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Old December 27, 2012, 11:34 AM   #1
jwrowland77
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RCBS Precision Mic...Thoughts On...

I was thinking about buying a set of these. One for my .223 and one for my 7mm Rem Mag but wanted to get some thoughts on them first. Wanted to see if anyone uses them or had used them and see what they were like. Mainly asking because they are not cheap but I can handle that because I'm sure they are great in the quality department.

As always, I appreciate any thoughts or advice in these.
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Old December 27, 2012, 11:47 AM   #2
243winxb
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They are useful, if the operator understands the difference between Headspace & Head Clearance. Reads & understands the instruction. Studies the SAAMI website & drawing for there caliber. http://www.saami.org/specifications_...ex.cfm?page=CC

Last edited by 243winxb; December 27, 2012 at 11:58 AM.
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Old December 27, 2012, 12:17 PM   #3
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For measuring case head to case shoulder dimensions, these have the best consistency. However, the gauge they include for measuring bullet ogive contact with the throat is not as satisfactory as using an actual bullet of the type I will be loading in the Hornady LNL Overall Length Gage and then using their companion caliper comparator adapter to measure it with my calipers. They have case shoulder inserts for making case shoulder headspace measurement by using the comparator adapter, too. You can adapted it to different calibers and cases by changing the inserts and gage adapter case, which costs a lot less than a whole Precision Mic. The downsides are that the calipers have at least 0.001" less repeatability than the micrometer thimbles in most hands, though both have to be calibrated by measuring a good quality chamber headspace gauge if you want absolute rather than comparative accuracy.

I don't find that 0.002" seating length or headspace difference is a degree of error that is measurable on the target in most shooting platforms. I use the Overall Gauge and bullet comparator to find bullet ogive position on their adapter. I then use the case headspace insert to measure their adapter and samples of my resized cases, then subtract the gauge length from the average resized length, adding the difference (minding the sign) to the bullet ogive reading I use when setting up my seating die. This way, if my shoulder is, say, .003" longer than the gauge adapter, by adding .003" to the bullet ogive distance from the case head of a seated round, I have made the difference between the bullet ogive and case shoulder the same for my case and for the Hornady adapter case. That's the difference that determines how far the bullet will actually be off the lands when the cartridge is fired.
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Old December 27, 2012, 12:21 PM   #4
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Unclenick,

Yeah, I have the Hornady OAL gauge and comparator set that I use religiously, was mainly looking for something to help just bump the shoulders back a tad to help save on brass life from over working it. Thank you for your post. I really appreciate it. Gives me a lot to think about. Muchos gracias.
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Old December 27, 2012, 12:29 PM   #5
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No doubt the PM will make for easier shoulder length reading as it gets you out of having to balance the head of the case on the caliper. I have them in .223, .308, and .30-06, which are what I load most of, and find I turn to them for that reason after the Hornady gauge (mine's actually Stoney Point, from before Stoney Point sold the gauge design to Hornady) has given me base numbers. It's just that I usually need to average several measurements with the calipers before I fully trust what I'm getting. Once I establish that, I use the PM to double check what my seating die is putting out.
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Old December 27, 2012, 03:32 PM   #6
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Years before die companies made such case headspace gauges, a few were made by custom smiths that did the same thing. Some folks used a standard seating die with a micrometer caliper, like I did.

My first venture was with .308 Win. cases. Took a fired case that I'd trimmed to 1.900" long then pushed it into a standard RCBS seating die until its shoulder stopped against the die's shoulder. Trimmed that short, its mouth was well short of the crimping shoulder in the die's neck. Measured the case head to top of the die (no seating stem installed) and read the caliper. Set the caliper's dial to zero on that fired case. Now the caliper was set to zero at that fired case's shoulder headspace.

Repeated this with a few other fired cases and noted all their case headspace varied a couple thousandths about established zero.

Then I set the full length sizing die in the press and full length sized a fired case. Trimmed it back to 1.900" then put it in the seating die to measure it. The reading was + 3 thousandths from zero. The die had to go down another 5 thousandths to set fired case shoulders back about 2 thousandths. Resetting the die down and resizing that case got the die where I wanted it. The cost of two cases to set up the sizing die was close to nothing.

