RCBS Precision Mic...Thoughts...

Never used one. I use the nut from Sinclair to accurately measure the headspace of my ammo.

$20 here

The Sinclair tool does not measure headspace, but rather bullet seating.

The RCBS Mic measures the headspace.

Go into Sinclair and RCBS, read the instructions.

I've got 3 of them (.308, .30-06 and 6.5x55). The tool is great for setting up your FL sizing die to get the amount of case headspace that you want. The other feature of the tool for determining case base to bullet ogive dimensions is worthless, and I use the Sinclair hex nut for that operation.

Don

The RCBS case mic works quite well in determining your chamber dimensions,I could not see reloading without one as I use it to set up my resizing die. For instance I have a Rem 700 bolt gun in 308 that has a tight(.001 chamber where the brass blows out to only.002 after firing.My M1a & M1chamber starts at .004 or so and expands the brass to around .009.My .223 ar rifle is similiar.Using the mic tells me this,and I check the cases after full length sizing(for auto loaders) so I don't sqeeze them down to "zero" needlessly overworking brass.The bolt guns get neck sized.The bullet seating length function of the rcbs doesn't seem to work for me,I got a Hornady tool with dummy case that works well.Basically to use the case mic you take a case fired from your gun ,or out of resing die, and insert it into tool,Micrometer readings on tool let you know whats going on. I checked some once fired military 308,it blows out to .014 in the M-60's.One thing puzzled me is checking new 300 win mag brass before firing,It showed it was ..005 UNDER "zero" on the mic,after firing in rem 700 5-r bolt gun it still was.003 under.?

Have one in .223 and the other in .30/06, use them both and really like them.

I have 2, 308 and rem 7mm mag. Like said they are great tool to help setup the sizer so you don't overly set the should back more than necessary. It's also great if you have more than one gun. I have 2 7mm mag each with a different head spacing. I used it to setup the shorter one then use shims to set the other one back (0.005") so I don't have to change my die settings. These are great aids.

I have found the Stoney Point, now Hornady, case comparator to be more useful for my purposes. It has the added benefit of being less expensive.

Most importantly, rather than using the dummy bullet to determine your leade measurement, I use the actual bullets I will be seating, including the vagaries and dimensional differences inherent in the differing ogive designs.

I think the Stoney Point is superior to the RCBS precision mic.

Devices to measure cartridge head clearance

Problems when trying to measure the Head to Datum of a fired 223/5.56 case. 1. The measurement may become shorter as the case body expands, giving a false reading. 2. The firing pin strike may set the shoulder back .006" 3. An AR15 bolt slamming on the long cartridge may set the shoulder back .001" or more. 4. Shell plates may be a different thickness at each station, resulting in a very different shoulder bump. 5. There may be flex in the press/linkage. 6. You have no idea of the true headspace measurement of your firearms chamber. 7. Using "range brass" that has very different springback after firing. 8. To get a good reading, neck size only for 3 firings. These must me maximum or near maximum loadings to fully expand the brass to the chamber. 9. You (no one on this forum) can't follow instruction, read a micrometer or vernier caliper. Sorry had to put in #9

Problems when trying to measure the Head to Datum of a fired 223/5.56 case. 1. The measurement may become shorter as the case body expands, giving a false reading. 2. The firing pin strike may set the shoulder back .006" 3. An AR15 bolt slamming on the long cartridge may set the shoulder back .001" or more. 4. Shell plates may be a different thickness at each station, resulting in a very different shoulder bump. 5. There may be flex in the press/linkage. 6. You have no idea of the true headspace measurement of your firearms chamber. 7. Using "range brass" that has very different springback after firing. 8. To get a good reading, neck size only for 3 firings. These must me maximum or near maximum loadings to fully expand the brass to the chamber. 9. You (no one on this forum) can't follow instruction, read a micrometer or vernier caliper. Sorry had to put in #9
Last edited by 243winxb; Today at 01:38 PM.

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Are you familiar with this tool and how it works? Almost none of what you posted is relevant to the operation of this tool. Although this tool is not a requirement, and there are alternatives, it is a pretty handy tool and will help to extend the life of your brass.

Insert fired case. Take measurement (doesn't really matter what that measurement is). Adjust sizer (or shellholder height) until the desired setback is achieved. Check the fit by chambering the brass. You're done.

Understood Walkalong but this is a simple tool and some posts tend to over-complicate things which can be a turnoff for some of the new folks around here.

Yes, many details are important but there is a lot of extraneous noise around what many feel is required for 'precision reloading'.

I wish I would have had something like this 40 yrs ago when I started loading.

Yes, I know what you mean I am sometimes guilty of over-complicating things myself. We are very fortunate to have so many simple tools to accomplish our goals these days. Many of them are 'just do it and it works' which is great for beginners.

I think that it's just part of what makes us reloaders that makes us dig into every aspect of every step the the Nth level. We just have to know how, why, and if we can make it a little better.

All I'll say is, beats the heck out of using die instructions that say "screw die down until it contacts the shellholder, and then turn a 1/8 to 1/4 further down". Kind of like doing surgery with an axe.

Don

All I'll say is, beats the heck out of using die instructions that say "screw die down until it contacts the shellholder, and then turn a 1/8 to 1/4 further down".

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Don, I can't tell you how many years I did that! And always wondered why my brass didn't last very long.