Trim Length Variation

[JDinFbg]

I have a Forster manual case length trimmer. When trimming brass, there always seems to be some variation in the trimmed case length even though everything is locked down tight. I am now trimming some 6mm Rem brass (formed from military 30-06 brass) and am seeing case length variations of up to 0.001". I have read several posts of folks who have experience similar variations and have followed all the tips suggested, but am still seeing the same variations. I think I have finally discovered the reason. I measured the rim diameter of several of my cases and found they varied. I observed that the cases with the largest rim diameter tended to come out of the trimming process longer than those cases that had a smaller rim diameter. My conclusion is that the cases with the smaller rim diameter require the collet to shrink down further to grip the case rim resulting in the collet (and therefore, case) being moved farther forward, causing the case to trim shorter. So, my questions:

• Does this analysis seem reasonable?
• Has anyone else ever measured case rim diameter and found variations?
• Are there any tools or processes to machine case rims to a consistent diameter?

 

[Ruger 15151]

I have several case trimmers and they all vary. .001 is excellent. I think the variance is caused by differences in the case head and how its held in the trimmer chuck.

 

[peels]

My experience is the same with the Forster trimmer. How hard you push on the trimmer is the other cause of variation. But 0.001 variation is not bad at all.

 

[azrocks]

I have absolutely nothing to compare it to, as this is my first-ever trimmer, but the Frankford Arsenal Universal Case Trimmer, which uses a hand drill to do the dirty work, trims a large range of bottleneck calibers without the use of collets. The micrometer feature doesn't really work for absolute precision, which I kind of expected, but once you tweak it to get it trimming the length you want, it stays there. Using brass consistently sized, everything comes out exactly the same length. The trick is to turn the brass while you trim, as well as to use adequate pressure and trim until the blades are no longer making any contact with the case. It indexes off the case shoulder, so as long as your cases are sized consistently, it produces great results. The only thing I can't comment on is longevity, as it's fairly new to me, but so far I love it.

https://www.frankfordarsenal.com/ca...universal-precision-case-trimmer/1092514.html

 

[edwardware]

. . . and am seeing case length variations of up to 0.001".

1) If you're using a caliper, you're sort of kidding yourself at 0.001" resolution. It's just not repeatable.
2) Since you must be using a micrometer, realize that an imperceptible misalignment will produce that much variation.
3) Do you have any reason to believe that a 0.2% variation in neck length matters enough to measure?

Have you air-gauged that premium aftermarket barrel, and chambered the large end? Turned your necks for consistency, and are annealing every firing?

If not, you're kidding yourself here.

 

[JDinFbg]

I have absolutely nothing to compare it to, as this is my first-ever trimmer, but the Frankford Arsenal Universal Case Trimmer, which uses a hand drill to do the dirty work, trims a large range of bottleneck calibers without the use of collets. The micrometer feature doesn't really work for absolute precision, which I kind of expected, but once you tweak it to get it trimming the length you want, it stays there. Using brass consistently sized, everything comes out exactly the same length. The trick is to turn the brass while you trim, as well as to use adequate pressure and trim until the blades are no longer making any contact with the case. It indexes off the case shoulder, so as long as your cases are sized consistently, it produces great results. The only thing I can't comment on is longevity, as it's fairly new to me, but so far I love it.

https://www.frankfordarsenal.com/ca...universal-precision-case-trimmer/1092514.html

I watched the video link you posted and a couple of others I found on YouTube. That trimmer is really slick, and certainly takes less effort than my Forster. I think one of the Frankford trimmers needs to go on my shopping wish list.

 

[azrocks]

I watched the video link you posted and a couple of others I found on YouTube. That trimmer is really slick, and certainly takes less effort than my Forster. I think one of the Frankford trimmers needs to go on my shopping wish list.

Yeah, I like it a lot. I keep checking the resultant cases waiting for it to require readjustment, & it just never happens. The plastic shell holders may have longevity issues, but time will tell. If so I hope they're available separately. Only complaint is that holding the cases gets a little tiring for the hands after a bit. But that bit was over 100 cases in a sitting, so maybe I just overdid it.

 

[JFrank]

Yes
No
I don’t know

Last I heard .001 variation is like a human hair thickness, I believe there are bigger things to worry about

 

[Walkalong]

My experience is the same with the Forster trimmer. How hard you push on the trimmer is the other cause of variation.

Yep, my RCBS is the most susceptible to this, my Forster flexes less, and my Wilson none at all. The operator can influence how much.

But, as posted, a .001 spread is excellent, and +/- .001 is very good.

 

[Hanshi]

Bottle neck cases or most any case that's not meant to be crimped don't need precise dimensions. The 6mm, as long as it doesn't exceed max length or fall much below min, can be off a good bit, as an example. The cartridge OAL is what's important to watch.

 

[MikeInOr]

I have a Forster manual case length trimmer. When trimming brass, there always seems to be some variation in the trimmed case length even though everything is locked down tight. I am now trimming some 6mm Rem brass (formed from military 30-06 brass) and am seeing case length variations of up to 0.001". I have read several posts of folks who have experience similar variations and have followed all the tips suggested, but am still seeing the same variations. I think I have finally discovered the reason. I measured the rim diameter of several of my cases and found they varied. I observed that the cases with the largest rim diameter tended to come out of the trimming process longer than those cases that had a smaller rim diameter. My conclusion is that the cases with the smaller rim diameter require the collet to shrink down further to grip the case rim resulting in the collet (and therefore, case) being moved farther forward, causing the case to trim shorter. So, my questions:

• Does this analysis seem reasonable?
• Has anyone else ever measured case rim diameter and found variations?
• Are there any tools or processes to machine case rims to a consistent diameter?

What is the purpose of the ammunition? If it is hunting ammo .001 is completely irrelevant! If you are loading for 1000 yard bench rest matches there are MANY other factors that have a MUCH bigger affect on the repeatability of your ammo before that .001" in length difference has any affect at all. Just the variations in other case dimensions are going to cause variations that will WAY out weigh a .001" case length variance.

 

[mrawesome22-250]

The average human hair is .004" thick. Now section that hair into 4 even slices...

Pretty irrelavent in my mind. Then the accuracy of the measuring tool comes into question. There is also the issue of repeatablity skill of the person operating the tool.

 

[Bat Rastard]

When I read about .001 variation, I thought he was bragging!

 

[Skeptic13]

I have the same trimmer and love it. My gives some slight variances but overall it is very consistent. Within .002 on all cases which I think is fantastic.

 

[chamokaneman]

You're probably on to something with the rims. I sometimes wonder if the thickness is uniform, i.e. case length is a little different depending how it sits on the calipers. In any case, .001 is pretty damn good. I wouldn't worry about it if it were me.

 

[JDinFbg]

I started this post after measuring only 10-15 trimmed cases. After I completed trimming all 102 cases I had to process, I entered the data in a spreadsheet and found I actually have 0.0155" length variation between the longest and the shortest cases after trimming. The standard deviation in length is 0.0035". All are shorter than they started and now under the specified maximum length, so I'll see what results I get the next time I go to the range.