Gunsmiths : Short Fire Forming

[Wolfdog91]

Aight question for the gunsmiths, since this has be curious down to a mechanical level. Gotta gun that fire forming brass short not long.
Shot 14 red of FACTORY PMC .223 FMJ though it too em measured base to shoulder and averaged out a fire form to 1.450 of cool. Well measured the remaining comple rounds and their 1.454 "....what could cause this from a gunsmithing perspective? Possibly a loose chamber making the brass expand outwards to fit the chamber this bringing the shoulder in orrr what ?
Gun is an H&rR Ultra Varmint BREAK ACTION ( not bolt)
And I know I'm over thinking this but that part of what makes this enjoyable for melol

[megasupermagnum]

First I have to ask, are you measuring these with the primers still in them?

[Wolfdog91]

First I have to ask, are you measuring these with the primers still in them?

Fired formed : without. Decapoed them with a Lee universal de capper. Measured before anneling. Factory ammo, weeellllll didn't have much of a choice lol but primers seem to be under flush

[megasupermagnum]

It's hard to say with factory ammo, but the problem is most likely not the rifle. Some variation is normal, as I'm sure you know. .004" is a bit much, but not unheard of. What it boils down to is that brass doesn't fully expand to fill the chamber. If it did, that's what a sticky case is. That ammo is either lower pressure, so it isn't expanding very well, or it has poor annealing, which is allowing varying amounts of expansion.

You aren't overthinking it. The issue is accuracy is a rabbit hole. Headspace is a tough one with normal brass, and normal dies.

[Greg S]

You won't get a true readings until the shoulder is bumped back .001-2, reloaded and shot a second or third time.

The reasoning behind this is that the shoulder on factory brass is generally on average SAMMI minus .004 and occasionally as much as .006.

If you fire 5-10 rounds twice with minimal resizing you should have a real good idea.

I found this out trying to adjust a sizing die for my SCAR in 7.62 x 51. On the first firing of M118 match, my cases came out SAMMI +.002-.005 at the datum mark on the shoulder of my case gauge. I bumped the shoulder to .003 and checked for proper function, reloaded and shot the 20 ctgs again. This time, they came out +.004-.006 and one at +.007 Hmm..

I then looked and measured a new box of M118 in my chamber gauge and found my factory ammo averaged SAMMI -.004 average. So, my cases on initial firing were growing an 'average of -.004 to + .002-.005, minimal shoulder bump on first reload to +.003 and on second firing and upon firing, they came out at +.006-7. I now FL size for the SCAR to SAMMI plus .005 for reloading.

Since this light bulb moment, I have checked all factory new bottleneck rifle brass and found they one out of the bag at an average of -.004.

Another interesting this I discovered was that some M118 brass starts to spring back when resizing after the second firing and a majority does after the third firing. Food for thought.

[Wolfdog91]

You won't get a true readings until the shoulder is bumped back .001-2, reloaded and shot a second or third time.

The reasoning behind this is that the shoulder on factory brass is generally on average SAMMI minus .004 and occasionally as much as .006.

If you fire 5-10 rounds twice with minimal resizing you should have a real good idea.

I found this out trying to adjust a sizing die for my SCAR in 7.62 x 51. On the first firing of M118 match, my cases came out SAMMI +.002-.005 at the datum mark on the shoulder of my case gauge. I bumped the shoulder to .003 and checked for proper function, reloaded and shot the 20 ctgs again. This time, they came out +.004-.006 and one at +.007 Hmm..

I then looked and measured a new box of M118 in my chamber gauge and found my factory ammo averaged SAMMI -.004 average. So, my cases on initial firing were growing an 'average of -.004 to + .002-.005, minimal shoulder bump on first reload to +.003 and on second firing and upon firing, they came out at +.006-7. I now FL size for the SCAR to SAMMI plus .005 for reloading.

Since this light bulb moment, I have checked all factory new bottleneck rifle brass and found they one out of the bag at an average of -.004.

Another interesting this I discovered was that some M118 brass starts to spring back when resizing after the second firing and a majority does after the third firing. Food for thought.

Well that's actually the funny thing umm I can't bump these shoulders lol. The gun is fire forming .223 factory ammo DOWN to 1.450" base to shoulder which just doing though range brass and brass from my AR is nuts . Everything's elese is fire forming down two 1.454 max. And with my dies set up I can only bump down to 1.451. I am considering taking one of my extra dies and lapping it down a few though with some 500 and 1000 grit sand paper and a flat surface but not sure . Sooo. Yeah this is a quirky little deal I did do a test though. Took 4 cases labeled them messed them before annealing after Anneling. Nothing would chamber ,just get stuck.Ok . Full length resized in roughly.004 incriminates of shoulder bump and after 1.458" the gun would not easily lock the breech of I guess imma call that the outer limit. It it's still crazy to me it'll take anything from 1.450" to 1.458" I don't think that normal lol. But I'll try what your talking about. I mean all I'm really doing right now any way is sizing the sides down so we'll see if there's any movement after the second firing

[Winger Ed.]

And I know I'm over thinking this but that part of what makes this enjoyable for melol

That's good to hear.

After awhile, I quit over thinking and splitting hairs on most of this stuff.

