223 Rem Base to datum measurement

Nathan · February 1, 2017, 11:52 AM

So, i have some 223 rem that the base to datum averages 1.456 on fired cases. When i measure xm193, i get average 1.448.

Im setting up my fl die. What should i target? Touching the shellplate lightly, i get about 1.456. A hard camover gets me 1.450.

Dufus · February 1, 2017, 02:05 PM

I would depend on your firearm.

A semi-auto needs a little space, so size a little more.

A bolt gun would require a chamber measurement to determine a proper sizing.

T. O'Heir · February 1, 2017, 02:16 PM

"...I'm setting up my fl die..." You're over complicating the whole thing. Forget base to datum averages altogether. Isn't necessary.
Screw the die in so the shell holder just kisses the bottom of the die when the ram is all the way up. Lock the die. Then adjust the decapping pin so it pops the spent primer out and lick it. Done.
You don't need to much around with the off-the-lands stuff either.

AllenJ · February 1, 2017, 02:32 PM

+1 T. O'heir

Adjust your die so the empty fits in the chamber then lock it down.

Nathan · February 1, 2017, 03:52 PM

That all sounds good until some don't fit. AR's arent real forgiving with don't fit. They jam up something nasty and usually require a hammer to open the action! Ask me how i know!

I might try the empty case thing.

CarJunkieLS1 · February 1, 2017, 04:20 PM

If the cases that you have were fired in your rifle. Set your die up so the shoulder is bumped back .004-.005 from fired case measurement. You don't want to "overwork" your brass if you don't have to.

mehavey · February 1, 2017, 04:35 PM

I wouldn't depend on gasgun-fired cases to give you a reliable case headspace dimension.
Too many ejection process stresses & strains going on.

Instead, size gradually until the bolt fully closes with minimal effort (if the extractor
doesn't muck things up) then use that dimension to baseline on for a coupla thou less
than that for your standard.

As a test, measure a few sized/loaded cases before loading; load (let the bolt go home normally);
then measure again to see if chamber pushed the shoulder back to any degree.

ps: every loading session/brass lot/fired-case work-hardness is different.
Plan on measuring again at the start before locking the die down... and then measure again occasionally.

243winxb · February 1, 2017, 05:38 PM

Each station of the shell plate may give a different shoulder bump.

ShootistPRS · February 1, 2017, 06:44 PM

The on-going recommendation is that semi-autos need to be fully sized. Set your die to kiss the shell holder when the handle is stroked. There is no one measurement that is the magic key to getting reloads to chamber. The case has to be sized in circumference, length and neck to positively chamber. If you have problems after full length sizing then you have the luck to need a small base die.
I just bought my first AR and I will do some experimenting with my own brass and chamber to see if I can extend the life of the brass without affecting the function of the rifle but I can't recommend that anyone tries it until I find out whatmy gun can use.

Nathan · February 1, 2017, 08:25 PM

I just fit my rounds to the actual gun. My chamber is pretty tight. To eject loaded rounds without a hammer, I need to be under 1.454" according to my gun.

This is not kissing the shellholder. Kissing the shellholder yields 1.458". I need firm cam over to get a nominal of around 1.451". Funny thing is, increasing cam over force keeps shortening the case!

February 1, 2017, 11:52 AM	#1
Nathan Senior Member Join Date: July 1, 2001 Posts: 6,331	223 Rem Base to datum measurement So, i have some 223 rem that the base to datum averages 1.456 on fired cases. When i measure xm193, i get average 1.448. Im setting up my fl die. What should i target? Touching the shellplate lightly, i get about 1.456. A hard camover gets me 1.450.

February 1, 2017, 02:16 PM	#3
T. O'Heir Senior Member Join Date: February 13, 2002 Location: Canada Posts: 12,453	"...I'm setting up my fl die..." You're over complicating the whole thing. Forget base to datum averages altogether. Isn't necessary. Screw the die in so the shell holder just kisses the bottom of the die when the ram is all the way up. Lock the die. Then adjust the decapping pin so it pops the spent primer out and lick it. Done. You don't need to much around with the off-the-lands stuff either. __________________ Spelling and grammar count!

February 1, 2017, 02:05 PM	#2
Dufus Senior Member Join Date: December 10, 2014 Posts: 1,965	I would depend on your firearm. A semi-auto needs a little space, so size a little more. A bolt gun would require a chamber measurement to determine a proper sizing.

February 1, 2017, 02:32 PM	#4
AllenJ Senior Member Join Date: December 11, 2009 Location: Northern California Posts: 1,766	+1 T. O'heir Adjust your die so the empty fits in the chamber then lock it down.

February 1, 2017, 03:52 PM	#5
Nathan Senior Member Join Date: July 1, 2001 Posts: 6,331	That all sounds good until some don't fit. AR's arent real forgiving with don't fit. They jam up something nasty and usually require a hammer to open the action! Ask me how i know! I might try the empty case thing.

February 1, 2017, 04:20 PM	#6
CarJunkieLS1 Senior Member Join Date: October 1, 2013 Posts: 686	If the cases that you have were fired in your rifle. Set your die up so the shoulder is bumped back .004-.005 from fired case measurement. You don't want to "overwork" your brass if you don't have to.

February 1, 2017, 04:35 PM	#7
mehavey Senior Member Join Date: June 17, 2010 Location: Virginia Posts: 6,894	I wouldn't depend on gasgun-fired cases to give you a reliable case headspace dimension. Too many ejection process stresses & strains going on. Instead, size gradually until the bolt fully closes with minimal effort (if the extractor doesn't muck things up) then use that dimension to baseline on for a coupla thou less than that for your standard. As a test, measure a few sized/loaded cases before loading; load (let the bolt go home normally); then measure again to see if chamber pushed the shoulder back to any degree. ps: every loading session/brass lot/fired-case work-hardness is different. Plan on measuring again at the start before locking the die down... and then measure again occasionally.

February 1, 2017, 05:38 PM	#8
243winxb Senior Member Join Date: July 26, 2011 Location: USA Posts: 1,730	Each station of the shell plate may give a different shoulder bump.

February 1, 2017, 06:44 PM	#9
ShootistPRS Senior Member Join Date: January 3, 2017 Posts: 1,583	The on-going recommendation is that semi-autos need to be fully sized. Set your die to kiss the shell holder when the handle is stroked. There is no one measurement that is the magic key to getting reloads to chamber. The case has to be sized in circumference, length and neck to positively chamber. If you have problems after full length sizing then you have the luck to need a small base die. I just bought my first AR and I will do some experimenting with my own brass and chamber to see if I can extend the life of the brass without affecting the function of the rifle but I can't recommend that anyone tries it until I find out whatmy gun can use.

February 1, 2017, 08:25 PM	#10
Nathan Senior Member Join Date: July 1, 2001 Posts: 6,331	I just fit my rounds to the actual gun. My chamber is pretty tight. To eject loaded rounds without a hammer, I need to be under 1.454" according to my gun. This is not kissing the shellholder. Kissing the shellholder yields 1.458". I need firm cam over to get a nominal of around 1.451". Funny thing is, increasing cam over force keeps shortening the case!