RCBS Precision Mic Question - Resizing Brass

[HK Dave]

Howdy folks, I'm new to reloading and am trying to resize once fired brass.

I took a bunch of once fired brass from my precision AR in 223 and measured it with the Precision Mic. The Mic reads +1 on average. I'm resizing the brass until my Mic reads -1. That sound right?

I measured my factory match ammo and it reads -2 on the Mic.

I also measured some once fired brass from another AR and it reads +5 to +6 on the mic, so it seems the chamber on my precision barrel is tight at +1?

Thanks!

[GeoffLinder]

Not the right way to do this. Measurements are not what you go by although you can measure properly resized brass against fired brass and get a benchmark of measurements for YOUR rifle.

The size dies most important job is not about the external case dimensions but about the ID of the case mouth so it is brought back to proper ID to grip the seated bullet with proper tension. 2nd job it performs is to bump the case shoulder back a couple thousandths to provide proper headspace when the round is chambered. 3rd function is to slim the case down a little to allow some clearance when chambering the loaded round.

In a bolt action you really only need item number one done, number two (bumping back the shoulder) is less important and item 3 means little as long as the reloaded round is going back into the same rifle.

For a semi-auto, all 3 are important and you want to use SB (small base) size dies to provide full case OD resizing to help insure proper function during auto-loading process.

Proper way to do this all is to trust the resizing die and do the die setup properly.

The shortcut way to setup a size die is to screw the die in until it just contacts the shell plate when ram is all the way up, then maybe add an eighth turn more and lock it off. This will generally provide satisfactory results.

A better way to do this is to use a case gauge designed to tell you how much shoulder setback you are doing and use the gauge to setup size die for the correct amount of shoulder setback instead of an arbitrary amount like the shortcut method will provide.

A good case gauge will tell you several things about your cases and finished loaded rounds. First, it will tell you if shoulder setback is within spec (1.5 to 3 thousandths). Second, it will tell you if case length is correct and if it needs to be trimmed (.223 should be trimmed back to 1.750" after several firings). Third, it will tell you if the overall case OD is correct for proper chambering. Remember, cases have a slight taper and you really don't want to verify this with measurements as much as you want to verify this by dropping the loaded round into a case gauge and seeing if it fits well. Case gauges are cut to the same spec chambers are cut to so this is your go, no-go.

A good case gauge to use is the LE Wilson one. There is a pdf copy of instructions on how to use it at the following link.

http://www.lewilson.com/casegage.html

Here is a previous post of mine about the process that happens when brass is fired and then re-sized.

http://www.calguns.net/calgunforum/showthread.php?p=10462545#post10462545

Hope this helps :-)

BTW, I am assuming (and hoping) that you have a reloading manual AND have read it all the way through first. This will all be covered in a good reloading manual.

[HK Dave]

Thank you so much for your detailed response. I'm new at this so please forgive me if I seem dense.

Basically what I did was use an RBCS precision mic and measured all the once fire brass... it reads +1 on the mic.

I then screwed in my full length resizing die until it touches a .008 shell holder and gave it another 1/8th turn or so.

After resizing the brass... i put it back into the precision mic and it gives me a reading of -1... so i guess that means i've bumped the shoulder back 2 thousands?

I then verify that it's good to go by dropping the case into a Wilson case gauge and making sure it sits flush. I noticed my once fire cases wont seat all the way into the Wilson case gauge.

I do have a multiple reloading manuals... guess i better go read them again.. it just feels like a foreign language right now. haha

[HK Dave]

I think I understand the "case gauge method", and it's what is described in my reloading manuals, however then I wonder what is the purpose of the RCBS precision mic?

[GeoffLinder]

I think I understand the "case gauge method", and it's what is described in my reloading manuals, however then I wonder what is the purpose of the RCBS precision mic?

