters that do not shoot much. If you follow those directions, ammo will fit in any factory rifle, but case life is almost assuredly going to be quite short, due to repeated stretching thinning cases near their heads. Problem occur when folks do not have a proper understanding of what takes place when a round is fired. Back in the day, fellows that I knew who had belted magnums always thought that the "magnum pressures" were to blame for their short case lives, when it was their following the die makers' instructions for setting their FL dies that was the cause. There is no SAAMI spec. for datum to head dimensions of belted cases, for that reason, new cases are very short in this dimension to take in a wide variety that will be encountered in chambers. Similarly, dies for these calibers will have the potential for bumping shoulders way back for the same reason. I have measured new and once fired brass, from factory chambers for a couple of magnum calibers, and in both cases the increase in datum (on the shoulder) to head dimension was .021! I think that you can see why setting those shoulders all the way back would lead to very short case life.

 

[Brians356]

Make sure to either reseat the primer (to get it well below the head) or remove it before measuring the fired case.

I always do. Even when primers, on close inspection, appear level with the head, they often sit slightly proud of it.

I have measured new and once fired brass, from factory chambers for a couple of magnum calibers, and in both cases the increase in datum (on the shoulder) to head dimension was .021! I think that you can see why setting those shoulders all the way back would lead to very short case life.

And the reloader is missing out on the "free" extra case capacity afforded him as well - however slight.