using redding instant indicator comparator to measure shoulder bump

[thebigunit]

got a bunch of AE 5.56 LC brass that was fired in my stag and mp15. brass is mostly LC 11 and was from two multi box purchases. I checked about 20 pieces of brass shot from both guns and measurements were in the same range for both. all the brass is mostly in the +.0025 to .0035 range over minimum chamber based on set up gage, with an occasional piece closer to .000 or .005. according to instructions, I should bump .004-.006 for auto loader based of the largest DL(.005) which should move me back to -.001 to .001.

I'm getting most of my pieces to size back to desired bump but then I'm getting a piece that moves back -.004 and sometimes with little to no bump. I'm using my Redding T7 and very lightly coating cases with Imperial. There seems to be no correlation between ease or resistance when sizing to the amount of bump.

I've cleaned the dies several times to make sure nothing inside was causing this. I then decided to size using a small base redding, redding body with lee collet neck sizer and RCBS FL die to see if there was any difference and to check neck concentricity between the 3. run-out for the redding body/lee collet and the rcbs FL sizer staying in the .000-.001 range while the redding small base was noticeably off more in thee .002 to .004 range. I assume that is possibly due to the .002 extra body sizing of the small base die and brass movement resulting.

I check the pieces that bump over/under desired amount and they easily fit in my Wilson headspace gage and chamber in both guns. I know that I am using brass from two different guns and potential chamber variance, etc and the the new brass hasn't fully formed.

I have tried to induce a situation to get the comparator to take different measurements but have been unable to do so. Tightened my turret and linkage bolts just to make sure but didn't seem to move any.

Is this an example of, "The more you know, the less you know?"
what is causing these shoulder bump variances? different brass hardness? brass movement due to one chamber having slightly larger body area of chamber?

did a search and didn't find anything and not in any of my manuals that I could find.
so any help would be appreciated. I would like to better understand what I am actually doing and know that I have a lot to learn.

 

[Bart B]

Two things cause what you're seeing.

One is the amount and type of case lube used as well as its lubricity. The easier the case goes into the die when sized, the more the shoulder will get set back. I use a 40-60 mix of Hoppe's No. 9 bore cleaner and STP engine oil treatment for case lube.

The other is the amount of spring in the press. The harder the case goes into the die, the higher the die will be when the ram's at the top of its stroke. Most full length sizing dies set properly in the press to bump fired case shoulders from a rifle .002" to go back into the rifle will have a few thousandths clearance between the shell holder and die bottom at the top of their stroke. When folks lower the ram as soon as it toggles over at the top, the spring in the press doesn't have enough time to pull the die back down on the case a thousandth or more.

Best thing I know of to have all fired bottleneck cases resize to the same case headspace (head to shoulder datum length) is to use Redding's competition shell holders. These come in .002" steps above the standard .125". Use the one that when the die's set to just barely press against it when the press cams over pushing the case the same distance into the die every time and bumps the shoulder of a fired case from your rifle's chamber back about .003". This makes sized case headspace have consistant dimenions and correct for your semiauto chamber.

In second place, what I do, is to tumble cleaned cases in a foam lined can so the case lube's evenly distributed on all cases. That keeps their sized case headspace to a .002" max spread; good enough.

Your sized cases should have their case headspace about 3 to 4 thousandths less than actual chamber headspace your rifle has. Unless you have an adjustable chamber headspace gauge to measure yours, use a fired case from your chamber then set the die to make sized case headspace .003" shorter than what the fired case has.

 

[thebigunit]

Bart, thanks for your response. I've read hundreds of your posts over the last 3 years and value your knowledge.

as to the lube, I'm using Imperial, just running the body between my fingers, very light coat. I pulled most of the cotton off my Q Tip and using it to lightly lube inside of neck on about every 5th case, could this be part of the cause in regards to lube amount/resistance? will see how thin a coat I can get on. I also read something about the expander possibly being a culprit? I may just use the body die to eliminate any expander/neck sizing issues.

will the RCBS precision mic measure my actual chamber headspace? or does it just measure the datum line like the redding or hornady comparator? I know you can take a cast of chamber but what other method/tool would you suggest. I've read many posts about this but can't seem to recall them all.

my other thinking is that I can reload the brass and fire it another time or two. Keep the brass from both guns separate. Then take measurements with my comparator from both lots and get a better reading from there.

like most of us, I am trying to improve my reloading skills and knowledge and hope I am not wasting your time. I'm transitioning from the old "set the shoulder back and check to see if it chambers" from my bolt guns to newer and better methods

 

[Bart B]

All those gauges will only measure case headspace; there's no way one could fit in the chamber. Besides, depending on the load used and the case brass metalurgy properties, how much its body shrinks back from being pressed hard against the bolt face, chamber and shoulder walls will vary a thousandth or more. And their reference is tyically the datum diameter on the bottleneck case shoulder. The RCBS ones are made with a loose thimble that's super-glued in place after closing the gauge on a chamber headspace gauge then aligning its zero mark with the gauge index line. But there's a .001" spread (or more) across all chamber headspace gauges for a given cartridge made on this planet. My RCBS Mic for .308 cases reads -.0006" when closed on my Forster .308 Win. GO gauge.

I've seen adjustable chamber headspace gauges with graduations marked in + and - thousanths from the standard GO guage reference. Actually saw one made for .30-06 chambers. I don't know if any are still available. There may be one or two companies that would make you a set of gauges from GO - .003 to NO GO in .001" increments.

I wouldn't get concerned about your chamber's exact headspace. Just set fired case shoulders back 1 to 2 thousandths then shoot the darned things. There's ample tolerance for head clearance (bolt face to case head when rimless bottlenck case is hard into the chamber shoulder when fired). A couple thousanths spread's not made any difference in my match rifles shooting well under 3/4 MOA at 36,000 inches down range. All the cases have their shoulders at the same place for every shot; hard against the chamber shoulder. I don't care about a couple thousandths spread in how far away the case head is from the bolt face. Especially with the firing pin having .060" plus protrusion from the bolt face.

Well, whadda you know; found this web site but have no idea how good they are:

http://www.manthei-mess-systeme.com/gb_headspace_gauge.htm

 

[thebigunit]

on the topic of the lube you use, interesting mix. but the question that keeps haunting me, what is this foam lined can that you speak of? can size? foam type? could you elaborate more on it's construction and how you use it?

thanks again

 

[Bart B]

I used a specially made can out of some exotic metal that was about 6" diameter and 7" tall. Bought some 1/2" foam at an army surplus store then cut it to fit the can's inside circumference and height. Finally wrapped friction tape around the outside where it lined up with the two pairs of rollers on my Thumbler's Tumbler. After dribbling a few drops of case lube on the foam and filling it half full of cleaned cases, I snapped the custom made cover on it. Then laid on the tumbler rollers and turned it on for 20 minutes.

Oh, I forgot, that can is exactly the same as the one labeled below. The other picture is that can on the right next to its black hard foam bottom liner standing beside the other can with its hard black rubber side and blue rubber bottom liner used to tumble fired cases to clean them:

Please, no remarks about the wild colored floor mats in my shop.