As far as case lengths go, I trim down to listed 'trim to' length after sizing, and call it good until they go over it.
Also keeping batches of 100 (for rifles) together for consistency, has worked out real well.

[megasupermagnum]

It all sounds good to me. I'm sorry if I missed it what a new case measures, but fired cases can have increased headspace from new. I wish I could give a more detailed answer than, it is the way it is. What's even odder is that you can size a case and get less headspace. For example, if your fired case measures 1.450" to the datum line, you can size that case and get that to 1.452", maybe more.

Then once you figure that out, you will find your sizer might not produce perfect concentric case necks, and your expander ball will produce some headspace variations. It really boggles the mind to think of the precision it takes to get to where rifle shooters casually talk about "bumping a shoulder back .002"".

[Winger Ed.]

It really boggles the mind to think of the precision it takes to get to where rifle shooters casually talk about "bumping a shoulder back .002"".

Me too. But it depends on at what level of the game you want to play at.
Getting a 2" group can be pretty easy, a 1" gets harder and requires more effort,
cloverleafs take more effort, and so on.

People who are in shooting competitions, and play to win, will go all out- down the last little ga-zillionsth of an inch.
It's the same for them as car racers where the difference in 1st and 2nd place after hundreds of miles might be half a car length.

[M-Tecs]

For the .223 the minimum headspace is 1.4636" and the maximum headspace is 1.4736". The RCBS Precision Mic and the Mo Defina type gauges are direct measurement meaning they read the actual headspace. The Compass Lake/Hornady Lock-N-Load Headspace Gauge is a comparator and they do not read direct measurement unless it is used in conjunction with and actual headspace gauge.

What are you using to get your measurements?

How repeatable is your method?

You are stating that your case headspace is .004” LESS after firing?

If that is correct brass has to move somewhere so I would think your assessment of radial expansion is the most viable explain. As the case body OD gets bigger the case will get shorter.

[Wilderness]

Aight question for the gunsmiths, since this has be curious down to a mechanical level. Gotta gun that fire forming brass short not long.
Shot 14 red of FACTORY PMC .223 FMJ though it too em measured base to shoulder and averaged out a fire form to 1.450 of cool. Well measured the remaining comple rounds and their 1.454 "....what could cause this from a gunsmithing perspective? Possibly a loose chamber making the brass expand outwards to fit the chamber this bringing the shoulder in orrr what ?
Gun is an H&rR Ultra Varmint BREAK ACTION ( not bolt)
And I know I'm over thinking this but that part of what makes this enjoyable for melol

I think what Wolf is saying is that he has brass that is 1.454" head to shoulder (new ammo). He and we would expect that firing it would take up any spare chamber space, resulting in a fired case measuring MORE THAN 1.454" head to shoulder. Instead, the cases are coming out SHORTER at 1.450".

If you size down the body of a case, e.g. make a .32-40 out of a .30-30, the new case comes out longer. Likewise, if you expand the body of a case, e.g. expand .30-30 to .375, it comes out shorter. In the examples we are talking .025" or more both ways. Change diameter and you change length.

In firing the cartridge, opposing influences will be at work on the head to shoulder measurement. The "blowing out" influence will push the shoulder forward by stretching brass, and the "expanding body" influence will pull it back. The net effect then depends on which is the greater. Resizing the body of the case will reverse the "expanding body" influence, depending on the degree of resizing. Head to shoulder measurement may then go back to 1.454", or perhaps further if the die is set to allow a longer measurement.

The "blowing out" influence can be quite weak. I have experience of blowing out .30-30 brass to headspace on the shoulder for a rifle with spare headspace. Mostly I hold the case back with headspace circlips under the rim. I am looking for .015" to .020" of shoulder advance. With full power ammo, some cases get there in one shot, a lot need two, and some need three.

Another influence on head to shoulder outcome is the combined effect of firing pin strike and primer backing out on firing. I have undone my good work on blown out brass by using the cases for very light loads - one shot and the case is back to headspacing on the rim.

In the more general sense, if we were to compare only the new case and the resized case, we would probbly not see the effect that Wolf has picked up. Resizing would have restored the lost length. What Wolf has noticed may be more common then we think, and we do not see it only because we do not look.

[Wolfdog91]

For the .223 the minimum headspace is 1.4636" and the maximum headspace is 1.4736". The RCBS Precision Mic and the Mo Defina type gauges are direct measurement meaning they read the actual headspace. The Compass Lake/Hornady Lock-N-Load Headspace Gauge is a comparator and they do not read direct measurement unless it is used in conjunction with and actual headspace gauge.

What are you using to get your measurements?

How repeatable is your method?

You are stating that your case headspace is .004” LESS after firing?

If that is correct brass has to move somewhere so I would think your assessment of radial expansion is the most viable explain. As the case body OD gets bigger the case will get shorter.

https://i.postimg.cc/0rGNZWqd/FB-IMG-1644008566567.jpg (https://postimg.cc/0rGNZWqd)

https://i.postimg.cc/64h533h1/FB-IMG-1644008572476.jpg (https://postimg.cc/64h533h1)

https://i.postimg.cc/MvWpL76k/FB-IMG-1644022431041.jpg (https://postimg.cc/MvWpL76k)

[jsizemore]

It's the springy break action.