Basics: Headspace

Setting Proper Headpace on Resized Cases
by Germán A. Salazar
Setting the headspace on a resized case is important to ensure reliable functioning of the rifle, avoid damage to the rifle, maximize case life and achieve consistent accuracy. The objective of this article is to show you a method of setting up a full length resizing die that allows you to quickly adjust its setting to achieve the desired amount of shoulder set-back (headspace) for a number of rifles chambered for the same cartridge or for different lots of brass.

"Headspace" in the context of the rifle chambered for a rimless cartridge is actually the distance from the bolt face to the point in the chamber that is halfway up the shoulder. In the reloading context we often use the term to indicate how much clearance have we built into the brass case as compared to that distance. Simply stated, if the headspace measurement on a resized piece of brass is 0.002" less than the chamber headspace of the rifle in which it was fired, we often say the brass has 0.002" headspace. That's not a technically correct use of the term, but it will do for most reloading purposes.

Brass is a fairly "live" metal, it conforms to the chamber dimensions under the pressure of firing and as the pressure drops, it springs back towards its original dimensions, although not all the way back - that's why we resize cases. If it didn't spring back at all, extraction would be very difficult; you get an idea of that with an over-pressure load where the brass, in fact, does not spring back enough to allow easy extraction.  Brass fired at normal pressures springs back in both length and diameter, both need to be addressed in resizing in order to make the brass hold a bullet again and go into the chamber properly.

Brass that is only neck sized will hold a bullet, but will chamber hard and with each firing will chamber harder yet. The reason for this is that is has essentially no headspace, just a little from the spring-back, and the radial dimensions also have insufficient clearance. Hard chambering tends to wipe the lube off of the locking lugs, leading to galling of the lugs in a relatively short time. In addition to damaging the rifle, hard chambering and extraction are hardly what a competitor needs in the middle of a match.

Let's look at a method of full length sizing that allows you to control the headspace of your brass and to easily adjust the die for proper headspace from one rifle to another or even one lot of brass to another. Yes, different lots of brass might require a slightly different setting of the die to achieve the same headspace because the metallurgy of the brass may be slightly different. The same can happen as the brass hardens a bit after many firings.

First, you'll need a method of measuring the headspace of the fired brass and the resized brass. Hornady makes a tool for this and there are others as well such as Wilson and Forster. I usually use a piece of barrel steel into which the shoulder area of the appropriate chamber has been cut. Either way will do a perfectly good job. The gauges made by Mo DeFina are also very useful for this process. Whichever method you use, the objective is the same, we want to measure the headspace dimension of the brass before and after resizing to make sure that we've created the headspace clearance we're after.

Second, loosen the lock ring on the die and move it up on the die. Get a #17 O-ring (7/8" x 1/16") from you local Ace Hardware and slide it over the die so that it will go between the lock ring and the press. Run the ram in the press all the way up and screw down the die until it makes contact. Now, lower the ram and screw the die in 30 degrees more (make a mark at 6:00 and turn it to 7:00). now, raise the ram again and you will feel it bump, but it will still go all the way. Next, with the ram still bearing against the die, lower the lock ring and once it begins to compress the O-ring keep turning until it feels slightly firm, that should be about 1/8 of a turn. Lock the lock ring, lower the ram and with a Sharpie type marker make an index mark on the press. Erase any index mark you may have made on the lock ring from the earlier 30 degree setting. At this point there should be a mark on the press and none on the die.

If the shellholder doesn't come into solid contact with the die solidly enough to make it cam over (that's the bump feeling) then it won't set the shoulder back. That was the first part of what we did. Second, by using the O-ring method, what we've done is created a setup in which the die will slightly float under pressure from the ram and this keeps things more centered. Thirdly, the O-Ring setup allows you to tweak the die up and down a tiny bit as needed to really set the amount of shoulder setback properly for your rifle and brass. Resized brass headspace should be between 0.001" to 0.002" less than the headspace of the fired case.

Using and Adjusting the O-ring Die
Wih the die now set up in the press, take a few fired cases and deprime them - not by running them into the die, use a manual decapping stem or a decapping die. Decapping prior to this initial measurement is important because any slight primer cratering or protrusion will affect our readings. Using your gauge, measure each case for headspace and make a note. During setup, I'll write it directly onto the case with the Sharpie. With the die at the initial setting, size two of the cases and check the headspace again. Ideally the headspace will be between 0.001" and 0.002" less than the fired cases - this is for a bolt action rifle, for a semi-auto use 0.002" to 0.003" headspace. The consistency of your case lube application will affect the consistency of the headspace dimension.

Adjust the die in or out as appropriate to get to the desired dimension, check two cases per setting to make sure of the setting. Once you have the setting where you want it, make a Sharpie mark on the die's lock ring to index to the mark on the press. The setting will repeat very well from session to session. You'll find that older brass or brass from a different rifle will call for a slight change in the setting, but you'll have a good place to start. I check headspace each time I reload brass, while it may seem like a lot of work, it's really just a couple of minutes of effort - well worthwhile for enhanced reliability and brass life.

Here are a few pictures to clarify things.

1. The die with the O-ring, self-explanatory.

2. The die on the press with the index mark visible. The second index mark is for another rifle with a shorter headspace.

3. A homemade (by a friend) headspace gauge that is simply a piece of steel with the exact angle of the shoulder cut into it and a hole big enough for the neck to pass through.  The same thing can be done nicely with a cut-off piece of the barrel and the chambering reamer when your new barrel is installed.  Wilson and Forster also make useful gauges for this purpose.

4. The gauge with a true chambering headspage gauge in it. You'll see the pointer on the caliper rests on zero. There's no critical thickness for the gauge, it could be anything since what you'll do is simply compare fired brass to sized brass and check the difference. Since it looks nice with the pointer at 0 my friend ground the gauge to the thickness that yielded that setting. Rotating the dial on the calipers, or setting digital calipers to zero would achieve the same result. You don't need the chambering headspace gauge for the die setup, I just used it to show "true zero". All that really matters is the difference between the fired cases and the sized cases.

5. The gauge with a piece of sized brass in it. The rifle it was fired in gives brass that reads 0.004" when fired, so I try to size it to read between 0.002" and 0.003" (set it back 1 to 2 thousandths). This piece has already been sized and will slip in and out of the chamber with no resistance and is not overworked as it would be if I had taken it all the way back to 0. It will last longer this way.  When using this type of gauge, be sure to remove the primer before taking the measurement.  Obviously, if you do that in the sizing die you won't get the reading you needed, so use a simple decapping die or a primer punch such as those sold by Wilson or Lee.

Headspace measuring tool from Mo's Competitor Supplies.  The case goes inside the gauge and the cap screws on, you read the amount over or under "0" directly on the cap against an index mark on the gauge body.  MCS Inc., 34 Delmar Dr., Brookfield, Connecticut 06804 ph:203/775-1013

For more information on this and related topics, see Measuring the Case.

 Update - October 28, 2009.
I've just become aware of a very nice looking tool to measure headspace.  While I haven't tried it, it looks to be well thought out and nicely made.  If anyone tries it, I'd like to hear about it.  The tool is made by Innovative Technologies.


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