Why I Have to trim only once?

[gifbohane]

So, I pick up 100 empty cases on the range . Take them home and trim etc.. I load and fire them and afterwards measure them with a view toward trimming again. They are 99% measured a 1.751, about where I trimmed them to in the first case. Obviously I do not need to trim them again before firing.

Why? I know that they are fireformed to my chamber, but do they not lengthen after each firing?

Thoughts please?

 

[cfullgraf]

What cartridge?

Bottleneck cases when full length sized are a bit smaller than the chamber. When fired again, the case will expand to the chamber. When resized again, the brass will flow during the sizing process, most likely in the length.

The amount is frequently small and you may get several firings before the case needs trimming again.

RCBS has a sizing die, I forget the name, that is designed to minimize/eliminate trimming. There is a process for the initial trim but then the case does not need trimming. I have no experience with the die.

If you neck size, cases need trimming less. But, neck sizing is not to be done with auto loading rifles and it is falling out of favor for bolt rifles.

 

[jonas66]

Yup.

When I shoot my .223, or 'loan' ammo to friends, I make sure to get the cases back, because I can usually get at least two or three reloads out of each case before needing to trim.

Cuts back on the prep work.

It helps that I usually only load light target ammo, seems when I load heavier charges then the case needs to be trimmed after that firing.

 

[gifbohane]

Sorry. Forgot to mention I am talking about 223.

 

[higgite]

Did you resize them before measuring them after you fired them?

 

[Phantom 309]

@cfullgraf is correct, measure for case length growth after resizing, not just firing.

Also, the RCBS die mentioned is the X-die. There is a "stop step" above the expander ball that the case mouth runs into during sizing to prevent growth. You initially size the cases with the stop mandrel threaded out, then trim the cases a bit shorter than normal (example: 1.740" for .223). Then a trimmed case is run up into the die and the stop mandrel is threaded down to touch the case mouth. The stop mandrel is then left in that position for all subsequent resizing on that batch of brass.

It does work pretty well and was the first die set I bought when I started reloading and it seemed like a good way to cut down on trimming. After some experience I realize that I could get by without it for the volumes I load, but it's what I have and works just fine. I keep a case that is trimmed to exactly 1.740" and labeled "X-die setup" in my toolbox to set the mandrel back to the same position each time.

The x-die info doesn't exactly address your question, but since it was mentioned I figured that I would share my experience with it. If you don't like trimming it may just be the bees knees.

 

[AJC1]

The closer you size to the size of the chamber the less brass movement you get. If you have a small die and a large chamber you will get maximum growth, and shortest life.

 

[PapaG]

Measure after full length sizing. They won't be the same.

 

[gifbohane]

On taking them home from the range after they have been fired by me for the second time, When I measure them after firing and before Sizing/depriming they measure 1.744. I then size/deprime and they stretch to 1.751 which is pretty much where I trimmed them to in the first place.

Will they stretch (elongate) after 3 firings, 4, 6 or never again?

 

[Bfh_auto]

Measure after full length sizing. They won't be the same.

I agree. I trim after sizing. Not before.
I do trim every time because I do my prep work in down time. Like when my boys are watching Thomas the train or Cars.

 

[FROGO207]

It depends on how sloppy your chamber is and how much pressure you use when shooting them. A lesser charge will stretch them less. You will have to fire some and measure them each time, then you will know roughly how many times you can go before trimming again,

 

[TEXASJD]

I don’t trim every time even in gas guns I trim back about ten thousands when I do first work up on brass, flash holes, uniforms pockets it will be several reloads before you have to do it again even FL sizing. In bolt guns it may never need trimming again if you are neck sizing.

 

[Blue68f100]

On taking them home from the range after they have been fired by me for the second time, When I measure them after firing and before Sizing/depriming they measure 1.744. I then size/deprime and they stretch to 1.751 which is pretty much where I trimmed them to in the first place.

Will they stretch (elongate) after 3 firings, 4, 6 or never again?

Is this a bolt gun or AR?

AR's can be hard on brass, if it's not tuned properly. Longer barrel (rifle length gas system) AR's are usually a little easier on the brass since the port is at a location where the pressure is lower. Length after the gas port has a big play on function. This what set the time required needed to operate the bolt.

Your brass should stretch a little each time. How much depends on the load and chamber.The RCBS-X die has a shoulder that holds the case mouth in place to minimize stretch during sizing. But with this die your instructed to trim your brass 0.010" short to start with.

I have a Custom 224V AR that I did all the machining on. I have yet to trim any brass that's run through it. Have over 10 reload cycles on the brass and a few are getting close but I expect the primer pockets to fail first.

 

[gifbohane]

it is an AR

 

[cfullgraf]

it is an AR

I find cases need trimming more frequently when fired from an AR and probably most semi-auto rifles. I figure cases are ejected from the chamber before it has time to shed heat into the chamber wall like it would in a bolt rifle. (quickly grab a fire case ejected from an AR and another ejected as fast as possible from a bolt rifle and see which one is hotter, the AR case).

I feel the case ejected from the AR continues to expand once outside the chamber and then does not contract in size as it air cools as much as a case restrained by the bolt rifle's chamber.

Then, when resized, the AR case needs to be sized more that causes the brass to flow more making the sized case longer.

To add to my first post and as others have said, case length measurement needs to be done after sizing.

 

[gifbohane]

case length measurement needs to be done after sizing.

Yes I do the same.

 

[jmorris]

Why? I know that they are fireformed to my chamber, but do they not lengthen after each firing?

Because you have your sizing die set right for the chamber. If you size them more, they will “blow out” to match the chamber, then when sized again the extra brass will turn onto a longer neck, splitting the case in to 2 parts once it gets thin enough.