Headspace: Most Accurate Method : Determining Best OAL

Gary H · August 21, 2001, 08:14 PM

What is the most accurate method of determining headspace & OAL in a bolt action rifle? Folks have suggested the RCBS Micrometer and fired brass for headspace, but cases change size as they cool. Stoney Point has a direct way to determine OAL, but dummy case may not be exactly same as your favorite case. Various Bullet Comparators are available to measure loaded round.

If someone could simplify this issue and provide a practical approach to quantifying a bolt actions chamber and subsequent sizing operations.

swifter... · August 21, 2001, 10:16 PM

I've had poor results with the RCBS, much better with the Stoney Point. To maximize accuracy, simply send them a couple once-fired cases from your rifle. It costs about $12 to have 'em converted, and is about as precise as you're going to get.

Especially if you have an Ackley chamber... Ask me, just ask me

.
Tom

Bogie · August 22, 2001, 04:12 PM

Are we talking about headspace, or about the bullet/throat relationship?

Anywho - if you've got a bolt action rifle, and you want to FL size, take your bolt apart, and remove the firing pin and spring assembly (Sinclair can sell you a tool). Notice now how easy your bolt closes - it isn't trying to cock the firing pin, etc...

Now, screw your die into your press. Unscrew it a turn or two. Now, with a nice lubed case, size the brass. Put it in the rifle. Does the bolt handle close with a moderate amount of difficulty? Screw in the die a quarter turn, and size the brass again. Still the same bolt feel? Repeat... Then you'll come to a point where the bolt will just _drop_. Back the die out the quarter turn, then back in about an eighth or even less, and size a fresh piece of brass. You know you're dead on when the bolt sort of drops, but then you've got to gently press it to close it. At this point you're sizing your brass the minimal amount needed to easily chamber.

Now, for determining overall length. First, realize that all bullets are different lengths - even match bullets. What you wanna do is measure off the bullet's ogive. Get the little gizmo from Sinclair that looks like a big ol' 6-sided nut with holes in the faces... One will probably fit your bullet.

Now, take a bullet, and seat that sucker in one of your UNPRIMED minimum headspace cases. Seat it pretty long. Now, take some fine steel wool or an indelible marker, and either brush the surface of the bullet or color it or something - the idea is to make any change noticeable. Now, with the firing pin still out, chamber that sucker, and try to close the bolt. You may have to seat the bullet a little deeper, but you're right when it closes hard... Hopefully you've got enough neck tension going that the bullet doesn't pull out of the case when you extract the case. If it does, repeat with a little more neck tension...

Now, measure the round using the ogive tool. Write the number down. Load the round in your press, and run it into your seater, and screw the seater down until it touches the bullet (you can feel it). Now, seat another dummy cartridge, and measure it. If the numbers are slightly different, adjust our seater stem until they're the same. This is your "bullet jam" length. Many rifles will shoot very well at this length, but you have to work up your load from MINIMUM START LOADS, because you can experience peak pressures very rapidly due to the bullet jam. You may wanna back off a few thousandths.

Many folks think that the best accuracy comes when the indentations of the lands on the bullet (that's why you changed the appearance - to make 'em stand out more) are roughly square.

Gary H · August 24, 2001, 10:09 AM

I'm putting together my equipment and your responses are greatly helpful.

August 21, 2001, 08:14 PM	#1
Gary H Senior Member Join Date: April 26, 2000 Location: Texas Posts: 2,267	Headspace: Most Accurate Method : Determining Best OAL What is the most accurate method of determining headspace & OAL in a bolt action rifle? Folks have suggested the RCBS Micrometer and fired brass for headspace, but cases change size as they cool. Stoney Point has a direct way to determine OAL, but dummy case may not be exactly same as your favorite case. Various Bullet Comparators are available to measure loaded round. If someone could simplify this issue and provide a practical approach to quantifying a bolt actions chamber and subsequent sizing operations.

August 21, 2001, 10:16 PM	#2
swifter... Senior Member Join Date: March 16, 1999 Location: So. CA Mountains Posts: 540	I've had poor results with the RCBS, much better with the Stoney Point. To maximize accuracy, simply send them a couple once-fired cases from your rifle. It costs about $12 to have 'em converted, and is about as precise as you're going to get. Especially if you have an Ackley chamber... Ask me, just ask me . Tom __________________ There are damn few situations so screwed up that adding government to them will not make 'em worse... The best homeland security is an armed & informed citizenery! Molon Labe!

August 22, 2001, 04:12 PM	#3
Bogie Senior Member Join Date: June 5, 2000 Location: Job hunting on the road... Posts: 3,827	Are we talking about headspace, or about the bullet/throat relationship? Anywho - if you've got a bolt action rifle, and you want to FL size, take your bolt apart, and remove the firing pin and spring assembly (Sinclair can sell you a tool). Notice now how easy your bolt closes - it isn't trying to cock the firing pin, etc... Now, screw your die into your press. Unscrew it a turn or two. Now, with a nice lubed case, size the brass. Put it in the rifle. Does the bolt handle close with a moderate amount of difficulty? Screw in the die a quarter turn, and size the brass again. Still the same bolt feel? Repeat... Then you'll come to a point where the bolt will just _drop_. Back the die out the quarter turn, then back in about an eighth or even less, and size a fresh piece of brass. You know you're dead on when the bolt sort of drops, but then you've got to gently press it to close it. At this point you're sizing your brass the minimal amount needed to easily chamber. Now, for determining overall length. First, realize that all bullets are different lengths - even match bullets. What you wanna do is measure off the bullet's ogive. Get the little gizmo from Sinclair that looks like a big ol' 6-sided nut with holes in the faces... One will probably fit your bullet. Now, take a bullet, and seat that sucker in one of your UNPRIMED minimum headspace cases. Seat it pretty long. Now, take some fine steel wool or an indelible marker, and either brush the surface of the bullet or color it or something - the idea is to make any change noticeable. Now, with the firing pin still out, chamber that sucker, and try to close the bolt. You may have to seat the bullet a little deeper, but you're right when it closes hard... Hopefully you've got enough neck tension going that the bullet doesn't pull out of the case when you extract the case. If it does, repeat with a little more neck tension... Now, measure the round using the ogive tool. Write the number down. Load the round in your press, and run it into your seater, and screw the seater down until it touches the bullet (you can feel it). Now, seat another dummy cartridge, and measure it. If the numbers are slightly different, adjust our seater stem until they're the same. This is your "bullet jam" length. Many rifles will shoot very well at this length, but you have to work up your load from MINIMUM START LOADS, because you can experience peak pressures very rapidly due to the bullet jam. You may wanna back off a few thousandths. Many folks think that the best accuracy comes when the indentations of the lands on the bullet (that's why you changed the appearance - to make 'em stand out more) are roughly square. __________________ Job hunting, but helping a friend out at www.vikingmachineusa.com - and learning the finer aspects of becoming a precision machinist. And making the world's greatest bottle openers!

August 24, 2001, 10:09 AM	#4
Gary H Senior Member Join Date: April 26, 2000 Location: Texas Posts: 2,267	Thanks I'm putting together my equipment and your responses are greatly helpful.