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**Unclenick** · April 8, 2011, 10:45 AM

Deepcore,

No, trimming does not affect headspace except in pistol rounds that headspace on the case mouth. In a chambering that headspaces on the shoulder, it is the position of the shoulder that affects headspace, so the sizing die can change it, but not the trimmer.

As Dmazur said, you are really trimming to control how far the case neck sticks out forward of the headspace determinant (the shoulder). Cartridge length is an indirect measurement of this, where the difference between your headspace gauge and your bullet oigve seating depth gauge readings is what is an actual measure. The Giraud and Gracey motorized trimmers and the less expensive Possum Hollow hand/drill operated trimmers all provide that shoulder to ogive measure.

Hatcher showed that just closing the bolt rapidly on an Enfield rifle could shorten headspace up to around 0.006", IIRC. Some of that is the case fattening to fill the extra diameter in the chamber. A little may be neck lengthening. To allow for shoulder setback from this perhaps growing the neck a little, most modern commercial chambers have some extra neck length. 0.020" isn't uncommon. However, there are too many chamber differences out there to count on that, so you really have to do a chamber cast or measure it by other means to learn what your own chamber has available. You're safer sticking with the SAAMI maximum until you do.

When you fire a cartridge the case fills out into the chamber. When you run it into a sizing die, the die first starts to narrow the case, which actually lengthens its headspace (the brass has to go somewhere), then pushes it shorter again at the shoulder. Again, the brass has to go somewhere. In standard sizing dies for bottleneck cartridges it flows into the neck from the shoulder, growing the neck same as it did when the case shoulder was formed at the factory originally, and forms the "dreaded donut", a small internal bulge of brass at the base of the neck. When you withdraw the case over an expander it will try to iron that donut forward a little which contributes to neck growth a bit.

To see this for yourself, measure the length of a fired case, then use a headspace gauge and measure that, and record the difference in the numbers. Then resize the case with the decapper/expander removed, and find the difference between the two again. The increase in difference is in neck growth. Do this with a dozen cases to get an average. Put the decapper back in, and repeat the experiment with another dozen fired cases. The difference in the difference of the two methods is due to the expander. Note that expanders are famous for pulling necks off-axis, too, and that can affect your readings by making the shoulder slightly asymmetrical.

You can avoid a lot of the brass moving using an RCBS X-die, which maintains neck ID with a mandrel that doesn't pull anything anywhere. It does allow the neck to thicken slightly and I expect the donut brass will move down into the case, since the mandrel will block it. I haven't played with one long enough to be sure.

The other method, popular with accuracy shooters, is to resized the case body separately from the neck, using a Redding Body Die, then a Lee Collet Die, which also uses a mandrel and also does not upset the coaxiality of the neck, as demonstrated in this UK YouTube video.

If you want an inexpensive way to measure the headspace size on a case that's been fired in your chamber, you can use a caliper and a spacer or a journal bushing (as I show below). The big fat spacers or a journal with a flange (shown) are easier to keep flat against the caliper anvil. I like the bigger jaws on a 12" caliper for this, but anything can be made to work with some practice. A height gauge and surface plate are even better. Keep in mind that the hole in the spacer is not precisely the headspace datum diameter, so your result is relative. You can compare it to new brass measured the same way to get a vague idea of where you are, but comparison to a headspace gauge (shown in the first picture) is better for zeroing.

Brass will spring back a bit after firing, so measuring headspace on brass is not as precise as using a headspace gauge set in 0.001" increments or, done carefully, the Sinclair tool mentioned earlier. To improve the precision, neck size and refire the same case several times. When it is beginning to feel a little snug on chambering, measure it then. You can also make the same measurement after each of several firings and find a trendline regression toward its limit, but that's maybe a little more math than most shooters want to mess with (though Excel's trendline function will do most of the work for you).

Zeroing on the headspace GO gauge:

Comparing to a fired case (or to a resized one for sizing die setup):

So that case is 0.003" over SAAMI chamber minimum to go in a chamber that is 0.005" over minimum.

