RCBS Precision Mic


Well-Known Member
Jul 15, 2009
Hello All,

Anyone want to share an opinion of your experience with the RCBS Precision Mic to check for OAL to the ogive?

My plan is to use it to measure the the oal and then use a comparitor to transfer the measure to a bullet.


I have a set for my .300 RUM and really like it. One of the neatest features for me is checking the length of case from the head to shoulder. That tells me if I need to bump the shoulder back or not. If i do bump the shoulder, it will tell me where its at and how far I need to go. It will also measure how far your cases have stretched.

As far as the comparator goes, once I measure the OAL to the ogive, I keep on using the mic for my measurements and dont bother with a comparator. Basically you can keep checking the length on your loaded ammo with the mic. Just remember to write down what the case length is on your mic.

Its a shame they don't make them for a 338 Lapua...
Hey Tom,

I use them and I like them a lot. I'm a real believer that not enough folks use gages in their reloading, so anything in that direction is likely to get a positive nod from me. There's actually a seating/OAL tool that is part & parcel of the Precision Mic. With that said, I'll have to say that this is the only portion of the Mic that I have no use for. Doing what it sounds like you're doing here (assuming I'm reading you correctly) sounds like a much better way to go. Give it a shot and let us know how it's working for you. Several other methods of determining an ideal OAL out there too, if this one doesn't work out to your satisfaction.

Hope it helps,

Kevin Thomas
Lapua USA
I second that. I've used them for .270 Winchester and 22-250 and believe it helped dramatically in setting up a new load. I use it mainly to measure the case length from base to shoulder before and after sizing and the seating depth of the bullet. The only part of the mic kit that I have found to be inaccurate was the part with the fake round to measure the distance from the base to the rifle lands. I gradually seat the bullet further and further down, "coloring" the bullet with a marker each time, until I see it isn't touching the lands and then measure it with the mic to get a distance to rifling. I will say the fake round measured more accurately when used in my bolt action than in my Encore, so maybe it has to do with the type of action you use.

One of the neatest features for me is checking the length of case from the head to shoulder. That tells me if I need to bump the shoulder back or not. If i do bump the shoulder, it will tell me where its at and how far I need to go. It will also measure how far your cases have stretched.

Can someone please explain what bumping the shoulder back means?

Is bumping a shoulder relative to full length sizing?

I neck size, so I'm almost always shooting fire formed brass. Even when I hunt as I've never had a problem. Does this mean I don't need to bump my shoulders?


Hi Tom,

Whenever we size a case, there's several different dimensions in the mix that we're changing. Full-length sizing does the most thorough job of returning the case to nearly it's original, unfired dimensions. Since the case stretches longtitudinally as well as circumfrentially, there's generally some degree of interference between the shoulder of the case and the shoulder of the chamber. Neck sizing (as the name implies) alters only the neck dimension. You may not have had any problems with hunting ammo from just neck sizing, but eventually, you will. Mr. Murphy will virtually guarantee it, and I'll add that it'll come at the most inopportune time. Sorry, just works that way. If you choose to neck size your hunting ammo (and I'd strongly recommend against it) you will still need to full length resize occasionally. Otherwise, that interference will continue to worsen, and you'll find youself pushing a bit to get the bolt to cam over and close. Full length sizing is just insurance in the field, to make sure this never becomes an issue for you. Otherwise, at some point, you won't be able to get the bolt closed. Long way to say it, but yes, you need to bump those shoulders.

The secret to best accuracy and long case life, is simply not working the brass any more than you need to assure easy and reliable chambering. But that doesn't mean doing so to the extent that you risk chambering problems, at least in my opinion. Elsewhere, we have a thread going concerning chamber gages, like the RCBS Precision Mic. A gage such as this (or any of several others out there) will allow you to bump those shoulders back by a set amount (one that you KNOW, and can measure) and you'll know you're not working the brass to any excessive degree. The best of both worlds, and I can't recommend these highly enough. Well worth the $, and the time it takes to use them. Hope that helps!

Kevin Thomas
Lapua USA
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Thanks for getting back to me. Really appreciate it.

So does the gauge tell you that the shoulders need bumping and then you do it with a sizing die? A full length sizing die?


Hey Tom,

Bear in mind that all chambers are slightly different. The gage allows you to measure this dimension and establish a baseline for YOUR chamber. Sizing, from that point, is a matter of rechecking the sized case until you see the head to shoulder dimension make some movement. Varys with each application, but even .001" can now be measured and set when you adjust your dies. For some applications, you may want to induce more, say. .003" or .004" to assure reliable function in all circumstances. This is, with the gage, it now becomes entirely YOUR call. And yes, you'll use a FL sizing die, or a body die to do this.

Alway liked the quip from LE "Sam" Wilson; "You don't know what you think, you only know what you measure!"

Kevin Thomas
Lapua USA
Hey Tom, Kevin is spot on with this one. I too like to neck size of some of my rifles, but I have gotten to where I've been purchasing 3 die neck sizer set from Redding. This set also includes a body die which will only work the body of the case and bump the shoulder if you need to. It wont ever touch the neck which is the beauty of it. The cases will stretch eventually and become hard to chamber. Of course you can always purchase a body die seperately if you'd like.