Cheap, accurate and very usable case headspace gauge. But it's easier and faster with my RCBS Precision Mic's.

One can also drop a GO headspace gauge in the seating die then measure it with the caliper. The dial could be zeroed on it as a reference to compare case headspace to the SAAMI chamber headspace spec.
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Old December 27, 2012, 03:33 PM   #7
243winxb
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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 board) can't follow instruction, read a micrometer or vernier caliper. Sorry had to put in #9
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Old December 27, 2012, 05:18 PM   #8
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Watch it, bud! I've MADE Vernier scales (well, OK, one; for my lathe's tool post scale).

Below is a very exaggerated illustration I cooked up awhile back that shows what Bart described about the case growing as the sizing die starts to narrow it. The brass has to flow somewhere. It also shows why most neck growth occurs during sizing.



Many folks have good luck just turning the sizing die to get the desired shoulder setback, but a press that is light and flexes much or a die that's not solidly in place can allow some variation in results when two cases don't require the same seating effort (getting a range foundling or mixed history brass into the group, or uneven case lubing can do this). In those instances using the Redding Competition shell holders or a shim on top of the shell holder so there is some over compression of the die against the shell holder at the desired case size, and that can also help make the result more uniform.

When the unevenness is due to springback being greater in one case than another, I've found that a case that comes out too long can be run back into the sizing die and held there a few seconds while it relaxes. Often one or two times will shorten it another 0.002". This can be useful with your first time putting range foundlings and once-fired brass through it.
Attached Images
File Type: gif shoulder setback and neck growth.gif (45.9 KB, 255 views)
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Last edited by Unclenick; December 27, 2012 at 05:32 PM. Reason: typo fixes and clarification.
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Old December 27, 2012, 05:28 PM   #9
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Sweet. Much appreciated all.
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Old December 27, 2012, 06:20 PM   #10
Bart B.
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Unclenick, great graphics.

Tell that story and run the pictures backwards and it's almost exactly what happens when the round fires. Case body expands, sucks neck brass back into the body part, shrinks back down after pressure drops and bullet exits. Result is a fatter case with a bit more case headspace and a shorter neck. The case is now shorter from head to mouth than before it was fired.

Such are the life cycles of a new brass case from shooting, resizing reloading, reshooting, resizing, reloading, reshooting resizing. . . . . . .

After each cycle for a given shot, the case overall length grows a thousandth or so.
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Old December 28, 2012, 09:15 AM   #11
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Bart,

Good idea. I could make that a series. I have a different set of sectioned drawings showing the primer backing out and being reseated as the pressure ring stretches thin during longitudinal stretch of the case. Maybe I will get time to combine them or even animate them.
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Old December 28, 2012, 11:10 AM   #12
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Unclenick, a set of drawings showing reality will help a lot of folks. But you'll catch a lot of flack showing primers backing out as pressure builds before it gets high enough to stretch the case back then reseat the primer. Too many people believe the case head's held very tight against the breech face and the primer stays in place when the round's fired.

I've oft times thought about making a set of drawings showing how a rimless bottleneck case centers its front end in the chamber shoulder and its back end against the chamber wall, not perfectly centered floating in space or laying in the bottom of the chamber when fired as claimed by thousands. I've dodged all the flack from others saying it isn't so.

Best wishes in your endeavor.
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Old December 28, 2012, 01:20 PM   #13
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Bart,

Thanks for the heads up. I wonder what they'd think if they looked at Garand's original primer-activated semi-auto rifle design. Probably suffer a denial conniption. I first ran into primer back-out loading wax bullets decades ago. I didn't know to drill the flash holes out to an eighth of an inch, and the primers quickly jammed the rotation of the cylinder of the Charger Bulldog I was firing them in.
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Old December 28, 2012, 04:52 PM   #14
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The short answer is-- if you are going to use this tool to help measure cases for use in factory rifles using the most commonly used bullets--and really pay attention to and follow the directions--these are very useful and cost effective tools.
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Old December 29, 2012, 09:48 AM   #15
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Unclenick, Old Roper posted this link in another thread that shows how the case and primer change shape and position when the round's fired:

http://www.varmintal.com/a243z.htm

I'd seen it before but totally forgot about it until he referenced it.
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Old December 30, 2012, 04:59 PM   #16
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Yes! I'd forgot Al put the primers in those animations, too. It's a good reference.
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Old December 31, 2012, 12:45 PM   #17
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After buying an AR10 clone (Remington R25) 4 years ago, I bought an RCBS Precision Mic in .308, and quickly found that I wanted the Hornady/Stoney Point tool as well, after finding the bullet gauge RCBS includes for measuring bullet ogive not all that useful.

