Case trim question

N

nonliberal

Guest
I noticed that I was getting inconsistant brass for my 6br after trimming by as much as .005 and finally figured out why.

The rim thickness on the Lapua brass is inconsistant by as much as .005, it ranges from .045-.050. My case trimmer uses a shelholder to hold the brass so it is referenced off the inside of the rim and not the headstamp.

My question is which measurement is more important? OAL from the headstamp to mouth or from the inside of the rim to mouth which means the necks would be the same?

Or should I buy more brass and sort it by rim thickness which obviously is the more expensive option?

Thanks.
 
Why would inside of rim give a better measure?

I would think on the 6br your measurements shoiuld start from the headstamp.

Jim
 
Eliminate the problem

I would suggest a Wilson. It eliminates the problems induced by other types of case trimmers.:)
Centerfire
 
Ditto what Centerfire said. For live varmint shooting and other non-competitive use just about any trimmer will work, for precision work get a Wilson.
 
janderson0 said:
Why would inside of rim give a better measure?

Jim
Click to expand...

Well I don't know that it does or not but I can tell you what I was thinking even though it may be wrong.

I was thinking if the discrepancy is in the rim, it might not be a good idea to compensate for that by taking material off the necks, which would make all the necks uneven from the get go.

I figured the thicker rim might make the headspace tighter but since the shoulder stops the case from going deeper into the chamber, the necks trimmed to length based off the headstamp would still be uneven by the amount of the discrepancy in the rim. I thought by bypassing the measurement of the rim all together the necks would be the same after that.

I dont know the answer so Im not arguing the matter, just giving my way of thinking so I can be corrected if Im wrong.

I will look into the Wison trimmer.
 
Wow

You lost me some where in there.
All criticle dimensions, as far as your chamber is concerned, originate at the bolt face. So that should be the starting point for overall length, and for headspace. If you look at your reamer print, you will see that all dimensions regarding lengths originate at, or are backfigured from, the bolt face.
I also think you need to get another brand of case trimmer, one that uses the head of the case as the positive stop.
I am curious. What are you using?? And what were they thinking?????.......jackie
 
Wilson trimmers, especially with the carbide cutter, are dead nuts accurate, and as fast, if not faster, as most other trimmers. I'd think that the Gracey could beat one on speed, but it'd still be fairly close.
 
Its a Lyman and I dont know what they were thinking. :)
I never thought to look at the cartridge diagram, but I see what your saying with everything being based off the headstamp.

With that being established my next question would be, would'nt a .005 variance in the rim throw off all the dimensions from that point foward? Or are all the measurments independent from another? Now Im just curious.

The Wilson will be ordered this week. Thanks for the recommendation guys.
 
Hmmm

I fail to see where the thickness of the rim has anything to do with anything as far as the rest of the dimensions are concerned. The rest of the case doesn't even know that the groove cut that forms the rim is there. It is a non factor.
The only time rim thickness would become a factor is if it was so thin that primary extraction suffered, or too thick so that the extractor could not hook over it.
The other criticle measurements are NOT idependent of each other. As I said before, everything originates at the bolt face, which is THE constant among a whole bunch of variables, and does not change.
For instance, if I say my chamber has a overall length of 1.510, that is the distance from the bolt face to the last part of the neck diameter before it dives into the bevel to the lead. If I say my reamer has .045 freebore, that is taken from a dimension that originates at the bolt face.
The only reason these lengths will vary from one reamer made off of the same print to another is manufacturing tolerances.......jackie
 
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From memory the Lyman trimmer uses a couple of holders that go over the rim to lock the case in but the case is still held back against the headstamp which is what is used as the reference surface for the trim length.

What I noticed with a Lyman trimmer I had back in the day was the alloy body had a bushed sleeve for the cutter bar. If you slid the cutter in and out a bit firmly (in my case the cut was smearing brass under the pilot and sticking the cutter to the case mouth so they "pop" free and the cutter would hit the sleeve firmly) it would hit that bush/sleeve. That would drift it out and a bi of pressure on the hendle when cutting could slide the sleeve back in. Did that all make sense ?? Long and short the trim length could alter because the sleeve was the stop and it wa smoving.

Either way, the rim thickness isn't the problem, I'd get a new trimmer. The Wilson like has been suggested or I quite like the Forster one. If you hold the case on the stop when locking the case holder the trim length is usually dead on and not worse than + or - 1 thou.

Bryce
 
Understood. I guess the only other time the rim thickness matters is when you have a junk trimmer that was poorly designed.
The Wilson is on the way.

Thanks guys.
 
I Am Wrong On One Point

There is one instance where the thickness of the rim is of prime importance, that being, on cases such as the 30-30 where the case headspaces off of the rim. But, I assumed we were talking about modern rimless chamberings.......jackie
 
You were correct, thanks Jackie. Sometimes I get an idea in my head and it takes someone to get out the crayons and draw me a picture before it clicks.
 
Just another thought

If you like call Sinclair for your Wilson. They have verrrry knowledgeable people who will advise you of the couple of items you will need besides the basic trimmer. Can't get this info just anyplace.
Ie. the shell holder is case specific. You may want to chamfer with the Wilson tool etc.:)
Centerfire
 
Nonliberal,

Sounds like you got it sorted.

Just wanted to check, what model Lyman did you have ? If it was the normal orange one with a universal case holder the rim thickness wouldn't matter. However, based on your shellholder comment, it sounds like it may have been an Accutrimmer model.

That model uses a normal press shellholder with a plunger that pushes against the head stamp area. As such that model does indeed reference off the inner surface of the case rim. That is just crazy. You were peobably 100% correct that the variation in trim length would coincide with variation in rim thickness.

A new trimmer is a good plan !!

Bryce
 
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