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GUNS Magazine July 2011 Digital Edition - Page 12

• G L E N Z E d I K E r • ChAMbEr hEAdSPACE ertainly, if you do your own barrel installations, safety of the end result is of crucial importance. Of course it is. I really don’t think turning your head, closing your eyes and firing a new installation from the hip into the dirt constitutes a certifiable safety check. Well, OK, laugh (it was supposed to be funny), but I’ve had people tell me that’s what they do…. It’s true tolerances among parts are generally tight enough that swapping bolts and bolt-carrier assemblies normally won’t result in parts pairings, resulting in a headspace concern. Question is how you value the difference between supposing and knowing. We get pretty complacent after enough builds because it is very unusual to encounter a problem using mil-spec and commercially-produced bolts and barrels. to confirm. There are huge differences in NATO and SAAMI-spec .223 Remington chambers. Headspace, in essence, is the distance from the bolt face (when the bolt is in battery or fully forward, rifle ready to fire) to the location of the “datum line” along the case shoulder. On .223 Remington, this line is a .330" diameter. Height to this line will be in the vicinity of plus 1.4636" (SAAMI headspace spec). When the chamber reamer is run, the builder or manufacturer stops forward progress of the cutting tool with respect to this. Headspace can vary a little from maker to maker depending, among other things, on their beliefs as to what “playing it safe” means. If the reamer were run too far, then the datum line measure would be farther into the chamber. That would allow too much room ahead of C Check it. Get the tools Since headspace checks are easy enough and a pair of gages is an easy enough investment, I surely recommend this routine to anyone who works at all with barrels and bolts. You will need to know what you’re looking for before you invest in a gage set. By that I mean the chambering specifications you are out Here is what we’re working with, and for. The case datum line on a .223 Remington is .330". From the point of that diameter (dotted line) back to the bolt is headspace in the chamber. Likewise, from that line to the base of the cartridge case is our concern in sizing cartridge cases for reuse in a rifle. Get resized case headspace to read .003" under the actual chamber height to this line and we’re good to go for handloads, by the way. the cartridge case shoulder, and that defines excessive headspace. If the reamer wasn’t run far enough, then the height to the datum line would be shorter, and that is insufficient headspace. Both are bad. Excessive headspace opens up the potential for a case failure, what we can call “blowed-up.” Insufficient headspace means the bolt might not close fully on a chambered round, and that opens up the potential for an out-ofbattery discharge, what we can also call “blowed-up.” don’t blow It We want nothing to blow up! Headspace gages work just as intoned by gauging whether there is, one, enough, and, two, not too much, space. You will need two (you can’t run with just one). A “NO-GO” gage is too long in order to replicate excessive headspace. If the bolt closes on this one, then don’t fire the rifle. A “GO” gage replicates correct headspace. The bolt is supposed to close on this one. If the bolt does not close on a “GO” gage, then the chamber is too short. Don’t fire the rifle. My gages range 0.003" difference. Not much. Maybe you can borrow, but you sorely need to have a gage set available. These are from Forster. For a new barrel installation, you’ll need a “GO” and a “NO-GO.” Make sure the chamber and the gages are clean. It doesn’t take much at all to impede and corrupt gage operation. There are SAAMI-spec .223 Remington gages and NATO-spec 5.56mmx45mm gages. Know which you need, and certainly which you are using. A “Field” gage is a specialty tool to check suspected well-worn rifles for excessive headspace. Some take a field gage along to gun shows, but it’s an unnecessary purchase if we’re concerned with new parts. 12 WWW.GUNSMAGAZINE.COM • JULY 2011

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