If I were you, and this if what I do. Before I go in the field, I will use my Mic to check each and every case that I will be taking with me. That will give you piece of mind that you won't have any feeding or chambering problems.

By the way, the mics are calibrated according to what the SAAMI specs are on the case. If the case is exactly to spec, then the mic. will read 0. If it reads +.002 then you know the shoulder is .002 longer (or farther away from the head) than what the spec. is. If it reads -.001, then you should never have any trouble chambering that case.

But you do have varibales there. It depends on your rifles chamber and how deep the chamber is cut..
"My plan is to use it to measure the the oal and then use a comparitor to transfer the measure to a bullet."

If you mean you wish to use the dummy bullet/"free bore" device to find the max cartridge length to the ogive and then measure that with a seperate gage I think you would be spinning your wheels.

As the others have said, I love the Case Mics for measureing cases to the shoulder and then setting FL dies AND for measuring cartridges to the ogive to adjust seater dies but that "freebore tool" sucks. I find it impossible to get any usable consistancy with mine and I have three of them. (The Hornady comparitors also work well and can be much less expensive if you load for very many cartridges.)

I use a wood dowel rod I use to mark and measure the max OAL length with my dial calipers. Seat a dummy to that OAL with that bullet, then use the seating tool to find what the ogive length is and work backwards from that.
I really wish to understand this better, which is why I'm going to continue asking questions.

Is it safe to assume when you guys say shoulder length it refers to the angle of the shoulder relative to the body?

Is this a summary of what you guys are doing with the bump the shoulder thing?

A full length sized case is saami spec. However the chamber may not necesarily be cut to spec. So first you measure a piece of fire formed brass and find out just how out of spec your chamber is using the mic, then you adjust your full length sizer to size your brass where it just ever so slightly "bumps" the shoulder down from the fire formed sized. So you brass is actually moving just not to much. Then you use a neck sizer to adjust the neck.

Does that sound about right? Or do you size the neck first and then bump the shoulder? Or am I way off?

I need to know the answer to the above before I can ask my next question which relates the whole thing to a strickly neck sizing perspective.

I use the mic similar to others that have posted. I shoot a round with new unfired brass and then use the mic to find the measurement of the chamber. In my case it usually hovers around +.0035" on the mic. What I've found is the best compromise for me is to adjust the full length sizer die to where it pushes back the shoulder on every round to +.0025".

I used to only neck size until the measurement grew to +.0045" or where it was hard to chamber. But I have found this method: 1) ensures the same brass dimensions every time, 2) my loads are always easy to chamber and extract and 3) I have had no problem getting 6+ reloads from full house loads with several brands of brass.

Hope this helps. The tool is well worth the money.

sorry for the confusion here, but it IS a complex topic. The dimension we're taking into account here is the actual "play" or space between the shoulder of a chambered round, and the internal angle of the chamber. Or, if you prefer, the space between the head of a chambered round and the face of the closed bolt. In otherwords, the headspace. This reference is used to describe the portion of a firearm's chamber, or, a cartridge, that prevents it from entering any deeper. This can be the rim on a rimmed case (44 Magnum, 30-30 WCF, 38 Spl, etc.), the belt on a magnum (458 Win Mag, 375 H&H, etc.) or the case mouth on a straight walled rimless case (45 ACP, 9mm or 40 S&W, etc.). It has to coincide with some mechanical point of the firearm that it engages and stops that forward movement. In the case of a rimless, bottlenecked case like we've been discussing here, that point is where the shoulder angle of the cartridge case contacts the shoulder angle of the chamber. The distance between the head of the case (in any of the different styles of cartridges I've mentioned so far) and the pont that stops forward movement is called headspace and refers to the distance to the datum line. This isn't the number to worry about, and it's more the concern of a gunsmith, not the reloader. The headspace dimensions, that .001" to .003" or so of slop that allows the cartridges to chamber and fire properly, are what we need to consider. Not enough space and you have tight chambering issues and will eventually run into a situation where the rounds won't chamber. Too much, and you've got brass that fails very quickly, potentially hazardous case head separations, accuracy problems and misfires. Pays to get it right, and the case gage is the best way to do this. Once you've had a chancve to use these tools just a bit, this stuff will all become second nature to you, and you'll have a very clear understanding of what's going on. It's just new to you right now, and we've all been there.

Kevin Thomas
Lapua USA
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Thanks to all that replied. I'm very interested in fully understanding this.

I do know what headspace is. Just never concerned myself with it. I've seen diagrams for headspace that show a cartridge loaded into a chamer and then there is a little bit of white space between the shoulder of the cartridge and the shoulder of the chamber. Please correct me if I'm wrong here because everything else hinges on this.

What you guys are doing is taking a fire formed brass and setting your full length sizer to bump the shoulder back a specified amount. Is this correct?

Does setting the FL sizer up this way correctly size the neck of the brass?

Do you ever end up with inconsistent neck tension?

Is the only advantage of bumping the shoulders with a FL sizer over a neck only sizer the eventuallity that I'll not be able to chamber a round?

Can someone please describe to me what is happening to the brass everytime I neck size only that will cause the brass to no longer chamber? I would assume the shoulder is getting longer but I'd figure that the neck would control for that since it must always be sized to the caliber, in my case .308.

Thanks again to all

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