Brass not expanding after firing?

[guy_with_boolits]

Newbie here
Loading 223 for an AR

Did two loads with the intention of fire-forming them so I could then size them down to a much smaller shoulder bump.

To my surprise, after I fired them, about half seemed to have barely any shoulder movement from fully sized!

The other half seemed to have expanded fully with a shoulder to head increase of about 14 thou (thats right about where this rifles shoulder is)

The two loads did differ in bullet seating depth, but were otherwise identical. It did appear the longer seating depth (with a bullet jump of only .020" to the lands), was higher pressure because the primers seemed much more flattened.

Unfortunately I did not keep track of which cases came from which load, as I was shooting them both at the same time and didnt anticipate this difference.

So my question is, can reasonable loads that act totally normal, not have enough pressure to fire-form the 223 case up to the shoulder from fully-sized? (fully sized being LEE die setup per instructions)

Both loads shot 1.5 moa and cycled the AR just fine, so they are not super low. They are not super hot either (21gn RL7 w/ 55gn FMJBT)

[littlejack]

Are you checking your fired cases with the primers punched out? If you try to check case length with the spent primers in, you will get measurements all over the place.
If some of the rounds have bullets seated deeper, the pressure will rise. If it rises enough, the case will get blown out tighter in the chamber. That is why when firing a load that is way over pressure, you get hard extraction of the case. You've actually stretched the chamber. When the chamber springs back, it grips the case, as the case did not have enough spring back to have some clearance in the chamber. If the bullet is seated out to long, and bumps the lands, again, you will have higher pressure.

[guy_with_boolits]

Are you checking your fired cases with the primers punched out? If you try to check case length with the spent primers in, you will get measurements all over the place.
If some of the rounds have bullets seated deeper, the pressure will rise. If it rises enough, the case will get blown out tighter in the chamber. That is why when firing a load that is way over pressure, you get hard extraction of the case. You've actually stretched the chamber. When the chamber springs back, it grips the case, as the case did not have enough spring back to have some clearance in the chamber. If the bullet is seated out to long, and bumps the lands, again, you will have higher pressure.

Its not the primers sticking out because I'm using an instrument that uses a shell holder to hold the case, so the primer is taken out of the measurement.

It makes sense to me the loads have different pressure, and also that pressure would impact how much the case stretches. However, if the load has enough pressure to cycle the AR and get up to 2800fps..isnt that enough to fully stretch the case? Thats what I dont know here. How much pressure is required to stretch a fully sized (SAAMI shoulder to head distance) 223 case all the way to a 5.56 chamber shoulder (about 14 thou longer than SAAMI in my rifle)

[TimD]

It is not surprising that the higher pressure load stretched your cases more, or that the lower pressure load cycled your action.
Small changes in seating depth can result in sudden large increases in pressure.
You should always reduce your load when changing seating depths, then work back up while looking for signs of overpressure.

[littlejack]

Ok, sounds like the primer protrusion is out of the equation.
I'm not sure I understand your last remark? Are you saying that your chamber is .014 longer than SAAMI specs? For all practical purposes, the 223 and 5.56 chambers are identical. The only real difference, is that the 5.56 chamber has a longer throat/leade, to allow for the higher working pressure of the 5.56 and it's mil-spec loadings. This is one of the reasons for overpressure when firing 5.56 in a 223 chamber. Correct me if I'm misunderstanding you.

[guy_with_boolits]

It is not surprising that the higher pressure load stretched your cases more, or that the lower pressure load cycled your action.
Small changes in seating depth can result in sudden large increases in pressure.
You should always reduce your load when changing seating depths, then work back up while looking for signs of overpressure.

ok thanks. I'll just mark this up as a lower but still perfectly functional load not stretching the brass enough for purposes of using it to adjust shoulder bump. I had no actual reason to expect all functional loads to stretch brass so I suppose this should not be a surprise.

now the question is: do I even need to control shoulder bump if the brass isnt expanding more than a couple thou?

[littlejack]

I normally bump the shoulder back on my cases .004 or there bouts. Understanding that unless one is firing way hot loads, the spring back from the fired case, is already shorter than the chamber. My cases vary in length after firing. So, if I shoot (pardon the punn) for .004 for bump back, the shorter cases will still chamber in my rifle. That's why you can get away with neck sizing only for a few reloads, before the case stretches to far and needs partial resizing.

