Case Sizing/Headspace Question

[Justin Holder]

I'm new to reloading and need help understanding something.

While setting up an RCBS FL sizing die in .223rem. for a CZ bolt action I found something odd.

When comparing factory unfired ammo headspace to once fired case headspace using a Hornady Headspace Comparator, the once fired cases headspace is .003 shorter than unfired factory ammo. All cases have been fire formed in my gun.

When I set the sizer die to full length resize the headspace measurement stays the same but OAL stretches out by .005

What is going on? I was under the impression that factory ammo was already at a minimal size and headspace would stretch upon firing, not get shorter.

 

[918v]

I think your expander is pulling the shoulder forward. Are you lubing the inside of the necks prior to sizing?

 

[243winxb]

On firing, the body of the case expands outward. The shoulder takes a different shape. The degree of shoulder angle changes, giving a different measurement. The headspace (head to datum) of the chamber & die are very close to being the same. But the fired case body, when sized back smaller, pushes the oal (trim length) longer. The die stops the shoulder from moving forward.

 

[Walkalong]

The case gets fatter and shorter upon firing, and skinnier and longer when sized. the shoulder moves with this process as well. The brass is not the dimensions of your chamber after its naturally elasticity works its magic when the pressure dies down.

 

[steve4102]

the once fired cases headspace is .003 shorter than unfired factory ammo.

When measuring ammo with seated primers make sure they are not proud, affecting measurements.

 

[243winxb]

Shoulder set back on firing.

The firing pin strike can push the shoulder back .006" or more. Testing was with 2 Savage bolt actions in 223, using annealed & factory brass when sized for a crush fit & measured. The extractor type & fit may be a factor. Test -Take 3 brass that have been FL sized so there is a crush fit when closing the bolt. Seat used/fired primers in the empty brass. Make sure the extractor is not pushing the shoulder back on chambering. Measure head to datum with your tool. Record measurement. Chamber/firing pin strike/measure/Record. How much was the shoulder pushed back?? Do test 2nd time on same brass, shoulder should be set back even more. When a loaded round is fired, the shoulder set back may not be seen , as the shoulder blows out/forward. Soft annealed brass may cause a misfire at times.

[/URL][/IMG] Click to read & see larger photo.

 

[ranger335v]

Case length and OAL are not the same things.

 

[fguffey]

OAL, Overall length? COL, case overall length, then there has to be case length, then there is COAL, I suppose adding the C to the OAL means including the bullet.

The .005 has to do with the effect the chamber had on the case when fired added to the effect the die had on the case when sized or returned to minimum length/full length sized, meaning the .005 has nothing to do with head space? Or as I say the length of the chamber from the bolt face to the shoulder when compared to the length of the case from the head of the case to it’s shoulder.

"I'm new to reloading and need help understanding something.

While setting up an RCBS FL sizing die in .223rem. for a CZ bolt action I found something odd.

When comparing factory unfired ammo headspace to once fired case headspace using a Hornady Headspace Comparator, the once fired cases headspace is .003 shorter than unfired factory ammo. All cases have been fire formed in my gun.

When I set the sizer die to full length resize the headspace measurement stays the same but OAL stretches out by .005

What is going on? I was under the impression that factory ammo was already at a minimal size and headspace would stretch upon firing, not get shorter"



Head space stays the same? The length of the case from the head of the case to its shoulder stays the same, I determine the length of the chamber first from the bolt face to the shoulder/datum, it makes no sense to anyone else, by knowing the length of the chamber I know the length of the case after firing, everyone else fires first then measures, as you can see by the responses there is a lot going on, case gets shorter, longer, then there is recovery, spring back, snap back.

Again, my chambers are not adjustable, my dies and presses have threads, threads allow me to adjust the die in the press, all I am required to do is eliminate wild guestimates. I use the feeler gage, the companion tool to the press, and I make case comparators, or put another way, I make case length gages.

F. Guffey

 

[jim243]

The case gets fatter and shorter upon firing, and skinnier and longer when sized. the shoulder moves with this process as well. The brass is not the dimensions of your chamber after its naturally elasticity works its magic when the pressure dies down.

^^^ +1. Your chanber will deturmine the case size upon firing. All within SAAMI specifications and well within safety standards. Different brands of brass will act differently because of metal content and you should expect to see variances.

Jim

 

[fguffey]

Sound great,

Quote:
The case gets fatter and shorter upon firing, and skinnier and longer when sized. the shoulder moves with this process as well. The brass is not the dimensions of your chamber after its naturally elasticity works its magic when the pressure dies down.

^^^ +1. Your chamber will determine the case size upon firing. All within SAAMI specifications and well within safety standards. Different brands of brass will act differently because of metal content and you should expect to see variances.

Jim
jim243 is offline" src="http://images.thehighroad.org/statusicon/user_offline.gif">


but when a little thought is put into the part where the case gets shorter from the case head to the shoulder, sound like air is trapped between the chamber shoulder and case shoulder, after firing the trapped air pushes the shoulder back?? and we all know that does not happen.

I have had cases shorten .045 thousandths in length, but! I have never had a case shorten in length from the case head to the shoulder/datum.

F. Guffey

 

[gamestalker]

Ranger335v took the words out of my mouth, OAL and case (brass) length are not the same thing. Making this distiction is important when attempting to explain to us your circumstances, and must be understood by the reloader in order to properly understand what the books are relating.

Casing Over All Length - OAL, COAL is the measurment from case head to the tip of the bullet of a loaded cartridge.

Case length is the measurement from case head, to the mouth of the brass. Maximum case length and trim to length is determined only after resizing has been accomplished.

Head space is the distance from the bolt face to the shoulder in the firearm. On the brass it is from the head to the shoulder.

I have never had a case get shorter from the head to shoulder after firing. Shorter from head to mouth, yes! There is a lot of expansion going on during firing. New or factory brass is a one size fits all until it has been fired from your chamber, at which point in time it begins taking on the characteristics of that chamber. But once you've run it through the die it changes in length and dimension, to what degree is determined by the type of die, and according to die adjustment.

Just a tip though, pick up your books and do some extensive reading so you can learn a bit more about reloading terminology / deffinition. Other wise your going to misinterpret what the books are referring to, and the advice being offered here as well. Please don't be offended, this hobby can be a bit much for a new guy, and especially when working with bottle neck cartridges.

GS

 

[zeke]

On the cz rifles owned in 223 and 308, the headspace is about .002-.003 shorter than most factory cases, which are tight fits. They are extremely accurate rifles. When fired, the shoulder is actually set back to conform to the chamber.

The OAL is likely increased by dragging the expander ball through the neck.

 

[ranger335v]

OAL and COL are the same thing; Overall Length vs. Cartridge Overall Length, both meaning the case and a seated bullet. Case length is case length, there is no "over all" to it.

Neck length is increased by FL sizing, not expander drag. Neck length doesn't affect OAL at all, any additional neck length simply means the neck overlaps the bullet a bit more.