Not being a gunsmith I would like to get a better understanding of headspace, specially rimless.

Rimmed catridges seem to be the easiest to under stand. With standard rim thickness the headspace gauge (s) mearsures the gap between the bolt face and were the rim meets the chamber. Excessive gap and you risk having a portion of the case exposed. You either have to much headspace or you would not be able to close the bolt. Correct?

Rimless is the one that I am having problems with. I understand the concept that the Go gauge will give you the reading that your chamber is deep enough to close the action with standard ammo. No-Go will give you the reading that your chamber is to deep. Correct?

Here is were I have the probem. When a barrel is installed on an action that uses a rimless cartridge, is there special care taken to ensure that the gap between the bolt face and the chamber is set to a minimum? Otherwise the headspace gauges could give a false reading when a unsafe condition could exist.

I have only helped install a barrel once, and I hope I can explain this simply and correctly. My apologies to you guys who really know how to do this.

They make two gauges a go and a no-go. Generally, the go gauge reproduces industry standard dimensions for a particular round. The no-go gauge is too long and you cannot close a properly head-spaced rifle on a no-go gauge. I don't think the no-go gauge has much place in setting headspace.

We used only the go gauge to set our barrel which was already fully chambered. We had already squared up our receiver and bolt, and started with a trial fit of the barrel because we were working with a Mauser 98 action and you are having to seat the base of the barrel on the inner ring and the same time the shoulder of the barrel seats against the forward edge or face of the receiver.

Essentially, the trial establishes the relationship between the inner ring and the receiver face, and, if you can close the bolt on the go gauge, you can take metal off the barrel base (and shoulder) until the bolt closes loosely on the gauge. Remember, the barrel isn't yet torgued down. The trick with a fully chambered barrel is to have the bolt close tight when you "crush" fit the barrel to the inner ring and face.

It is better to be a little loose on trial because, if you can't close the bolt, you have to re-ream the chamber. This, of course, is the same situation you have when you use a barrel that is "short chambered." You fit the barrel to the inner ring and receiver face then hand ream a few thousands out of the chamber to set the head space.

In both instances, the process establishes the distance between the bolt face and the chamber's shoulder. Ku-dude

[ 07-04-2002, 19:47: Message edited by: Ku-dude ]

Wildcat51---

"Headspace" is defined as the distance from the bolt/breech face and whatever in the chamber stops forward motion of the cartridge.

The GO guage represents the ideal of this dimension.

Headspace is set my measuring the distance from the face of the action to the face of the bolt and then chambering until the base of the GO guage is that distance from the stop shoulder on the barrel, LESS a thousandths or two for crush.

In 42 years of gunsmithing I've never owned a NO-GO guage. I use a GO and an indicator to measure slop.

wildcat51,

I think the problem you are talking about is the amount which the case protrudes from the chamber. Rem 700s have good potential in this department My preference is to have as much of the case in the chamber as possible rather than a lot of it in a bolt conunterbore.

Over to Mr Belk

Mike

[ 07-04-2002, 18:43: Message edited by: Mike375 ]

just my 2 cents
this is just a LENGTH messurement, and has NOTHING to measure throat or neck length. The gages look like decapitated cases, kinda.

headspace is the distance from bolt to "datum"point. for the 308 based rounds (and lots of other rimmless) it's midway up the shoulder, on belted carts, it can be belt or shoulder, and rimmed, it's the rim. it can be case mouth on straight, rimless.

the "go" represents the saami ZERO measurement (some makers) that is defined as a perfect chamber, without the +/- side.
the nogo is usually .005+ over saami. A go gage will close on an overlength (headspace) chamber, with ease. You can have grossly excessive headspace if one was to check just go, and not nogo. Results could be streched cases and case seperation.

most custom reamer makers, by default, make to minimum saami (x-.001). I think that you should always order, for a hunting or dgr, a perfect saami reamer, as you do NOT want a tight chamber on those.

field reject is usually .003 or so longer than a no go. if a field reject chambers, DO NO SHOOT the gun.

there are lots of ways of using either gage, or gage and round, or just a round.. always take out the firing pin.

hope this helps
jeffe

[ 07-04-2002, 19:33: Message edited by: jeffeosso ]

Mike375,

Yes indeed that is what am talking about. The rimless headspace gauge will only measure the distance from the bolt face to the shoulder area in the chamber. It could care less how much of the case is unsupported between the bolt face and the chamber. Since most reloaders normally just bump the shoulder, or just resize their brass, chamber depth would not be a problem if you survive fireformimg. The point I am trying make is that when installing a barrel on a action this critical area maybe overlooked or not a priority. Take a look at some of the responses. I believe that most people think that the rimless headspace gauge checks this critcal area. In the rimmed cartridge it does. But with rimless not all is equal. Like you said Mike375 my gumsmith likes to set the barrel back to cover as much of the case as possible. Please correct me if I am wrong.

[ 07-04-2002, 20:37: Message edited by: wildcat51 ]

wildcat,

My comment about having as much case in the chamber rather than the bolt counterbore was aimed at Rem 700s.

Ultimately, the bolt face/extractor will determine how far the case can be in the chamber.

I am no gunsmith, but I think they will have some specifications for each action as to the amount of case protrusion.

Mike

Since a rimless caliber headspace gauge is designed to ensure that a cartridge cannot enter a chameber to too great a depth, hence permitting a cse to stretch too much when fired (assuming it didn't go in so far that the firing pin cannot strike the primer with sufficinet force for the cartridge to fire), I doubt that anyone is under the illusion that the amount to which a case can protrude from the rear of the barrel is in any way related to the headspace measurement, which is the distance from the face of the cartridge head to the datum line, not the distance from the end of the barrel breech to the datum line!! The degree of protrusion of a cartridge case from the end of the chamber is a function of the action/breeching design of a given action type. Some protrude more than others, but in no case should the cartridge have any case wall ahead of the web left unsupported by the steel of the chamber!! For example, in a Mauser, the entire SOLID HEAD of the cartridge case protrudes, (basically, the extractor cut and the rim), and this poses no problem at all. But if more than this protrudes, the chamber has not been cut deep enough.