Reloading 308 Cal. Rem. 700 bolt action. When reloading for head space. Using RCBS Precision head space mic. Using the gauge with F/C brass fired case measured from zero to minus .001 head space, sized to minus .001, with different ADI brass, I still sized to .001 but after fired the brass measured minus .003 the case got shorter rather the longer. How could this be. The bolt closes on .001 just fine with no resistance.
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How could this be
- Thread starter cw308
- Start date
condor bravo
New member
Somewhat of a common thing and the answer is something like this: when the round fires, the case lengthens by stretching forward to fill the chamber, but at about the same time the case walls expand outward to also help fill the chamber, but in stretching outward the forward stretching is pulled back in some, sometimes resulting in reduced length. So it becomes a matter of forward stretching versus outward expansion and which has the greater influence on resulting length. Because of that, the cases may never seem to need trimming but you have to resize first and then check case length to know for sure. Often I never need to trim but only occasionally square off the case mouths by just removing the knife edge ridge that forms on the case mouth.
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WVMountaineer
New member
The explanation condor give is why neck sizing is such a great thing for bolt guns. Full length sizing is really working the brass in this situation. Brass is expendable. So, what works and makes you comfortable should take precedence over 308 brass. Just something to consider. God Bless
Thanks Guys, is it best to size to the fired shorter case minus .003 That would give me .004 head space. This is only happening with the ADI Brass. With the FC brass my head space was .001 sized down .001 from fired case, I trim necks every firing. My cases are all exact when finished.
condor bravo
New member
Wow, that seems like a lot of additional work to trim after each firing. The cases must lengthen quite a bit after sizing but may not need to be trimmed each time. Perhaps try experimenting with the die adjustments that might improve things. Maybe the shoulder is being set back but not needed. First try backing off some with the die. Then check the trimming after firing and resizing to convince yourself that it may still be necessary, or probably perhaps not, to trim that often.
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Sizing the body with a full length sizing die may also lengthen the shoulders. The Precision Mic's I have when measuring factory rounds they measure close to zero on the gauge. I wouldn't try for a minus reading. Fired cases that are mild to mid range loads don't always expand forward to the maximum length of your chamber plus after they expand forward the shoulders retract some. Most standard bolt rifle chambers I've measured usually show anywhere from .002-.004" headspace, yours may vary. So if rounds measure "zero" on the Mic you still have .002-.004" clearance. Maximum loads expand forward more than mild ones or like in your case can actually shorten the shoulder. The Precision Mic at "zero" is very close, plus or minus .001", to what a factory round measures and to saami minimum dimensions. Also, try some of your sized cases in your rifles chamber to find out what length that the bolt closing starts to get tight. Your fired cases may not be tight but some once fired cases from other rifles may be too long and you can size until the bolt just starts to be snug to close. No need to oversize the shoulders back and like you've found out different cases vary from fired dimensions plus will also vary from sized dimensions.
I still sized to .001 but after fired the brass measured minus .003 the case got shorter rather the longer.
As I have said many times, I have fired cases that shortened .035" from forming and firing. I have fired and formed cases for one chamber that resulted in the case getting shorter by .045", to describe 'shortened': The case got shorter from the mouth of the case to the head of the case.
At the same time the same case lengthened .202" from the the shoulder to the head of the case. The case body got longer and the neck got shorter. After firing the case neck measured .217", my opinion, that is too short. My opinion, the 300 Win Mag has a short neck, the 300 Win Mag neck is close to .267" long.
To explain lengthen: The case did not get longer from the shoulder to the case head, had that happened the case would have separated.
F. Guffey
CW308,
Ignore the trimming comments. I believe they've misunderstood that you are measuring from head to shoulder datum diameter, and think you are measuring case overall length. It's true that firing can actually shorten case overall length by pulling brass from the neck in order to expand the shoulder, but it shouldn't shorten the measurement you are making.
Federal brass is generally softer than most other, so it is less springy and will spring back less after reaching the shoulder than harder brass does. The ADI may be relatively hard (I don't have any to compare). Normal brass springs back 0.001-0.002 inches from actual headspace size. You can can get that by calibrating your RCBS PM to a precision headspace GO gauge, if you really want to know.
