Will amount of bump effect charge/pressure

M

Metal god

New member
I have a 308 AR that has a very long chamber , almost field gauge length . This means I size my cases for that firearm .006 to .008 longer then for my other 308's .

I have some specialty bullets that are limited in quantity that I worked up in the AR-10 and I'd like to reduce the amount used to find loads . I'd like to load that same charge in cases sized for my other rifles . How much difference is there if any in pressure when charging a case with .008 shorter case headspace ?

In my head I think very little because many people size there cases way shorter then you need just by following most die instructions so some are already .006+ shorter then needed and I don't see manuals pointing out this could be a problem as far as reducing charge weight if you do size your cases "short " BTDP .

I'll add these loads are not max but are a bit above the middle point of "safe" charges .
 
I doubt you'll have any bad issues if you set fired case shoulders back no more than .0015 inch average.

Keep cases resized for each rifle separate and set the die accordingly.

Pressure difference will be small with the same load but may need measuring to see real levels because of actual chamber volumes.
 
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considering the amount of gas generated and the small amount of case volume that .006 represents I don't think pressure is a issue. Shoot a couple over a chrono just to be sure. Do bear in mind that the maximum headspace SAAMI specs is .010 (their fieldgage spec). G0 and NO GO have a .007 difference if memory serves. The more the case is stretched and compressed the more the brass is worked so keep that in mind. Do internal and external checks for stress cracks developing
 
The long headspace is the issue . On all my other 308’s I bump .002 . If I fire those sized case in the AR it blows the shoulders out .010 . Those same cases fired in my other rifles only blow out .002 . This is why I size the AR case specifically for that rifle and the cases are longer head to datum .008 . This means they will not chamber in my other 308 rifles .

I did tests on the AR chamber . It closes on NO-GO gauge + .003 shim but not a field gauge . This tells me the chamber is just short of a field gauge . I don’t remember the exact numbers but i use my #10 comp shell holder and my cases are still being bumped something like .004 from fired size . The first barrel was sent back and replaced with this one . They both were cut to about the same length . The manufacturer offered to refund my money but after doing the math I concluded that I could just replace the barrel cheaper than trying to build a completely new rifle . So for now I’m just sizing the cases long so I can shoot it until I replace the barrel.
 
SAAMI specs lists 1.6300" minimum and 1.6400" maximum for headspace. SAAMI doesn't list nor specify any go, no-go nor field gauge dimension.

SAAMI term "head clearance" dimension is the space between bolt face and fully chambered case head.
 
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That may be true but my Forster field gauge says 1.638 on it . I didn’t know .002 longer then a field gauge was considered still gtg . First I heard of that , any reason why you think Forster’s gauge is short of saami spec ? It may explain Why the manufacturer kept telling me the chamber is within spec . I’ve always figured if it fails a no go gauge it’s out of spec but then again I’m not a gunsmith

My gauge’s read GO-1.630 / NO-GO 1.634 / Field 1.638
 
SAAMI specs lists 1.6300" minimum and 1.6400" maximum for headspace. SAAMI doesn't list nor specify any field gauge dimension.
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that's bogus info. There is no Spec for the NO GO. There are specs for minimum (GO) and maximum(Field length)

https://saami.org/wp-content/upload...99.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

page 225 column F give the min (GO) and Max (field length).

here's how Dave Manson calculates the length for his NO GO gages

https://mansonreamers.files.wordpress.com/2015/06/headspace-gauge-dimensions-and-tolerances.pdf
 
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SAAMI doesn't list nor specify any go, no-go nor field gauge dimension in their glossary nor use them anywhere in that document referred to. People outside of SAAMI made them commonly used.
 
Bart,

Follow the link to this SAAMI standard, document page 212 or Adobe page 224, Headspace Gauges. I think this changed over time. They didn't used to spec primer pockets and primers, either, but they've agreed on a standard now.


Hounddawg,

Yep. It's a 0.010" range, and that's pretty typical for a min to max. Commercial field reject gauges are sometimes just 0.008" over min and military can be 0.015" over min. Frankly, anything in that range is reasonable if you are setting the shoulder position two or three thousandths shorter after fireforming.

