How Precise Does Trimming Need to Be?

[watercam]

I have some 5.7x28 cases to trim and my chosen tool for trimming is hung up with Forster collets on back-order. I have used a file to come close but there is still some variation in overall length. Within +/-.015". Any guesses as to the what may happen out of a 5" barrel?
Just in case someone has come up with the same issue.
Thanks,
Mike

 

[Rocketvapor]

@watercam I do not have a5.7x28 but +/- 0.015" sounds like a rough cut. Did you mean +/- 0.0015" ?
For some cases that are 0.001"or 2 different in a batch I will often use a fine file to adjust rather than run them through the trimmer.
Your neck is about 0.182" long. 0.015" short should not cause any issues.
0.015" long may interfere with the chamber.
I would try for a little tighter result with the file.

FN 5.7×28mm - Wikipedia

en.wikipedia.org

 

[std7mag]

I just use a Lee case length gauge as part of my standard loading practice.
No guessing.

 

[GLO123]

.015 is a lot when you say + or -. Thats .30. Get them within .001 and you will be good.

 

[30378]

The real issue is whatever length you go with, get them all the same. Consistency is absolute.

 

[358WCF]

.015 is a lot when you say + or -. Thats .30. Get them within .001 and you will be good.

.015 +.015=.030.

 

[watercam]

What I will do is wait for the darn collet to be made!
Thanks for the input!

 

[cem]

I guess that the difference in length doesn't matter by itself. To the extent that different lengths crimp differently I'd expect results to show on the target and also to be lost in the noise for much short range handgun shooting. I'd think easy enough to test with existing results and you tell us.

For bulk I'll use a Forster, especially a Forster base in a drill press for large lots; commonly 5.56 range scrap. I do find the Forster collet takes a little extra attention for uniform results compared to a Wilson micrometer setup for ultimate precision. Case holders for the Wilson should be checked for the odd QC failure but are almost all entirely satisfactory. I think Wilson style case holders can be made from scratch or modified with appropriate reamers more easily than Forster collets.

 

[markm87]

I remember years ago... here... a few honest loader fessed up to not trimming any more for bolt gun stuff.

I don't trim for bolt gun. At that time I was bumping the shoulder too much and getting case separation. Now I Just chamfer the neck for seating. And every few years I'll trim .308 brass when it gets pretty long.

I run a Giraud for 5.56 ammo, so I do every piece on that thing just to keep from shaving jacket material on OTM bullets.

 

[Hohn]

Since precision is repeatability, you want it as precise as you can get.

But it need not be very accurate (close a particular number).

I think short brass carbon rings faster and worse, so I’ll generally go as long as I can.

 

[RickKr]

...snip... I think Wilson style case holders can be made from scratch or modified with appropriate reamers more easily than Forster collets.

They definitely can. Use of a reamer would be idea, but it can be done on a lathe. I recently made one for 300 WSM. The trick is setting the compound accurately to match the very slight taper of the case body. I used the technique described in this YT video. It takes some practice, but I use this method for a variety of short tapers. I have a taper attachment on my Monarch 10EE, but the way it is situated, it is difficult to get to for setting taper. Plus, a taper attachment is overkill, setting the compound works fine.

Rick

 

[Rsadams]

If your talking safety wise then if it's above the minimum and below the maximum lengths then you are good.... If your talking about accuracy then as with anything else you want them all the same...
In my AR platform rifles I trim in the middle of the spectrum and run them....

 

[Rocketvapor]

I check each of my rifle barrels for case length with a borescope.
Note the red dots in the pic below. Values are measured cases, short and long.
My target is 1.765" and will trim when they grow to 1.769"

 

[paulwolf]

I check each of my rifle barrels for case length with a borescope.
Note the red dots in the pic below. Values are measured cases, short and long.
My target is 1.765" and will trim when they grow to 1.769"
View attachment 1338824

I was just looking at a piece of trimmed brass in my rifle with a borescope just like that. Being on the shorter end, it had a bit of gap to the freebore? cut. Looking at it got me thinking about what is the ideal trim length? Would it be better to target a tighter but safe marginal gap for stretch, closer like your "target" trim length, even if its outside the considered max trim length. I didn't measure or do any work so I have no idea what the max is or what that would look like in the barrel just thinking theory.
For full length sizing it is recommended to bump the shoulder back .002-.003 from bolt close. For cases its just recommended to trim in the middle or max of a .020 window from max-min case length for example. Shouldn't there be a method of measuring your actual max chamber case length and knowing to trim a magic number off to achieve the ideal length from that?

I know I am over thinking it but thought it was an interesting question concidering how much effort goes into tuning a seating depth but I haven't heard anything about tuning a case length.

 

[AWS]

I use the Lee Quick Trim with the Economy Cutter(i like to chamfer myself). I keep an old turret press set up with an assortment of dies. The length they cut to is very precise. I trim every loading.

5.7x28 die body

5.7x28mm FN Dies

5.7x28mm FN Dies

leeprecision.com

I do like the Wilson/C&H/Herters with the case holder another that is pretty precise but slower than the Lee. I do keep an old Redding and Lyman on the bench for some older rounds like the 6.5x58R Sauer, 5.6x52R and 9.3x72R that they don't make case holders, dies or case length gauges for.

 

[6MMsteve]

I check each of my rifle barrels for case length with a borescope.
Note the red dots in the pic below. Values are measured cases, short and long.
My target is 1.765" and will trim when they grow to 1.769"
View attachment 1338824

that's how I chk mine lately, don't have a reamer print so there u go

 

[PhilC]

Shouldn't there be a method of measuring your actual max chamber case length and knowing to trim a magic number off to achieve the ideal length from that?

There is and they're called chamber length gauges, I've used these for over 20yrs.

I set the brass trim length for all my rifles .010" less than actual measured chamber length. YMMV.