Admittedly, I hadn't had ANY experience loading for military-style semi-autos before, and after reading a lot of references....including the very good section on gas guns in my Sierra manual, not to mention, an article from a well-known AR clone manufacturer, attempting to scare people out of reloading for AR's (typical), I realized that dealing with head space correctly with these guns was a pretty important key to keeping them (and me) in one piece using reloads.

Anyway, I quickly found, that for that rifle, OAL was a moot point since the limitation was magazine size, not chamber size....still, the Stoney point tool helped me find that out. The most important thing was setting the shoulder back enough, but not too far to overwork the brass.....and for that the RCBS tool was just the ticket.

A comment about Varmint Al's nifty article:

Primer setback should be an obvious result of primer ignition....the flash hole is tiny, like the nozzle on a rocket engine. All the thrust is behind it. Although the intent isn't to make the primer fly, it does.....as far as it can.

Reminds me of my first homemade rocket I built as an 8 year-old stupid kid. I didn't understand nozzles....I just packed in my 7-1 ratio of powdered sugar and potassium nitrate into a tube and lit it. The result was a 6 " lift and a big burned circle in my mom's prize blue grass lawn.
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Old December 31, 2012, 01:37 PM   #18
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GWS, one thing about reloading for semiautos, besides not setting fired case headspace back more than .001", is the big accuracy issue with case heads.

Decades ago, some of the US military rifle teams tried reloading their once fired arsenal and commercial match brass to save money. They did save some money discounting the labor required. But accuracy with those reloads was not good.

Nobody's ever (to my knowledge)squared up the bolt faces on US military rifles to improve accuracy with handloads. Fired case heads are typically offset to about the same angle as the bolt face was they expanded back against when fired. When the high point of the case head aligns with the high point of the bolt face, the barreled action whips its muzzle axis off in some other direction than when that case was new and first fired. So the military teams quite resizing fired cases from their M1's and M14's.

I don't think any AR type of rifle maker (or 'smith rebarreling one) ever squares up their bolt faces. So, I don't expect accuracy from one with reloaded cases to be as good as with new cases.
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Old December 31, 2012, 01:46 PM   #19
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I'm glad y'all brought this up, as I am in the process of building an AR15 and will hopefully be reloading for it within the new year. Gives me stuff to research and read about it.
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Old December 31, 2012, 03:08 PM   #20
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Bart, yes, I've heard that before....very interesting. The R-25 was built as a hunter's gun not a tactical weapon (although it certainty qualifies except for the really small chamber). That said, it was built by DPMS so I can't say whether they had the means or desire to square bolt heads. Maybe luck is more important here, as my rifle does quite well (as a hunting rifle) shooting my reloads. Typically, 1/2" center to center, with an occasional flyer in some groups. But then I'm not loading for Palma either.

I've only shot two boxes of factory in four years....they came with the rifle. One was Remington Core-lok...I think 165 grain... and I got inch-ish groups. The other was Federal Blue box. That ammo shot one round and then wouldn't cycle....failing to go into battery without help. That's when I discovered the tight chamber. So, I decided to use a small base sizer and have had no problems, excepting one double tap caused by me and the learning curve on a new Pro-2000 press (failed to notic 2 rounds in the first 100 with high primers....shot....one!). Nobody's perfect. The gun made a fine Christmas. The press came in January....and that's how I started the Obama years. (something had to lighten my mood)

Bottom line is the rifle does pretty good with progressive reloads....for my needs, and the Precision Mic certainly helped make it more safe and accurate...whether or not the head is square. Wouldn't use either for bench rest.....and make sure your primers are seated.

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