reading an RCBS Precision Mic

[sonnyt650]

If I understand the readings for my SPS Varmint correctly a fired round is spot on the minimum ANSI dimensions for headspace give or take a thousandth. The weird part is my sized and loaded rounds are longer by a couple of thousandths -- that's right I don't screw the guage in as far for the fully-sized, loaded rounds with new brass. It seems like this would be fantastic for brass life, but in fact I get just a handful of reloads. What am I doing wrong?

[G-forceJunkie]

A couple of things. Brass does spring back a little, just because it reads "0" after firing, your chamber is probably a little longer. As for your loaded rounds, sizing the brass squeezes everything in and up. It sounds like the shoulder of your brass is not getting sized down enough. If your die is already contacting the shell holder, you need a shorter shell holder. The competition shell holders come in several thicknesses. Another possability, your expander ball is pulling the neck back out on the down stroke. Depending on what dies you are using, you can remove it on some, or polish it down to a smaller diameter.

[damndave]

My fired brass read 0-1 on my Mic. I got the Competition Shellholders and size them back down to -2. Sounds like your brass is about the same.

I believe the instructions say to size the brass down -2 from the fire formed length.

[westcoastr]

putting a fired round into an RCBS mic only gives you an indication of your chamber size - and chamber get cut to different lengths, no where that I've seen does a rifle manufactured state that it is cut to the minimum, though match grade are typically very tight.

Putting a fired round in an RBCS mic for a bolt gun it should be close...but the only way to know is buy/borrow a full set of headspace gauges and measure your rifle headspace. Assuming you don't rack you bolt handle so hard your claw ejector (if you have one) bends your rims. And the plop the headspace gauges in your your mic as well as the "zero" is rarely actually zero. it is a tool that give your a relative measurement until you do this.

for auto loaders a fired round can vary WIDELY, my FAL will fire form cases from -8 to + 12 of my RCBS zero. higher pressures will stretch my cases and if my gas value setting is too aggressive it will stretch the cases AND bend the rims.

I think you might not be sizing your brass down enough...and the force of the bolt is enough to cram it in your chamber.

Mic a few factory loaded rounds, depending on caliber, they are typically -5 on the mic (at least 308 is)

[30Cal]

I don't think the RCBS precision case mic is calibrated. Even if it were, it's not a substitute for a chamber headspace gage.

If your sized brass is longer than the fired stuff, then the expander ball of your sizing die is probably pulling the shoulder forward. The cases often squeal when this happening. That is cured by appropriately lubing the inside of the case necks.

You didn't mention how your cases fail or how many firings your getting.

[sonnyt650]

My first set of reloads were once-fired Lake City brass which saw maybe three reloads with starting charges before several had obvious bright lines and even burn through -- could be due to whatever chamber they were in severely blowing them out of proportion. Other than a couple of rounds early on that I probably didn't size to the stop (first time sizing a larger cartridge on a Lee Classic Turret), the reloads didn't fail to load at all. Processing brass in bulk, I used the same setup to produce ammo that ran perfectly fine through my semi-auto .308 which should be pickier. However the mic indicates those are even shorter by a couple thousandths after firing and the Hornady LNL Headspace Gage confirms all these relative readings.

BTW: the RCBS instructions state the mic is calibrated, and my results w/ sized cases near zero show the Lee full-size die when adjusted as indicated in the instructions return cases to near factory dimensions. If the sized cases were off by some odd number like 23 thousandths I would question the calibration, but not when they're one or two away from zero.

I can use my lathe to cut down the dies where they touch the shellholder, but without any problems cropping up for the last 1K rounds or so I don't see the point. The readings kind of bother me as I can't make sense of them, but it's nothing to take a cutter to at this point.

[fguffey]

The companion tool to the press, the feeler gage is the only tool option, but, the feeler gage is as foreign to the reloader as the terms transfer, standard and verify. Purchasing Redding Competition shell holders will do nothing for reducing the length of the case from the head of the case to it's shoulder, the only option for that technique is to grind the shell holder deck height or grind the bottom of the die, an alternative to the Redding shell holders is the feeler gage, the feeler gage offers 5 more options between .000 and +.010, an alternative to grinding the die and or shell holder is education, anything done with a Redding shell holder can be done with a feeler gage, anything accomplished by grinding a die can be accomplished with a feeler gage.

Redding list a thickness gage for $50.00 + or - a few, they list in the description the thickness gage as "sometimes called a feeler gage". Feeler gages sell for $6.00 at Harbor freight and $11.00 + or - a few at other places. So avoid the thickness gage and save money.

With the feeler gage there is no such thing as a sized case not fitting the chamber, I form cases for short chambers that are .017 thousands shorter than a go-gage chamber in thousands to infinity or a more practical .009 thousands longer than a go-gage length chamber in thousands, that is 26 different length cases with out changing the shell holder, die or press.

F. Guffey

[sonnyt650]

can be accomplished with a feeler gage

The way I see this is you're shimming under the case to raise the case up in the shellholder -- is that correct? There is a bit of slop down there so I can see sliding really thin strips of steel beneath the case. I might just give that a go though I'd need to figure out which ones will slide into my existing shellholders without modification.

The sizing die setup will be complicated by having it float above the top of the shellholder instead of touching, but I guess I could use another feeler there too. Or I could just cut the die and figure out which feeler sets up the correct gap above the shellholder -- removes the need for the feeler for each case. Yeah I've got a few options but probably won't take any of them at this point.