Head space on a barrel switch

B

Bill Daniel

New member
Why do you not set the head space on the go gauge only? AGI recommends using tape to give the head space 0.002 to 0.004 inch excess over the go gauge.
Thanks,
Bill Daniel
 
Using the GO gauge alone won't work because the bolt will close on the GO no matter how much excess headspace is present. Using a GO gauge and tape works to make sure the bolt WON'T close, but is not very precise and takes more time and trouble than simply sticking in a NO-GO gauge.

FWIW, a bit of explanation. The GO gauge ensures that the rifle will close and operate with the longest cartridge that is within tolerances for the ammunition. The NO-GO gauge ensures that the shortest cartridge that is within tolerances will not be allowed to stretch far enough to exceed the elastic limits of the case material.

Jim
 
Headspace on a barrel switch

Let me ask the question in a different way. The difference between a go gauge on the Win 308 and the nogo gauge is 0.004". When I rebarrel my bolt action I will tighten the barrel to the action on the go gauge then back off "a bit" remove the gauge and make sure the bolt will no close on the nogo so that I know the head space is less than 0.004" over the go gauge. Will my brass last longer if the head space is less than 0.003" or less than 0.002" or less than 0.001" over the go gauge? The 0.004" difference to keep me from setting too tight of a chamber and crushing the shoulder of a round vs too long a chamber that will lead to excess stretch and early case head separation. Will 0.002" be adequate for the above and net me longer case life?
Thanks,
BillobviouslynotagunsmithDaniel
 
Bill,

Cutting the headspace so the Go Gauge just kisses the end of the chamber without compression when the action is closed is what most match rifle armorers try to achieve. It results in what is called a tight minimum chamber. They also often use a special match reamer that is nearer the minimum diameters for the chamber as well. In more extreme cases there will be a special match reamer for one particular bullet, such as the ones made for the military M852 match ammo for the M14, that used the 168 grain Sierra MatchKing bullet, and that have a shorter freebore than is standard for 7.62/.308 so that the MatchKing's ogive is a more favorable distance off the lands. Or else they'll make one with a narrower than normal neck that is tight around a neck that has been outside neck turned to a specific dimension, but that can't even chamber a round in unturned new brass.

The idea behind the minimum chamber is minimize brass stretch on firing (which maximizes brass life for those that reload a carefully prepared case many times), and to center the cartridges and bullet as closely as possible. That helps shot precision.

That said, a match chamber is also more finicky than one that's not quite so tight. A hunting rifle is normally expected to digest a number of different bullets and to be reliable when it's wet, cold, dusty, or has to chamber a round that fell to the ground and got dented and roughly wiped against a coat or pants leg. A gun that's too tight or fussy is therefore a potential cause of returns to the gunsmith. 0.002" of extra space makes enough allowance to satisfy the need for reliability under a broader range of conditions than match shooters usually have to contend with, and it's still pretty tight and still capable of excellent performance precision. If you don't over-resize your cases, its fit will still be tight.


Tobnr,

Savage rifles use a barrel nut and that allows you to adjust the headspace. Most barrels are turned tightly into a receiver and can't be made looser or tighter without using a lathe to adjust the shoulder position and thread length and a reamer to recut the chamber to the new position, as needed. Typically, when you screw a barrel into a conventional receiver the threads also stretch some, changing the chamber length. That has to be allowed for in the cutting and reaming.
 
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Headspace on a barrel switch

Thanks Unclenick, tobnpr and James K. tobnpr use Kroil and be prepared for a fight. The barrel nut if "factory tight" is a beast to loosen. Also if the barrel is facing away from you the nut loosens to the right not "leftyloosy". It will let women and children be around you when you start your project.
Bill Daniel
 
Actually, even the Savage will stretch the chamber, since the nut works against the receiver threads to tension it. Chamber draw on the order of one to two thousandths per inch of chamber length isn't an uncommon range to encounter in many rifles. It's just that with the Savage, if you don't like how the headspace ends up you can easily loosen the nut and rotate the barrel and re-tighten until you get it where you want it. On conventional receivers there's no such easy tweak. Assuming you stick to the factory barrel torque specification, on conventional barrels how deeply you cut the chamber is the critical determinant of your final headspace, whereas the Savage chamber cut could err a few thousandths and you'd simply adjust it out when you set up the barrel.
 
