Brass expansion

[Area Man]

As I was driving home today, I was thinking about brass expansion during and after firing.

We know that during firing, the brass expands to fit the chamber. It then contracts or springs back slightly allowing the extraction of the brass.

And so it seems that the brass would remain slightly smaller than the chamber, no matter how many times it is fired. But we know that if only neck-sized, the dimension of the brass from the head to the shoulder gets too long after a few firings and the shoulder must be bumped back to allow the case to be easily inserted into the chamber.

I wonder how that happens.

 

[tonedaddy]

I have fired some brass 20 times and never had bump the shoulder back. Sized about 1/3 way down the neck with a bushing die. I do not shoot hot loads though.

 

[CatShooter]

I also have not had that problem... I neck size unless I get brass from another rifle.

 

[JohnHenry]

Area Man, The cause for the need to bump the shoulder that one experiences is "bolt flex". The following link to an article by Dan Lilja may be of interest. http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm

 

[Area Man]

Thanks John Henry for the interesting link.

Cat: I neck size only, but after a number of firings, I've had rounds that needed the shoulder bumped to get the bolt to close without a lot of effort.

Haven't I read advocates of FL sizing every time, or bumping the shoulder 2 or 3 thou every loading on this site?

 

[CatShooter]

Thanks John Henry for the interesting link.

Cat: I neck size only, but after a number of firings, I've had rounds that needed the shoulder bumped to get the bolt to close without a lot of effort.

Haven't I read advocates of FL sizing every time, or bumping the shoulder 2 or 3 thou every loading on this site?

I once, while loading at the range, loaded one 308 case 28 times, with no problems, and no need to "bump"...

I bought a 22-250, 40XB Single Shot in 1975 and got 400 Win cases with it. Shot the first barrel out (neck sized only) rebarreled it, FL sized the cases just enough that they were a tight crush fit for the first firing, and then I neck sized them and shot that barrel out... did the same for a total of 5 barrels, and annealed every 3 to 5 firings.

I finely got tired of the 22-250, and had it rebarreled to .244 Rem (6mm Rem for the young kids). The cases had been fired over 45 times each, and FL sized 4 times when I tossed them in the trash... they were still good.

 

[Area Man]

and JohnHenry: What was really scary was that I could actually follow the GWBasic code in the linked article.

 

[hogan]

Pressures and shoulder design has a lot to do with it; that is your question about why brass flows forward. There's also physics, where else can it flow in an otherwise closed environ?

Shoot cast bullets in a centerfire rifle at under 2000fps, probably never need to FL size your brass or trim.

Guys who load to the max also tend to experience loose primer pockets, that being the other variable environ at the other end of the case. Takes a couple repeated firings, but the boltface is typically a bit over .473/.533 or standard ctg head dimension since not all brass is uniform.

Look into cast bullet benchrest for the details as to perpetual brass longevity.

If you're wearing your brass out with expansion and trimming; think what you're doing to your barrel...

 

[jonbearman]

The shoulder of your brass is way thinner than the base and it grows the length of the braas as physics says that the path of least resistance is where pressure goes and this case like water will take the path of least resistance period.

 

[Area Man]

My question was how the brass grows to a size larger than the chamber that contains it. I guess the bolt compression is the answer.

 

[Area Man]

If you're wearing your brass out with expansion and trimming; think what you're doing to your barrel...

I never load hot. I never trim unless it's just to make the cases uniform again. I do find that shoulders need bumping occasionally, although I usually only neck size.

 

[LHSmith]

Haven't I read advocates of FL sizing every time, or bumping the shoulder 2 or 3 thou every loading on this site?

