I just fireformed a 100 pieces of brand new never fired starline 5.56 brass. After i fired them they measured at several different measurements from the datum line. Most were 1.4575"-1.458" and a couple at 1.459 but some didn't change from what they were when unfired at 1.452. Weird thing is that when I resize my brass I size them for 1.4565"-1.457" since I thought my chamber was at 1.460" and never had some stuck cases and will stick a piece of brass sized ar 1.457" and it ejects fine. So if the majority are ar about 1.4575 is that the size of my chamber even though some get as big as 1.460" on occasion? Not as much as they used to since switching out my bolt but I go/no goed my new bolt (jp enterprises fmos) and it still will spit out brass at that 1.460" but it usually stays at around 1.4575" on average. What would cause such variations even with brass at multiple firings?
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5.56 Brass headspace variations
- Thread starter JRBullock1987
- Start date
When you fire your cases and measure the datum dimension it will likely always be shorter than the actual chamber dimension. This is due to springbuck. We can get into a discussion of how much and why but that is what happens. It is likely your chamber is slightly longer than 1.459".
When the case is fired the case expands under pressure to fill the chamber and as the pressure continues to build the case and chamber expand. When the pressure decays the chamber returns to its original dimensions but the case was stressed beyond its yield point and therefor does not return to its original size but for normal pressures returns to less than the camber datum dimension. The point that it returns to depends on the maximum pressure, the case properties and to some extent the friction between the case and chamber.
When the case is fired the case expands under pressure to fill the chamber and as the pressure continues to build the case and chamber expand. When the pressure decays the chamber returns to its original dimensions but the case was stressed beyond its yield point and therefor does not return to its original size but for normal pressures returns to less than the camber datum dimension. The point that it returns to depends on the maximum pressure, the case properties and to some extent the friction between the case and chamber.
I have read that also in numerous places but I've never seen any data to support it.
I've just gone through an almost identical process (200 cases of Starline fireforming) in a new AR-15 .223Wylde build. Shoulder height (not really "headspace") was quite variable, and this was not unexpected with the AR-15 platform. Of note is that as I experimented with gas amount with the adjustable gas block, fired case shoulder height varied considerably.
I have AR15s that need .004 or .005 bump on brass to get it to chamber but I cant say it's not body diameter that stops it from chambering.
I am of the opinion that the best approach is to use a small base die for most AR's to avoid issues with the base sizing question/issue. It isn't a requirement but it helps especially if you are trying to keep the datum dimension close to the chamber headspace.
When you have checked the shoulder datum length and you are still wondering... try checking the diameters.
Brass diameters at the neck, 200 line, and shoulder are something you should study when starting up a new chamber or setting up your sizing process.
Brass diameters at the neck, 200 line, and shoulder are something you should study when starting up a new chamber or setting up your sizing process.
Yeah I was running a couple load work ups with aa2495 and it's a very low velocity powder in my experience and the high the charge weight for the most part the long the "headspace" or shoulder blew out. Their were 1 or 2 exceptions but most got longer the higher the charge weight. I did get a 5 shot group in the .3s with 75gr hornady match and most in the .5s and low .6s. Same with the 69gr smk. Accurate powder but velocity sucks. I'm making a video about it right now for my yt channel reloaderscodex if anyone wants to check it out. Should be done soon just need to finish editing. I think I'm gunna stick with lake city ammo but now that the starline has been put through my rifle I'll see what the accuracy is really like. I noticed that the shoulder runout out of the box was pretty bad. I'm leaning towards not really caring for it but we will see.
Does having a wide variability in shoulder datum length have anything to do with lock up? Like could the lock up of the bolt be slight different causing the pressures to change in the chamber? Or would it be like super obvious if the bolt wasn't locking up completely? I always wondered if that was what was going on with my rifle but I'm a dult so I have zero clue lol
