Strange fire form results

[gamestalker]

What could cause shoulders to get bumped back .006"-.009" after being fired. The cartridge is .243 win using a charge of 41.6 grs. of H4831, 100 gr. Speer BTSP, and CCI-200's, a load I have been loading and shooting for a very long time, and with great results. All were fired from the same rifle.

The brass that experienced this was all 40 pieces of FC brass, the 40 pieces of each Winchester and RP brass maintained their fire formed dimensions, very consistently I might add. All the head stamps were on their second necking, and I was preparing to load the third. Nothing about the brass looks suspicious, no incipient separations, and no signs of excessive or low pressure either, which there wouldn't be, considering this is a long time load I've used for years with all 3 head stamps mentioned above.

I'm not really concerned, just really curious if one of ya all with more in depth knowledge concerning internal ballistics could share some insight? RC, Arch, Blarby, BDS, anyone, what's your take on this?

GS

 

[blarby]

I have some thoughts- but we're gonna need your calipers for most of it.

These could or could not be the cause, but its a place to start :

I. There's been some reports lately about weird rims on "ammopocalypse" brass. Given that rim thickness isn't something even I regularly check, it might be worth checking out. Headspace= on the shoulder, but rimspace would allow room for deviation, right ? Maybe. This might have happened in the sizing process and you didn't know it. Did you set your dies for shoulders on the WIN, and then move on to the RP and then the Federal ? Or did you set it for each stamp ?

II. You got carbaceous buildup in that chamber thats sucking your brass further into the chamber than should be allowed ? Lol. Probably not. Right up there with : Did you get REALLY aggressive with chamber cleaning ?

III. You mentioned the WIN and RP brass turned out ok. I think your culprit is bad/inconsistent FC brass. I don't like FC brass, I figured that out when I first started loading rifle. Its not very consistent, I have neck splits to no end, and you can tell their emphasis is on that first shot outta the box, not the stuff we do with it.

Honestly, any of the problems that are mechanical to the rifle and not brass in nature would present on ALL of your brass, not just the Federals. Any of the problems that were present in sizing would deviate from headstamp to headstamp. That leads me to believe its a crappy lot of brass in some form- either something you can easily measure, or worse, something you cant.

Grab some rim thickness measurements, and lets talk this problem out one failpoint at a time.

 

[gamestalker]

Thanks Blarby.

I. Actually, I didn't touch the shoulders at all. I necked the brass and chamber checked each piece. But I did de-prime a couple sample pieces of each head stamp and then measured, which is my usual process.

II. My chamber gets completely and thoroughly cleaned following each range session. No build up what so ever, and true, it would have presented on other head stamps also.

III. I haven't measured the rims as yet, but I'll do that and get back with that info. I think this will be the culprit.

Being that I normally chamber check only after necking, measuring has been unnecessary unless bumping is needed. But I do usually measure 1 or 2 of each head stamp prior to necking, just because I'm pretty detail orientated as to how each head stamps is doing in the fire forming process.

GS

 

[gamestalker]

Well, the rims measured very consistent with the other head stamps. So now I'm just chalking it up to very hard brittle brass, FC is known to be rather hard/brittle, so it's probably springing back, therefore not holding it's fire form well, this batch may be worse than others. I'll see how it does on this third loading, that is if it doesn't separate from excessive head space?

Thanks Blarby

GS

 

[blarby]

Sounds good ta me !

If the rims are consistent, and your chamber isn't magically changing on you, and you are only neck sizing....

Sounds like some scrap for the recycler !

Or, just stuff 'em and use them for the "rainy day" box, or for real hunting loads where losing them in the hunt isn't a big deal.

 

[gamestalker]

Ya, I'll run a few to see if they will stay together, no incipient separation signs as yet, odd, but they'll probably due fine for one more loading.

Thanks again!

GS

 

[243winxb]

My guess is ...............

When the body of the brass expands outward, the shoulder may get pullled back. When pressure does not peak fast enough,with slow burn rate powders, the brass may not fill the chamber. There is something different between the 3 lots of brass your using. I would guess its softer?? When only neck sizing, the case volume is greater by 4% when compared to factory sized ammo (5.56mm). I would guess more in the 243 win. Neck sizing will reduce pressure when using the same powder charge (Military testing results).

 

[243winxb]

What could cause shoulders to get bumped back .006"-.009" after being fired.

The savage axis firing pin strike will set the shoulder back .006" in 223 Rem. But you would only know this if you have a misfire, bad primer.

 

[243winxb]

From Lee FAQ Collet Dies on Rockchucker Press

If you are using the Collet die in an RCBS Rockchucker press, or a similar design that toggles over center at the top of the (ram) stroke, this applies far greater force than is necessary to resize just the neck of the case, and can damage the die or collapse the case.
or maybe push the shoulder back?? These 3 suggestions should keep you thinking for a while.

 

[gamestalker]

Nope, no savage Axis, no collet die, just an RCBS neck die and some FC brass involved.

But the brass expansion thing with slow burning powders may have some merit? That's actually some useful information I'll add to my knowledge base. And yes, there is something different about the brass, it's FC, and the other two that didn't experience shoulder set back were Win and RP. All head stamps had been loaded twice, no neck sizing was done prior to taking measurements.

I was actually thinking the FC brass may have been harder, more brittle, thus it excessively sprang back?

GS

 

[FROGO207]

Try annealing a few of them and see if the results are the same after shooting them again. That would rule out hard brass as a problem if nothing changes. I bet you find this is the problem.

 

[BCRider]

If fire forming at lower pressure I can see the cases stretching around the diameter and pulling back. But to stretch the length as well it likely need greater pressure. And if they swell in diameter but not length that would pull the shoulder back a little.

In a couple of months I plan on fire forming some 7.62x54R brass for my Mosin reloading. I've had good luck fire forming straight wall cases with light loads and paper wadding but I guess based on this thread that I'll at least use some small size cast bullets so I build up enough pressure.

 

[gamestalker]

I was actually thinking of trying annealing as well FROGO207. I'm leaning toward the brass being harder than RP or Win., more spring back maybe? We'll see.

The one thing that bothers me about the theory as being low pressure caused by body expansion, is two elements, well 3 actually.

First off, I seat up pretty close to the lands, within .010" off. I've confirmed over the years that at this charge pressures are noticeably higher than at shorter oal's. This is a long time established load of mine.

Second would be the charge I'm using (post #1), it isn't exactly on the light side for H4831. And although H4831 is rather slow burning, I would think that would have more of a gradual pressure curve and would sustain peak for a fraction longer. Just speculation at this point, I really don't know.

Then there is the brass involved. There were 3 different head stamps used with this load which was assembled at the same time, same lot powder, with the precise same oal's, components, and powder charges, and fired from the same action, and the firearm was properly cleaned following each 20 round session. So if the low pressure condition was the culprit here, I would think I would have at least one piece of RP or Win brass exhibiting a similar anomaly.

The closest any of the other pieces of brass came to that much of a variance of the FC brass, was .002". They all did what they are supposed to do, what they always do when formed with this load, which is between zero HS, which was almost a snug fit too, to as much as .002". I'm a meticulous reloader, so this criteria is from well tested, documented load development.

GS

 

[Walkalong]

Use a full load to fire form, not a light one, although that looks to be middle way as far as powder charge.

Anyway, I would do it gain with 5 cases and a full charge. See what happens.

Another trick is to load long with heavy neck tension, jamming the bullet into the lands, to try to hold the case back against the breech face.

Light charges won't work well to fire form cases.