Virgin Brass Datum Line

antiwhackos · June 25, 2010, 11:36 AM

Will FL sizing virgin brass change the position of the datum line on the shoulder? I have new virgin brass from Winchester for a .250-3000 and all I did was trim them and push the die button through the necks to uniform them. They now have all been loaded. I have since then purchased the Hornady Cartridge Headspace Gauge Bushing set. The cartridge headspaces range from 3.565" to 3.571". I measured some old, fired cases I have and they measure 3.572". It appears to me that I should aim for setting my sizing die to give me 3.570" which is .002 less than the fired case. If that is the case, will the new brass that is shorter than 3.570" harm the cases in any way. Thanks for taking the time to read this. Al

**Unclenick** · June 25, 2010, 12:14 PM

Not at all. It is normal for new cases to be 0.007" shorter at the chamber shoulder datum than the longest chamber. Chamber tolerances are often 0.005" or more, and the new cases usually start at least 0.002" below the short end of that range. They just blow outward and forward on firing to fill the chamber. That's normal. In fact, that's one of the features of P.O. Ackley's wildcat designs, that the parent cartridge may simply be fired in the Ackley chamber to get the new, bigger case. It's called fireforming.

Julian Hatcher once used special reamers to make a .30-06 chamber way too long to see how long it could get before new cartridges would start getting head separations? It seems to me he got out an eighth of an inch or even two tenths, maybe, but still never got separations in new brass (not that what he did get would be reloadable—it would be way too thin at the pressure ring for reuse). So this is not an immediate safety problem, but accuracy would deteriorate with a chamber that is big enough. Most often excess headspace is a problem for sizing cartridges to feed properly or can be a sign of a worn or stressed gun, assuming it started at normal length. The excess length also lowers peak pressure and ballistics.

Many benchresters seem to favor bumping the should back 0.001" for best accuracy, but bumping it back 0.002" is usually the minimum given if the finished round is to be fed from a magazine rather than singly loaded by hand as a benchrest round would be. The 0.002" is small enough to prevent feed interference in most guns, though sometimes, as with some semi-auto chambers they can need more or more narrowing, which is what small base sizing dies do.

All that said, you can safely return a case to original size. You just end up thinning and work hardening the pressure ring faster if you do. John Feamster's rule of thumb is that such brass, used in the M1A, should be ditched after the fourth reload for safety. In a bolt gun, neck sizing and setting the shoulder back 0.001 or less, you see cases last 50 reloads sometimes.

antiwhackos · June 25, 2010, 03:13 PM

Wow, Unclenick, thank you for the time and effort you put into your response. I was concerned that too much brass was going to flow from the web area. Your reply was great in that I no longer have that concern. Also, I will set my sizing die for .001" less than the fired case measurement. Thank you very much. Al

June 25, 2010, 11:36 AM	#1
antiwhackos Member Join Date: July 31, 2007 Location: Ocean Springs, MS Posts: 15	Virgin Brass Datum Line Will FL sizing virgin brass change the position of the datum line on the shoulder? I have new virgin brass from Winchester for a .250-3000 and all I did was trim them and push the die button through the necks to uniform them. They now have all been loaded. I have since then purchased the Hornady Cartridge Headspace Gauge Bushing set. The cartridge headspaces range from 3.565" to 3.571". I measured some old, fired cases I have and they measure 3.572". It appears to me that I should aim for setting my sizing die to give me 3.570" which is .002 less than the fired case. If that is the case, will the new brass that is shorter than 3.570" harm the cases in any way. Thanks for taking the time to read this. Al

June 25, 2010, 12:14 PM	#2
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,061	Not at all. It is normal for new cases to be 0.007" shorter at the chamber shoulder datum than the longest chamber. Chamber tolerances are often 0.005" or more, and the new cases usually start at least 0.002" below the short end of that range. They just blow outward and forward on firing to fill the chamber. That's normal. In fact, that's one of the features of P.O. Ackley's wildcat designs, that the parent cartridge may simply be fired in the Ackley chamber to get the new, bigger case. It's called fireforming. Julian Hatcher once used special reamers to make a .30-06 chamber way too long to see how long it could get before new cartridges would start getting head separations? It seems to me he got out an eighth of an inch or even two tenths, maybe, but still never got separations in new brass (not that what he did get would be reloadable—it would be way too thin at the pressure ring for reuse). So this is not an immediate safety problem, but accuracy would deteriorate with a chamber that is big enough. Most often excess headspace is a problem for sizing cartridges to feed properly or can be a sign of a worn or stressed gun, assuming it started at normal length. The excess length also lowers peak pressure and ballistics. Many benchresters seem to favor bumping the should back 0.001" for best accuracy, but bumping it back 0.002" is usually the minimum given if the finished round is to be fed from a magazine rather than singly loaded by hand as a benchrest round would be. The 0.002" is small enough to prevent feed interference in most guns, though sometimes, as with some semi-auto chambers they can need more or more narrowing, which is what small base sizing dies do. All that said, you can safely return a case to original size. You just end up thinning and work hardening the pressure ring faster if you do. John Feamster's rule of thumb is that such brass, used in the M1A, should be ditched after the fourth reload for safety. In a bolt gun, neck sizing and setting the shoulder back 0.001 or less, you see cases last 50 reloads sometimes. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

June 25, 2010, 03:13 PM	#3
antiwhackos Member Join Date: July 31, 2007 Location: Ocean Springs, MS Posts: 15	Wow, Unclenick, thank you for the time and effort you put into your response. I was concerned that too much brass was going to flow from the web area. Your reply was great in that I no longer have that concern. Also, I will set my sizing die for .001" less than the fired case measurement. Thank you very much. Al