Precision Mic Use

[03psd]

I have been experimenting with a RCBS Precision Mic in my LH LTR. I have been reloading with a buddy, but am trying to get educated so that I can venture out on my own. Do these numbers seem plausible:

Headspace 1.6305"
Bullet Depth .212"

I get the whole headspace concept but dont understand what the bullet depth measurement actually measures and its relationship to other things such as COAL. If 2.8" is the standard COAL for .308 where does this .212 measurement fit in? Should the OAL of the depth tool not equal the the maximum COAL once it is chambered in the rifle and extracted?

I am confused and the RCBS directions obviously assume a greater level of knowledge then I have.

 

[Ratbert]

Re: Precision Mic Use

The bullet seater measures distance +/- from the SAAMI spec for the ogeive of the bullet. So if it reads 0.00 then the ogeive is exactly @ SAAMI spec in relation to the length of the case. Depending on what bullet you use, your overall-length will vary while reading the same result on the MIC. This is because a VLD type bullet like the 155 Scenar has a long thin nose before it reaches the full diameter of the bullet. If you are using a short stubby bullet then the overall-length will be shorter.

At the same time the distance the bullet is sitting relative to your lands is determined by your individual chamber. If your chamber has a long throat (like a factory Remington) you may be 0.100" or more from the lands when your bullet is seated @ 0.00" on the MIC. In my two 308 Win Match chambers I am sitting about 0.040" off the lands when seated to 0.00". (FYI, each full rev of the mic is 0.050"). To determine how much jump you have use the provided plastic bullet/case (or make your own, which is what I did) set with the "bullet" very long and then let the closing of the bolt push the bullet back into the case. Carefully extract the case and measure it. This is the value of the total length to YOUR lands. From there you can, for instance, choose to seat your bullets 0.015" less for a very minimal amount of jump or even 0.005" more for a little bit of jam. It's entirely up to you.

I personally have begun just loading to 0.00" (which produces about 2.818 COAL with 155 Scenars) so that I have "generic" loads that I can theoretically use in any 308 rifle. They shoot fine with 0.040" jump in both my rifles (consistent 0.75MOA or better) and I like knowing that I don't have to keep track of which loads will shoot in which rifles.

I'm not sure where you got the 1.6305 number for headspace, but in general the MIC operates the same way, with 0.00 reading SAAMI spec and your readings showing the measurement + or - that. Measure once-fired cases as well as cases that just barely close to get an idea of the size of your personal rifle and if you want a nice tight fit then you know what range to size your cases to.

 

[03psd]

Re: Precision Mic Use

I measured a fired case in the gauge provided and it appeared to be .0005 so I added this to the minimum provided for a .308 of 1.63". Did I do this incorrectly?

 

[myerfire]

Re: Precision Mic Use

I only use the Precision Mic to measure the fired case. Some times it takes two firings to completely conform to your chamber. The reading you get from the fire formed case is your starting point to work from to bump your shoulder back, say .001 or .002. There is no relationship between the reading on your Precision Mic and cartridge overall length.
myerfire

 

[Ratbert]

Re: Precision Mic Use

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 03psd</div><div class="ubbcode-body">I measured a fired case in the gauge provided and it appeared to be .0005 so I added this to the minimum provided for a .308 of 1.63". Did I do this incorrectly?</div></div>

Sounds about right. Just keep in mind that this is just the size that that case sprung back to, not the maximum headspace in your chamber. To find maximum headspace (with this mic) then you're going to need to keep firing and neck-sizing the same case (letting the shoulder grow a little each time) until the case becomes somewhat difficult to chamber. Then measure THAT and you know your maximum useable headspace. Looking at that you can then decide how you want to setup your dies, though really if the gun is still safe (won't close on a No-Go gauge due to too much headspace) then there shouldn't be THAT much of a delta between 0.0000 and your maximum. Really you just want to make sure that you don't over-size them so that it's so far UNDER your maximum that you create an unsafe condition. Resizing to back to 0 on the gauge should create safe, useable ammo in any gun. If you are super bugged about getting every last ounce of accuracy then you can resize to just a couple thou under your maximum instead of all the way back to 0.

As an aside, maximum in one of my rifles is about 0.004 - 0.005 or so (I haven't bothered to find the definitive limit.) A young, once-fired case tends to spring back to 0.002. After that initial shot it will slowly grow to 0.004 or so if I don't resize each time. At 0.004 I can tell the difference between closing the bolt on a 0.003 and a 0.004. I have my dies setup to bump the shoulder back to 0.000 just because I like having "spec" ammo and I can live with working the brass that much since the primer pockets seem to wear out first anyway. Incidentally, new Lapua brass seems to generally come sized about -0.004 to -0.006 under spec and in that rifle I will see very slightly flattened primers every time with my fairly moderate load just (as best as I can determine) due to the excessive headspace in that chamber/scenario.