Generally, short range Benchrest competitors FLS EVERYTIME. Their sizer die is usually cut with a sizer reamer that very closely matches the chambering reamer. They usually set the die to bump 0.001". Most feel they get best aggregate accuracy when loading in the upper tier of the load window- Remember they have a finite time in which to shoot , and invariably one will be confronted with multiple conditions, so they have to have a load or loads that work in real conditions..... Not still air. Secondly, a smooth cycling of the bolt is crucial to maintain a rapid return to battery in order to get shots downrange to catch the desired condition.
Not all cases will grow, stretch, or expand the same....not even if they were from the same factory box, subjected to the same loads or fired the same number of times. This is why some in BR either shim the die or use a ratchet device to fine tune shoulder bump on a case that has more resistance when chambering.
Typically most group shooters will use the same 10-15 cases for a 2-day agg.

 

[Area Man]

Generally, short range Benchrest competitors FLS EVERYTIME.

That's exactly what I was wondering about. Thank you for the information, and most importantly, thank you for actually reading the post you were responding to.

 

[LHSmith]

The caveat is : the dies are MATCHED to the custom chamber.....very little sizing is taking place. With most factory guns and over the shelf FL dies you are pretty much fireforming once they are FLS.

 

[Area Man]

Is there ever any advantage except for speed of loading to resize the case body any more than is necessary to get the dang thing into the chamber and close the bolt?

 

[LHSmith]

If one does not lube the bolt lugs and cocking cam, it could accelerate wear on these parts. Also, some claim they get their best accuracy only when FLS due to case volume dynamics.

 

[M-61]

Is there ever any advantage except for speed of loading to resize the case body any more than is necessary to get the dang thing into the chamber and close the bolt?

Not in my opinion. In fact I have felt that FL sizing started you back to where you were the first time the case was fired. The case is no longer fire formed to any chamber. NS enough to hold the bullet has worked for me, albeit I am loading for accuracy and not a speed contest so I would say in general my loads are not hot. ( i.e. my .222 I lost count how many times some of the brass has been reloaded...over 20...and neck sized only). My response is related to bolt actions nothing else.

 

[Tim Singleton]

I know after several firings my ppc brass gets harder. The brass work hardens I'm not sure if this contributes to less spring back after numerous firings. That lead to the need to bump the shoulder back.
Just another thought

 

[CatShooter]

I threw away a bunch of 22-250 Winchester cases that I bought in 1975, and I shot out 5 barrels with them. They had been fired about 45 times each... and were still usable.

The only reason I threw them away was because Lapua had come out with that beautiful brass. I fL sized then four times - when barrels were changed.

 

[hogan]

Have heard the "gonna have my smith make me a custom sizer with the same reamer used to cut my barrel" for years, here.

Not withstanding that doing so produces a sizer die that is over dimension. Dave Kiff will sell you a set of reamers if your goal is to have a custom sizer made, from your excess barrel stock or steel of your choosing, but how many guys even spring for their own reamer when having a barrel done? Probably not too many.

Shooting has so many variables, guys who get a good-shooting rifle get almost obsessive/compulsive or superstitious trying to duplicate the dynamics. Doesn't work all that often; if it did every competitor would be shooting at the level of Tony Boyer or David Tubb.

Work-hardened brass is a result of pressure, or maybe FL sizing .0004 or more each time to allow for a semi-auto where concern is to avoid slamfire.

My perspective on a bolt-action rifle that will only be fired with ammunition I've assembled is that The Key to precise, accurate results is a tight oal chamber; something like .0005 over go-gauge or reamed to .0005 over the dimension of virgin brass. Got 250 cases? Probably never need to buy more if you keep that initial firing to a .0005" stretch.

How many even have a Wilson chamber gauge or RCBS Precision Mic? Look at what your virgin brass measure before and after firing.

Bolt lug and cam lubrication have nothing to do with brass stretch, unless your lugs don't even engage the receiver recesses. One thing that will affect accuracy is a boltface that is several thousandths off-true. One thousandth is no big deal, but 3 or more and you have a problem or series thereof.

Short oal chamber eliminates about all brass problems if all other variables are within reasonable tolerance. Of course, that means different things to different people....