Pressure with Neck-Sized Brass but not Full Length Sized Brass?

[MSLRHunter]

I have a load for my 280AI that shows no pressure with new or FL sized brass, however that same load with NS brass often shows an ejector mark. Both dies are bushing dies using the same bushing so neck tension should be the same. When FL sizing, the shoulder is set back 0.002". Any ideas what could cause this? Has anyone else experienced this?

 

[Canadian Bushman]

Id say that your running pretty close to a maximum load. Now that the case isn't absorbing some of that boom by expanding your experiencing some high pressure signs. Have you shot any through a chrono for comparison.

 

[Joe King]

Put up a pic of the ejector mark. I'm wondering if it's just a shiny spot or if you have some brass extruded into the ejector hole. I it's the latter I'd say your loads are pretty hot for your gun. If the former then you'd be getting a press fit when you chamber a round. How is your extraction? resistance on the bolt lift, hard, no resistance?

 

[MSLRHunter]

Put up a pic of the ejector mark. I'm wondering if it's just a shiny spot or if you have some brass extruded into the ejector hole. I it's the latter I'd say your loads are pretty hot for your gun. If the former then you'd be getting a press fit when you chamber a round. How is your extraction? resistance on the bolt lift, hard, no resistance?

I'll put up a pic tonight of the mark. It is near max with H4831sc and 168vld, however velocity isn't very high-2815fps. Chambering and extraction are a little snug with the NS brass only.

 

[Mikecr]

Do you have any REAL pressure signs?

 

[Bart B]

Right off the bat, I believe case head extruding into holes in the bolt face is a "real" indicator of pressure being too high. Typical cartridge brass starts to extrude at about 65,000 CUP to 70,000 CUP; that's about the same as about 85,000 PSI.

There has to be something different between your dies and techniques to cause that if the components are the same. Probably a dimension someplace. So you've gotta measure everything between the two sets of resized cases from each tool and process as well as the dimensions between the loaded rounds of each. Every detail dimension wise should be checked.

 

[Mikecr]

He would have every pressure sign known with a 280 at 85kpsi & the subject would be all together different. That's not what's going on, and it doesn't seem to me like a real problem(so far).

 

[Bart B]

He would have every pressure sign known with a 280 at 85kpsi & the subject would be all together different. That's not what's going on, and it doesn't seem to me like a real problem(so far).

Could be. There is one other cause of "ejector marks."

If the bolt face is not square with the chamber axis, the first time that load's fired in a new case will press the case head against the bolt face fairly evenly all the way around at peak pressure. And if the case wall thickness ain't even all the way around, some stretching on the thin side will also happen more so than on the thick side (cause of banana shaped cases?).

Resizing that round won't square up its head. When it's reloaded and chambered again and its high point on the head aligns with the high point on the bolt face, a shiny mark happens there due to zero head clearance of the round when chambered. But it's isn't noticed until after the round's fired and ejected.

So pictures of the actual case head will help. If the mark's the same shape and position on the case head as the ejector's hole in the bolt face, it's probably pressure. If that mark's just a swipe of brass against metal, it may be something else. Using a high magification loupe or glass to inspect the case head will reveal if the case head is higher at the mark compared to the rest of the case head.

 

[MSLRHunter]

Ok, here is a pic of a case head. The mark is on the ACK, and this is one of the worst ones. As far as measurements of the FL and NS cases go, they were trimmed to the same length, prepped the same, same bushing, the only difference is the FL cases have had the shoulder bumped back 0.002". NS cases measure 2.140, FL cases 2.138, and new cases 2.137 measured with a hornady headspace gauge. I went back and chambered some of the NS brass and it is pretty tight, hard to close the bolt. Of course, the new and FL cases chamber with no resistance. My theory is that with the NS cases that are a tight fit in the chamber combined with a near max load has nowhere to expand to, so it flows into the ejector hole? What do y'all think?

Also, it's harder than I thought it would be to take a pic of an ejector mark!

 

[Bart B]

If that rounded shape at the ACK letters is raised above the rest of the case head next to it and is the same shape and location as the ejector hole in the bolt face, to me that is proof the case was fired with too much powder in it. Looks to me like that part started extruding back into that bolt face hole.

 

[MSLRHunter]

It is slightly raised, I'll back off my charge and go from there , thanks for the help.

 

[Joe King]

I would try backing off about 2% on the charge and see what happens from there, I would also get a body and bump the shoulder back just enough you can't feel it chamber.

 

[backwoods83]

I have seen this a lot with Norma and Nosler brass, the case heads are softer than most, I think that your high end charge in FL sized brass is expanding the cases to much and causing a crush fit in the chamber, with no room for expansion it tries to flow back into the ejector hole. How many firings are you getting before the primer pockets loosen, that'll tell you how much case head expansion you have, if you only get 2 or 3 loadings you need to back it off. NK size vs. FL has more internal capacity, I measure the h2o of new cases then measure one that is NK sized and typically have to come up in charge weight to duplicate, that's why I think its the crush fit issue.

 

[Bart B]

I have seen this a lot with Norma and Nosler brass, the case heads are softer than most, I think that your high end charge in FL sized brass is expanding the cases to much and causing a crush fit in the chamber, with no room for expansion it tries to flow back into the ejector hole.

I don't think there's any difference in how much a case expands to the chamber dimensions between a new case, neck only sized fired one or a full length sized one. Even with a 5K CUP/PSI difference in peak pressure in the 50K to 60K CUP/PSI range that some claim will be the difference between these case types, the chamber is the same size and the case is pressed hard against its surfaces at peak pressure. They're all a crush fit in the chamber.

It's been my experience that unless the powder charge is at least 10% below max, there's no evidence (primer sticking out past the case head) that the case doesn't expand to the chamber dimensions