April 8, 2011, 10:45 AM	#15
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Deepcore, No, trimming does not affect headspace except in pistol rounds that headspace on the case mouth. In a chambering that headspaces on the shoulder, it is the position of the shoulder that affects headspace, so the sizing die can change it, but not the trimmer. As Dmazur said, you are really trimming to control how far the case neck sticks out forward of the headspace determinant (the shoulder). Cartridge length is an indirect measurement of this, where the difference between your headspace gauge and your bullet oigve seating depth gauge readings is what is an actual measure. The Giraud and Gracey motorized trimmers and the less expensive Possum Hollow hand/drill operated trimmers all provide that shoulder to ogive measure. Hatcher showed that just closing the bolt rapidly on an Enfield rifle could shorten headspace up to around 0.006", IIRC. Some of that is the case fattening to fill the extra diameter in the chamber. A little may be neck lengthening. To allow for shoulder setback from this perhaps growing the neck a little, most modern commercial chambers have some extra neck length. 0.020" isn't uncommon. However, there are too many chamber differences out there to count on that, so you really have to do a chamber cast or measure it by other means to learn what your own chamber has available. You're safer sticking with the SAAMI maximum until you do. When you fire a cartridge the case fills out into the chamber. When you run it into a sizing die, the die first starts to narrow the case, which actually lengthens its headspace (the brass has to go somewhere), then pushes it shorter again at the shoulder. Again, the brass has to go somewhere. In standard sizing dies for bottleneck cartridges it flows into the neck from the shoulder, growing the neck same as it did when the case shoulder was formed at the factory originally, and forms the "dreaded donut", a small internal bulge of brass at the base of the neck. When you withdraw the case over an expander it will try to iron that donut forward a little which contributes to neck growth a bit. To see this for yourself, measure the length of a fired case, then use a headspace gauge and measure that, and record the difference in the numbers. Then resize the case with the decapper/expander removed, and find the difference between the two again. The increase in difference is in neck growth. Do this with a dozen cases to get an average. Put the decapper back in, and repeat the experiment with another dozen fired cases. The difference in the difference of the two methods is due to the expander. Note that expanders are famous for pulling necks off-axis, too, and that can affect your readings by making the shoulder slightly asymmetrical. You can avoid a lot of the brass moving using an RCBS X-die, which maintains neck ID with a mandrel that doesn't pull anything anywhere. It does allow the neck to thicken slightly and I expect the donut brass will move down into the case, since the mandrel will block it. I haven't played with one long enough to be sure. The other method, popular with accuracy shooters, is to resized the case body separately from the neck, using a Redding Body Die, then a Lee Collet Die, which also uses a mandrel and also does not upset the coaxiality of the neck, as demonstrated in this UK YouTube video. If you want an inexpensive way to measure the headspace size on a case that's been fired in your chamber, you can use a caliper and a spacer or a journal bushing (as I show below). The big fat spacers or a journal with a flange (shown) are easier to keep flat against the caliper anvil. I like the bigger jaws on a 12" caliper for this, but anything can be made to work with some practice. A height gauge and surface plate are even better. Keep in mind that the hole in the spacer is not precisely the headspace datum diameter, so your result is relative. You can compare it to new brass measured the same way to get a vague idea of where you are, but comparison to a headspace gauge (shown in the first picture) is better for zeroing. Brass will spring back a bit after firing, so measuring headspace on brass is not as precise as using a headspace gauge set in 0.001" increments or, done carefully, the Sinclair tool mentioned earlier. To improve the precision, neck size and refire the same case several times. When it is beginning to feel a little snug on chambering, measure it then. You can also make the same measurement after each of several firings and find a trendline regression toward its limit, but that's maybe a little more math than most shooters want to mess with (though Excel's trendline function will do most of the work for you). Zeroing on the headspace GO gauge: Comparing to a fired case (or to a resized one for sizing die setup): So that case is 0.003" over SAAMI chamber minimum to go in a chamber that is 0.005" over minimum. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle Last edited by Unclenick; April 9, 2011 at 10:46 AM. Reason: Added a little info.