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Old 11-10-2019, 09:29 AM
mac1911 mac1911 is offline
Join Date: Oct 2009
Posts: 2,930
Wink Confusing about the RCBS gauge "headspace"

I think the big problem with the RCBS precision mic is the "headspace" wording.

The Gauge itself is not to measure "headspace" as you think of "chamber headspace". You can use it to compare a factory case to a fired case to see what your "fireformed" case now compares with the unfired cases. I think this is why Hornady calls their set up a comparator..

RCBS 30-06 in their instructions says the min Cartridge Headspace or dimension A is 0 on the gauge or 2.0487

Now where did they come up with that? I know thats min Chamber" spec but why zero on that?

pg 94

Says that the "CARTRIDGE" specs dont even call the measurement off the datum "headspace"

its just referenced at .375" the MAX is 2.0526" and gives you -.0070" allowed BELOW the 2.0526" or "min" of -.0070" = 2.0456"

Now this is all good if your gauge is made to very tight specs. We see from some posts they are not. With my simple tools my RCBS 30-06 case gauge short side that touches the shoulder is .373"

So basically if you have a dead nuts perfect min Chamber spec 2.0487" and a max Factory Cartridge "shoulder/headspace" spec of 2.0526" your .0039" over your min go gauge----so your cartridge will not fit right? no it will all but the weakest action will smash that .003" back and chamber.....wont it?

Its all very confusing until you just start using the gauge to compare "factory" brass with fired brass.

For example :
I measured all the HXP years I have and with the RCBS precision mike all samples 64/68/70/77 came in at 0 on the mic +- .001"

so I have always loaded my bulk 30-06 fodder to zero on the mic.
Then when I was looking to load more accurate ammo for my new barreled rifle I was just adjusting my dies until I got about .003" bump. Worked out just fine.

Later I bought the Hornady comparator. Measure the same HXP brass and it reads 2.044" mind you now you have another tool with its inconsistencies to deal with BUT again your using these tools to compare readings.

so with HXP coming in at 2.044" on my gauge that puts me .0016" below "SAMMI" min for the Cartridge shoulder/headspace min....

oh the horror...

As far as the bullet seating depth gauge. Its not that hard to use. It gets you close. I think some bulet designs it might not work well with ?
I ended up buying the horday gauge and the "modified" cases its easier and quicker PLUS you get to use the bullet you intend to.. I also still use the poor mans gauge with my cast bullet rifles. simple a slip fit case mouth and black sharpie on the bullet type to "see" contact.

Last edited by mac1911; 11-10-2019 at 09:33 AM.
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