My RCBS PM's are only accurate to about 0.002 inches from absolute. I've put GO gauges into them and I've got one that reads 0.002" high and another that's 0.001 inches high and another that's -0.001 inch low on their respective GO gauges (.30-06, .308, and .223). This is just the tool's precision limit, and without calibrating to a GO gauge it is pretty impossible to say what you actually have.
P.S., if you fire a cartridge at low pressure (about 30,000 psi or less) it will back up rather than stick to the chamber and not stretch. No idea what you are shooting.
Ignore the trimming comments. I believe they've misunderstood that you are measuring from head to shoulder datum diameter, and think you are measuring case overall length. It's true that firing can actually shorten case overall length by pulling brass from the neck in order to expand the shoulder, but it shouldn't shorten the measurement you are making.
Federal brass is generally softer than most other, so it is less springy and will spring back less after reaching the shoulder than harder brass does. The ADI may be relatively hard (I don't have any to compare). Normal brass springs back 0.001-0.002 inches from actual headspace size. You can can get that by calibrating your RCBS PM to a precision headspace GO gauge, if you really want to know.
My RCBS PM's are only accurate to about 0.002 inches from absolute. I've put GO gauges into them and I've got one that reads 0.002" high and another that's 0.001 inches high and another that's -0.001 inch low on their respective GO gauges (.30-06, .308, and .223). This is just the tool's precision limit, and without calibrating to a GO gauge it is pretty impossible to say what you actually have.
P.S., if you fire a cartridge at low pressure (about 30,000 psi or less) it will back up rather than stick to the chamber and not stretch. No idea what you are shooting.
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condor bravo
New member
Regardless of what measurement is being referred to by the OP, but probably meaning from case head to shoulder datum line, he states that he "headspaces" his brass to .003 and trims after each firing. If really necessary to trim that often, it is probably being caused by a problem that should be addressed. My guess is that the shoulder is being pushed too far backwards during sizing ( headspacing to .003) resulting in excessive overall case length after firing and sizing and requiring trimming. Try backing off some with the sizing die to decrease the case "headspace" length but disregard achieving any particular case headspace number. As rg1 mentions, could be desirable if you had access to oversized brass as fired in another rifle, and then just size it down slowly until achieving that slightly snug chamber fit and then use that die setting for your own brass.
As far as the improbable use of "headspace" relative to brass, why not use it anyway since everyone knows what is meant? Like the politically incorrect term "case neck tension", everyone knows what is meant so go ahead and use it also. For instance, the main reason I prefer the heavier and thicker Federal brass especially for '06 Garand loadings is due to its greater case neck tension.
As far as the improbable use of "headspace" relative to brass, why not use it anyway since everyone knows what is meant? Like the politically incorrect term "case neck tension", everyone knows what is meant so go ahead and use it also. For instance, the main reason I prefer the heavier and thicker Federal brass especially for '06 Garand loadings is due to its greater case neck tension.
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Thank you for your input, I trim not because I have to, in most cases it's .001 I just want every case to be the same. I do shoot at a very low powder range. IMR 4064 40.5 gr. under a 168 gr. Sierra HPBT bullet. The ranges on LI NY are max. 200 yards. That load is giving me a 1/2" group a that range I am just stumped with the difference in the reloaded case to the fired case. I can install a fired case in my chamber with no problem. .003 I resize to .001 F/L size, I banchrest shoot only. I don't like to resize to zero headspace, can cause gauling on the bolt locking lugs I keep it to .001 works for me.
condor bravo
New member
I would think you are fortunate to have 200 yard ranges at Long Island. Sounds like you have things well ironed out to your satisfaction. Refer to post #2 again for explanation relative to difference in case length between unfired and fired case. It's a matter that the case upon firing expands in all directions rather than just forward and neck/shoulder spring back fills in any void left from the case wall expansion/contraction which then often results in a shortened case. A benefit of this is that rounds that chamber with difficulty, usually extract easily upon firing.
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