Case trimming difference?

[AJC]

If I have a case I want trimmed back .010 I am pretty sure the same .010 will be removed whether I index that .010 off the shoulder or the case head

This is factually true and not something i would ever dispute. If using an OAL trimmer you would trim that way. I dont think a trimmer that uses the shoulder would. I believe that it will trim to a consistent neck length and give you small case to case oal differences. So which would be more important, creating consistent OAL or consistant neck length. Knowing that you may have different release pressures if the neck length is different. The bullet seaters are set to OAL so the position of the bullet in the case would be different. Each way seems to focus on a goal, leaving another parameter to vary. Speaking simply "i am trying to identify what the most critical things to control are". Really trying to reach understanding, and really appreciate all of your efforts.

 

[mikecr]

AJC, the variances you're concerned with are not a given compromise.
With a logical plan, each and every part of this can be controlled.

 

[Bart B.]

When cases are against their headspacing stop, their head is off the breech face an amount called head clearance. Often confused with head space.

I've often thought SAAMI should name the case mouth's clearance to the angled chamber mouth a similar term. A reference diameter on that angled shoulder may be used.

 

[Bart B.]

So which would be more important, creating consistent OAL or consistant neck length. Knowing that you may have different release pressures if the neck length is different.

If the bullet body contact length average .200" +/- .002" and neck surface tension and friction per .001" of neck contact is the same, bullet release force spread may be 2%. 1.6 ounce is 2% of 5 pounds of release force.

Very small compared to the force needed to push the bullet fully into the bore.

 

[JimSC]

Except the case mouth won't always be at the same place in the chamber neck upon firing when there's any spread in case headspace across all cases.
,.........
Does any bullet seating die index off the case shoulder?

to put it in it's simplest terms. If a case is trimmed .010 shorter the brass will always come from the mouth of the case and the neck will be .010 shorter than it was before trimming. Neither the shoulder or the case head moves anywhere. The only thing changing is the length of the case neck and the only thing case neck length affects is how much of the bullet is being grabbed when seated. Neck length will not affect the seated bullet length OAL, only bullet uniformity will affect seating depth uniformity unless the seating depth adjustment has changed

 

[Bart B.]

Put another way....

The critical dimension for consistent bullet jump to the lands at the throat (leade) on cases headspacing on their shoulder is from case shoulder to bullet diameter on its ogive.

That's best attained when the seating die's seater stem only touches the bullet diameter where the lands do and the reference is the case shoulder. Case neck length nor bullet seated depth in case neck doesn't matter, a few thousandths spread is okay.

Seat bullets shallower as the throat erodes down the bore.

 

[Bart B.]

This picture is the best way to explain lots of things. Kudos to Uncle Ed for posting it.

It shows case shoulder contact with chamber shoulder next to the case neck.

If the case shoulder angle was enough smaller, it would contact the chamber shoulder next to its body. Such cases won't be driven as far into the chamber from firing pin impact before firing.

Both instances could have case headspace the same at the standard reference diameter; .400" on 308 Win stuff, for example.

Does your sizing die make case shoulder angles the same as the chamber shoulder?

 

[stifffingers]

This picture is the best way to explain lots of things. Kudos to Uncle Ed for posting it.

It shows case shoulder contact with chamber shoulder next to the case neck.

If the case shoulder angle was enough smaller, it would contact the chamber shoulder next to its body. Such cases won't be driven as far into the chamber from firing pin impact before firing.

Both instances could have case headspace the same at the standard reference diameter; .400" on 308 Win stuff, for example.

Does your sizing die make case shoulder angles the same as the chamber shoulder?

At the beginning of the thread I thought the question was related to trimming the case length so a uniform length could be utilized to set the mandrel for uniform case turning. One way I have used is to fire form the cases a couple of times in my no turn neck barrel. Then trim to length in my Wilson trimmer and move on to setting up my turner. Most of us have old barrels we can have the necks opened up for fire forming.

 

[AJC]

At the beginning of the thread I thought the question was related to trimming the case length so a uniform length could be utilized to set the mandrel for uniform case turning. One way I have used is to fire form the cases a couple of times in my no turn neck barrel. Then trim to length in my Wilson trimmer and move on to setting up my turner. Most of us have old barrels we can have the necks opened up for fire forming.

It was exactly about that. Using a trimmer that indexed of the shoulder I thought would be easier, but as with many things there are a ton of ways to accomplish a task and dozens of opinions on each way. I ask a lot of questions I get from reading trying to understand the "why" or the thought process. I have all oal type trimmers now and was trying to figure out if it was really worth buying one of the other style.