hes@ccvr1.cc.ncsu.edu (Henry E. Schaffer) (06/13/91)
In this part I'll tell how I started with my new RCBS Precision Mic set, which is a very neat gadget (really a small gadget set). It comes with 4 major parts, one of which comes apart into 4 pieces, plus an Allen wrench. But let me start with the simplest function, the measurement of headspace. Headspace is the distance between the face of the bolt and the "datum line" of the chamber - the place where the cartridge bears against when it is chambered, or the place against which it would bear if it were a bit longer. Since this is often confusing, let me quote another definition, from the Hornady Reloading Handbook, headspace is, " ... measured as the distance from breech face to that part of the chamber which stops the case's forward movement." For a rimless rifle (bottleneck) case, the headspace is the distance from the bolt face to the middle of the shoulder portion of the chamber. The reason this distance is important is that if the headspace is not enough for the case then it will be difficult or impossible to chamber the round. If the headspace is too great, then the case can stretch too much, or even separate when fired. If it just stretches, then it can be made worse or even dangerous for reloading. The importance of this is emphasized by the very small range allowed for safe operation - the SAAMI specs for most rifles only allow a range of .01". However a handloader can usually arrange cases to be longer and so to be safe even in a rifle which has excessive headspace. But first this has to be measured. The Precision Mic headspace gage is in two pieces, a hollow cylinder - the "body". This has one end closed, and a straight scale on one side - running parallel to the axis. This looks a bit like the barrel of a micrometer. The open end is threaded externally. Then there is a "headspace nut assembly" which also is cylindrical, with an open end which has an internal thread. At that end, around the end there is a scale like the thimble of a micrometer. At the closed end there is a hole, just the diameter of the midpoint of the shoulder of the rifle chamber. Therefore if you put a case into this gadget, with the neck pointing out the hole in the headspace nut, and screw the nut on the body, it will close down until the hole bears on the case, and the base of the case is on the base of the body. At that point you read off the distance using the two scales on the body and nut. The body divisions are .050", and the 50 divisions on the nut are .001" each. So it is easy to read off the divisions, with the normal care to make sure that you don't get off by one division on the body when the nut edge is close to a division marking. Since it is hard to get to the inside of the chamber to measure headspace directly, the measurement is made indirectly, by transferring it via a case which has been fired in that chamber. This fired case has been fireformed to fit the chamber, and has only retracted a bit, and so can be used to measure the headspace, as described above. Actually, a group is measured and the average is used. For the .308 Win min/max headspace is 1.6300"/1.6400". The average of my fired case measurements for my rifle is about 1.6315" which is quite close to the minimum spec. This measurement can then be used to set case length when full length resizing. RCBS suggest that cases be sized to be .001-.002 less than the headspace measured. I don't know how much smaller the case is than the chamber in which it has been fired. In exactly the same way, the length of factory cases or various reloads can be checked to see what they are in relation to the minimum spec. My measurements on American Eagle (Federal white box) 150 gr. loads showed an average just a hair under the minimum spec (perhaps -.0002"), for Hornady 165 gr. it was just about at the spec, and some custom target ammo using Lapua brass was under the spec, about -.004". Since I usually neck-size-only for my reloads, this whole exercise is much more for my curiousity than for any real benefit. However it is generally felt that cartridges used for serious purposes as hunting should be full-length resized, in order to minimize problems of chambering ammunition. For such cases, making sure that the cases are short enough but avoiding overworking the brass means that the measurement will be meaningful. This measurement, plus most of the other chamber measurements which I'll be discussing can be done by making a cast of the chamber, or by pounding lead into it (as described to me by Norm Johnson). However, I'm reporting on what I did, and not claiming that this is the best way or the only way to do this. By the way, Sinclair International Inc. had a very good price on the Precision Mic ($26.50). They are at 718 Broadway, New Haven, IN 46744 telephone 219-493-1858. In my experience they are very nice to deal with and ship very promptly. --henry schaffer n c state univ
bercov@bevsun.bev.lbl.gov (John Bercovitz) (06/14/91)
In article <35567@mimsy.umd.edu> hes@ccvr1.cc.ncsu.edu (Henry E. Schaffer) writes:
#I don't know how much smaller the case is than the chamber in which
#it has been fired.
That kind of varies, but one thing's for sure: If you fire a nice
soft case like a Remington enough times so that the primer falls
out, it will be a pretty good fit to the chamber. 8-) I'm pre-
suming neck sizing only, of course. I think if you do this, the
case is ever so slightly _larger_ than the chamber; at least it
feels that way when you try to put the case back in the chamber.
JHBercovitz@lbl.gov (John Bercovitz)