To the original question (I think), unless you are shooting at peak pressures below around 35,000 psi, the volume of the case before it expands to fill the chamber does not have a significant effect on final pressure. The expanded volume determines that.
 
yet if look at any manufacturers Go Gage or Field Length gage they will be machined to SAAMI min and max chamber length. The NO GO's vary by .001 or so but not the GO or Field Length. They will always be to SAMMI min/max specs

Anyway ignoring the derail. MG if you look at that SAMMI link I provided on page 226 you will see your GO gage is SAMMI minimum. My Manson 308/260/243 GO gage is also 1.630 but my NO-GO is 1.636 which is still .004 short of SAMMI max but too long for my tastes. Like you I set my headspace with a piece of tape on my GO then double check with the NO GO. It makes for a tight chamber and I get excellent case life and performance

edit @ Nick

did not realize that military gages could be that long. Of course I don't own any military rifles so I don't have reason to own any Field Length gages anyway. Learned something new

edit 2 - On the pressure I bet someone with Quickload could vary the case volume and come up with a more scientific guess.
 
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Unclenick, what do I look for on page 212 in your link? I see only minimum and maximum gauge terminology.
 
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To the original question (I think), unless you are shooting at peak pressures below around 35,000 psi, the volume of the case before it expands to fill the chamber does not have a significant effect on final pressure. The expanded volume determines that.
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I don't know about pressure but I'm about 1.5gr under max which doesn't mean much . I did go to max 46gr ( load below ) and had slight cratering of the primers but still had some radius at the edges of flat-ish primers .

Case volume was a great reminder of what I should be looking for . I just so happen to have tunes of case volume tests with my other rifles .

Now this is just general averages with the other rifles compared to the new long AR chamber .

54.8gr for my other chambers

55.6gr for the AR chamber

A tad over 3/4 of a grain less volume if I shoot this load in my other rifles . Might as well post the load .

308 Winchester
162gr AP
44.5gr Win-748
CCI # 200 primers - I have concerns about these , I may need to go to the military primers
LC-15 cases sized .006 longer then my GO gauge .
 
Sometimes you just got to go with your gut. You been doing this long enough to know what to look for but here is what I keep in mind when load developing. Sticky bolt ? Do the velocities match up with what they should be ? In other words not 250 FPS faster than what they should be for that barrel? Does the rifle cycle ok? And of course does the load print acceptable groups on paper ? If it does not feel right it probably isn't and vice versa
 
Best answer I can give you is to forget what you load in other rifles and find the load for that rifle! Your fooling with high pressure. You may find by working you a load for that particular rifle that it can match the other loads fine or you could find it won't. Don't fool around with pressure!
 
I've been called every name in the book for mentioning this, but here goes because its seldom considered.

Everyone shooting the same rifle and ammo will not get the same level of accuracy or precision.
 
BartB said:
Unclenick, what do I look for on page 212 in your link? I see only minimum and maximum gauge terminology.
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I had to reread your post. You are correct they don't do anything under the names of GO, NO-GO, or FIELD REJECT. They just put headspace gauge limits on the tables that match their chamber drawing limits. An ideal GO gauge will match their minimum. An ideal REJECT gauge would match their maximum but are usually made a couple of thousandths under the SAAMI maximum, and the NO-GO length for newly cut chambers is between the two.

I got a lesson in why the REJECT gauge is typically a couple of thousandths short. Many years ago I got one of the armorer's gauge sets for .308 W that Brownells used to sell. I discovered, comparing them with an RCBS Precision Mic and later with the Stoney Point (now Hornady LNL) case comparator, the gauge labeled 1.608" consistently read -0.001" shorter than the gauge labeled 1.607". Two of the other pairs of gauges in the set, despite bearing a labeled difference of 0.001" were actually the same size. So I concluded the -0.002" below SAAMI max was because the industry didn't trust some gauges to be closer than 0.002" to their stated length. It's a manufacturing tolerance allowance. I'm sure the higher-priced gauge makers can do better than that, but watch out for the cheap ones.
 
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