A mic or caliper is used mostly for measuring things like COAL (Cartridge Over All Length), case length, crimp or mouth OD after crimping with pistol rounds and so on. A mic is no good at measuring shoulder bump directly, only a comparator gauge in conjunction with a caliper with depth probe will tell you how much you are bumping the shoulder back.

The best way to do this, especially if you have the LE Wilson gauge is to drop a fired, but unsized case into the gauge. It won't sit with case head flush but the case shoulder should go all the way to the shoulder in the gauge. You then use the depth probe on a caliper to measure how far below top of the gauge face (or above the gauge face) the case head is. Then you resize a case also fired in that same rifle and drop it into the gauge. Repeat the depth of case head measurement with the sized case and the difference from the unfired case is how far you have bumped the shoulder back. Keep adjusting the die in or out until you have bump in the proper range. You may need to have several fired cases available to do this right if you have the die in too far to start with.

You want to bump shoulder back no less than .0015" and no more than .0030" to .0035". Too much bump and you will be over-stretching the case and case life will be shorter. In an extreme case of way too much bump you will get case head separations when firing because you are stretching the case too much when it blows out during firing and the case head stretches back to meet the bolt face. This stretching is all done at the rear of the case wall just ahead of case head.

[afrancke]

HK Dave, your approach described in the original post is correct. Geoff is confusing the Precision Mic with a generic micrometer, which it is not. The way you are using it is as a case headspace gauge, which is exactly what to do in order to set your die up.

[HK Dave]

HK Dave, your approach described in the original post is correct. Geoff is confusing the Precision Mic with a generic micrometer, which it is not. The way you are using it is as a case headspace gauge, which is exactly what to do in order to set your die up.

Thank God! I thought for a second that I bit off more than I could chew. I was thinking, "This can't be this difficult... how is it everyone can figure this out but I can't?"

[stilly]

Good stuff here. I will stick it on mah tablet and read it while I am fiddling with my ar and mic trying to stick the one part in where the thingy goes...

[GeoffLinder]

HK Dave, your approach described in the original post is correct. Geoff is confusing the Precision Mic with a generic micrometer, which it is not. The way you are using it is as a case headspace gauge, which is exactly what to do in order to set your die up.

My apologies, I was assuming a standard micrometer (machinists type). I had totally forgotten what that little RCBS bugger was, looked at one a long, long time ago and had memory fade. I will lookup the tool next time before I 'splain stuff LOL

A public humiliation session can be scheduled at any time

And yeah, a -1 reading from a +1 reading will produce .002 setback which is exactly perfect. Your precision AR chamber is a lot tighter than the other AR. Brass from that rifle will last a lot less than the brass from the tighter AR if you share ammo between them. So you would be best off sizing separately for each rifle to avoid overworking brass by sharing ammo between them. Cases setback .002 for the sloppy chamber would probably not chamber at all in the precision AR.

[HK Dave]

Haha no worries Geoff... I appreciate your answers regardless. I do plan to have a different set of brass for each barrel i own.

[Norcalkid]

I have a RCBS precision mic and do it exactly as you mentioned in your first post. My once fired brass measures +3.

[mroels]

I also measured some once fired brass from another AR and it reads +5 to +6 on the mic, so it seems the chamber on my precision barrel is tight at +1?

Thanks!

Also remember that brass shrinks back a little after being fired, so your chamber is more than +1. You could probably size to keep the shoulder at the +1 reading and have it snug in your chamber and save the brass a little from being overworked. But I generally bump down to -1 as you are right now, just in case I end up shooting it from another rifle.

[GeoffLinder]

Haha no worries Geoff... I appreciate your answers regardless. I do plan to have a different set of brass for each barrel i own.

Thanks Dave :-)

Good plan to have brass loaded per chamber with AR's. My 3 AR's all want quite different shoulder bumps too. If I size for tightest, loosest blows the brass out big time. So I do two batches, one for the loosey goosey and the middle of the road ones, another for the JP chamber. More work but worth it IMHO