Headspace on a barrel switch

Unclenick, the gunsmith on the AGI video said that the Savage head space was about 0.002" longer than other gauges and recommended tightening the barrel on the go gauge with a piece of cellophane tape on the head. The tape measures 0.002" on my caliper and this is what I did. It functions well with my proving rounds from Brownell's but won't close on the go gauge with painters tape on the head, which measures 0.0038". Sound about right?
Thanks,
BillcheapbutnotsuicidalDaniel
 
When a friend and I rebarrelled our Savages, we turned the barrels down against a Go gauge and tightened the nut. No No-Go gauge or tape involved. The Savage board says a normal tightening without a hammer or cheater will stretch the threads about .0015". We have never had any trouble with case separation or chambering of neck sized cases.
 
Using only the GO gauge is fine IF you plan to control or screen the ammunition. But if no room is allowed for the occasional long lot of ammo, you might find that the box you take deer hunting won't fit without beating the bolt handle down.

Most people get hung up on headspace and prattle about thousandths and ten thousandths and either forget or never knew that the whole headspace business is about ammunition. And in spite of the pretty pictures in the reloading manuals, the space from here to there on a cartridge is not some exact figure, it is a +/- figure. It is to deal with that +/- that a rifle headspace also has to be a +/- figure.

Jim
 
Good point made.
I handload, and FL size every case, for every caliber we shoot. Factory ammo, particularly milsurp (I'm making an educated guess here) is not manufactured to such tight tolerances.
 
If you take a case headspace gauge to new ammunition you will find the cases are typically under minimum chamber headspace length. I find 1.628" pretty common in new commercial brass. The manufacturers know what they are up against, and despite the fact the SAAMI specs allow some overlap between maximum case and minimum chamber, I've never seen a case that even approached maximum. Indeed, even the surplus 7.62 I've had has never exceeded minimum chamber length, though it wouldn't surprise me if it happened somewhere.

Hatcher mentions that he'd measured .30-06 cases that shortened as much as 0.006" just from vigorous chambering in a 1917 Enfield, so this is nothing new or special. Closing the bolt on a long shoulder just displaces the brass into the extra width available for it in the chamber. If you need a hammer to close a bolt, then you have more interference than the extra chamber width can accommodate, so SAAMI specs are likely being exceeded in one place or another. SAAMI specs are all about compatibility.


Bill,

I'm not sure what the Gunsmith in the video meant. Perhaps he was referring to how much the chamber stretched when you tightened the nut? There is no special Savage-sized GO gauge any more than there is special Savage size .308 Wichester ammunition. The gauges, if made properly, are a single standard from the breech face to the 0.400" diameter at the shoulder, which is called the shoulder datum. Below are the case and chamber headspace specifications. The object is for the cartridge to fit in the chamber, even if that takes a little expanding, and all are alike in that regard.

I will say that I have some cheaper gauges that were off by as much as 0.002". Maybe that's what the AGI gunsmith was worried about. Dave Manson, Clymer, JGS, and Pacific are all good brands, and if you have one of their gauges the length isn't likely to be off.

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One thing to understand is that the GO and NO GO gauges are for a newly installed barrel. The reason the FIELD gauge is longer is to allow for bolt lug setback over time and for barrel thread burrs and surface finish all to settle out with some shooting. So your gauge and tape may fit after awhile and you could still be in spec, though you could also choose to get out the gauge and set it back again for longer case life.
 

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Headspace on a barrel switch

Thanks again Unclenick! This was a Forster gauge. I will measure it on my LNL head space gauge to see. Great explanation as usual!
Bill Daniel
 
I have had no problems with reloads in neck sized brass for my Savage at Go + .0015" or whatever tightening the barrel nug actually did.
If I ran into a brand of ammo or a batch of reloads that would not chamber at that setting, I would change my habits because that would be very poor ammo.
 
Headspace on a barrel switch

"Houston, we have a problem..." Now I measured the Forster gauge at 1.620" not 1.630" as it is labeled. I next measured some new unfired Lapua Win. 308 brass and averaged 1.619". When I checked my prior measurements on some fire formed Lapua brass it had measured 1.624". The opening in the Hornady headspace gauge measures 0.4" on this same caliper and the SMK's measure .308". So the Hornady gauge seems accurate and the caliper seems accurate. Where did I loose the 10/thousandths of an inch?
Billinovermyhead?Daniel
 
Bill,

The LNL gauge insert hole has a slight radius because a sharp edge won't endure, and because of that it always measures a little short. Mine does the same. It's good for comparative measuring, but not absolute.
 
Headspace on a barrel switch

Thanks again Unclenick. I need to go shoot some rounds and let the recoil clear my head.
Bill Daniel
 
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