[guy_with_boolits]

Ok, sounds like the primer protrusion is out of the equation.
I'm not sure I understand your last remark? Are you saying that your chamber is .014 longer than SAAMI specs? For all practical purposes, the 223 and 5.56 chambers are identical. The only real difference, is that the 5.56 chamber has a longer throat/leade, to allow for the higher working pressure of the 5.56 and it's mil-spec loadings. This is one of the reasons for overpressure when firing 5.56 in a 223 chamber. Correct me if I'm misunderstanding you.

Lets say "fully sized" is where I had the Lee 223 FL sizing die adjusted all the way to the shell holder + 1/3 turn.

Then lets say "fire formed" is where I took one of those and fired it at high enough pressure to where it fire formed to the chamber as much as possible.

Now if I measure shoulder to head stamp between those two cases and compare, the fire formed one is about .014" longer.

So in theory if I wanted .004" shoulder bump I'd adjust the Lee die away from the shell holder until the cases it produced were only .010" longer than the "fully sized" case.

[littlejack]

If you want .004 clearance to the shoulder. Adjust your sizer die to just bump the shoulder back .004 shorter on the fired case than what it is after firing.Theoretically, you will probably have more than .004 because the fired round has already sprung back some (very little) upon firing.

[littlejack]

By the way, if your fired case IS .014 longer after firing, you could have a headspace problem. Are your primers protruding, or is there a fracture line around the case above the case head?

[charlie b]

FWIW, when I was taught about fire forming one of the main things to do was to seat the bullet out enough to engage the lands. That would make sure the case is held against the bolt face when it is fired. If not you are relying on the extractor to hold the case when the firing pin hits the primer.

I wonder, are the rims of all of your cases the same thickness?

Sent from my SM-P580 using Tapatalk

[243winxb]

By the way, if your fired case IS .014 longer after firing, you could have a headspace problem. Are your primers protruding, or is there a fracture line around the case above the case head?

Good things to consider., posted by littlejack.


The .014" stretch can cause case separations. Parts guns where the bolt & barrel are NOT of the same manufacture can have excessively large chambers.

Do get the correct headspace gauges and check chamber Head Space .

[littlejack]

Charlie b
Seating the bullet out far enough to engage the rifling, is a good point.

[TimD]

I agree with littlejack and 243winxb.

The difference between minimum and maximum headspace for 223 Rem is 0.010
You may not be able to reload those stretched cases safely.
You should check your headspace.

[TimD]

FWIW, when I was taught about fire forming one of the main things to do was to seat the bullet out enough to engage the lands. That would make sure the case is held against the bolt face when it is fired. If not you are relying on the extractor to hold the case when the firing pin hits the primer.

I wonder, are the rims of all of your cases the same thickness?

Sent from my SM-P580 using Tapatalk

Good point. Do you use a minimum load when doing this?

[popper]

barely any shoulder movement from fully sized!
1) Were all cases the same head stamp? Different manuf. have different toughness. Stretch will be different.
2) case is supposed to HS on the shoulder. However, sizing die (LEE) and barrel 'shoulder' are different. So if using a case gauge to measure fired cases, you get different readings. Check shoulder 'blow out' - fired case with sharpie painted on the entire shoulder. Chamber it. If just a thin line of contact, shoulder is NOT blown forward. Generally, to fire form a case it needs to be annealed first. Firing work hardens the brass so it doesn't move easily.
3) AR chamber does NOT support the rear of the case - measure that OD to check for over pressure. Higher pressure loads will tend to flatten primer as the primer is forced against the bolt. Ejector swipes/extractor tears are bad gas setup, deep dimples are high pressure. Pressure is still too high to allow the case to rotate (with the bolt), bent/torn rims from case still jammed against the chamber.
One question - why fire form 223 brass? :veryconfu If your rifle HS is improper - you control it with the sizer die. Try something different. Adjust the die to partial neck size a fired case. You will see the 'end' of the sizing, stop before you get to the shoulder. Chamber the rnd. If it fits, good to go. Repeat as necessary. Load and fire that case (or several) Mark neck/ shoulder with sharpie and size again. See where the die touched the case.