Help with understanding Brass trimming 7MM SAUM

[T Searles]

I am in the process of putting together a 7MM SAUM

PROOF Research Carbon Prefit Barrel 24" Length, 1-8.4 Twist, Sendero Contour,

Tikka T3, Action

Barrel is throated to allow seating heavy for caliber bullets closer to magazine length - Max COAL w/ 180 Hornady ELDM at 3.208".



OK now for my question I am ordering a Custom Lee quicktrim die, in order to do this I must submit the below data I would like to understand the optium dimension for “D” with the set up above? How does altering the over all case length effect accurancy and performance?

 

[DaveTooley]

Small variations in case length don't have a measurable affect on accuracy in a hunting rifle. Not until the case length exceeds the chamber length then that can and will run chamber pressures up causing issues.
Competitive shooters will trim for consistency but even then small variations get lost in the noise.
Go to saami.org and find the specs for this cartridge. It will show chamber and cartridge specifications.

 

[fisherman987]

I would go ahead and fireform your brass 2 or 3 times, full length size then do the minimum trimming necessary to even them all out, chamfer, debur, and then take your measurement. I agree with Dave, the trim length won't be that important as long as all your brass is the same without being too long.

 

[RegionRat]

OK now for my question I am ordering a Custom Lee quicktrim die, in order to do this I must submit the below data I would like to understand the optium dimension for “D” with the set up above? How does altering the over all case length effect accurancy and performance?

Agree with the others. the overall case length isn't directly going to affect your accuracy or performance.
You must manage case prep and not get sloppy with anything, let alone case length, but the trim-to value is arbitrary as long as it falls within the specs.

You would be best off sticking with the SAAMI specs when you specify that length.
Going much longer or much shorter than the specs is opening a can of worms, and I won't go into that hole in this thread.

That said, if you are sloppy with brass management and case prep, you will directly affect your performance.

(BTW, nothing against Lee products in general and I have used them most of my life... but this isn't the case trim system you want. But that isn't what you asked....)

Here are the SAAMI specs. The chamber length (2.045") will show you their design margin. The cartridge drawing show you the trim length tolerance. In this instance your case max is 2.035" and the usual -0.020" tolerance would mean the min trim length is 2.015". Pick a number from 2.015" to 2.035" and stick with it.

The closer to the 2.035" you run, the closer you will watch them to make sure they don't exceed the chamber length. The shorter you trim them, the more room you leave for residue and carbon in the chamber if your cleaning habits are poor and you let the case length grow over that mess. Keep your prep and brass prep tight, keep the chamber clean, and no worries. YMMV

 

[Straightshooter1]

I am in the process of putting together a 7MM SAUM

PROOF Research Carbon Prefit Barrel 24" Length, 1-8.4 Twist, Sendero Contour,

Tikka T3, Action

Barrel is throated to allow seating heavy for caliber bullets closer to magazine length - Max COAL w/ 180 Hornady ELDM at 3.208".



OK now for my question I am ordering a Custom Lee quicktrim die, in order to do this I must submit the below data I would like to understand the optium dimension for “D” with the set up above? How does altering the over all case length effect accurancy and performance?


View attachment 1607484View attachment 1607486

I'm the type of person that like to control all that I can control, so I trim my cases with a 3 way trimmer as the final step in my case prep. There are two things that, in my mind, help with keeping case length consistent helps: 1. A consistent case length isn't going to have an issue with the build up of the carbon ring where the neck will get pinched in any way. 2. Having the case length the same keeps the timing for the amount of the blowby the same as the bullet moves forward past the case mouth.

It's a very small thing that I can't even measure in terms of performance differences in either velocity or on target. But I see it as just one small thing in a chain of things that all together adds up.

Because I typically load my projectiles long, there can be an issue with how much of the bearing surface is being held by the neck. I'll try and let my necks get long and then trim them there for a consistent length where the necks have a good hold, where a shorter trim length just might be a big problem.

 

[T Searles]

Agree with the others. the overall case length isn't directly going to affect your accuracy or performance.
You must manage case prep and not get sloppy with anything, let alone case length, but the trim-to value is arbitrary as long as it falls within the specs.

You would be best off sticking with the SAAMI specs when you specify that length.
Going much longer or much shorter than the specs is opening a can of worms, and I won't go into that hole in this thread.

That said, if you are sloppy with brass management and case prep, you will directly affect your performance.

(BTW, nothing against Lee products in general and I have used them most of my life... but this isn't the case trim system you want. But that isn't what you asked....)

Here are the SAAMI specs. The chamber length (2.045") will show you their design margin. The cartridge drawing show you the trim length tolerance. In this instance your case max is 2.035" and the usual -0.020" tolerance would mean the min trim length is 2.015". Pick a number from 2.015" to 2.035" and stick with it.

The closer to the 2.035" you run, the closer you will watch them to make sure they don't exceed the chamber length. The shorter you trim them, the more room you leave for residue and carbon in the chamber if your cleaning habits are poor and you let the case length grow over that mess. Keep your prep and brass prep tight, keep the chamber clean, and no worries. YMMV

View attachment 1607541

What Trimming system do you recommend???

 

[1911mag]

Henderson Trimmer best investment I made last year

 

[barefooter56]

I am in the process of putting together a 7MM SAUM

PROOF Research Carbon Prefit Barrel 24" Length, 1-8.4 Twist, Sendero Contour,

Tikka T3, Action

Barrel is throated to allow seating heavy for caliber bullets closer to magazine length - Max COAL w/ 180 Hornady ELDM at 3.208".



OK now for my question I am ordering a Custom Lee quicktrim die, in order to do this I must submit the below data I would like to understand the optium dimension for “D” with the set up above? How does altering the over all case length effect accurancy and performance?


View attachment 1607484View attachment 1607486

This will help you find your true chamber length and trim to length: https://www.brownells.com/reloading/measuring-tools/case-length-gauges/sinclair-chamber-length-gage/

 

[barefooter56]

What Trimming system do you recommend???

L. E WILSON.

 

[RegionRat]

What Trimming system do you recommend???

It is very difficult for me to make this specific recommendation without knowing your situation and goals. I also don't want your wife to have me on her dart board....
But since you asked....

In a nutshell, I have helped many folks who suddenly wanted to learn to reload who were adults with no technical or shop background. A few lived close enough to slow things down over many sessions and time, but some were from far away and it had to be done in firehose fashion.

I think I learned it the best way, in many small steps at a time, over a long time while I was a very little kid, and then I was formally educated as an undergrad when I was kidnapped by aliens and held prisoner in the defense business for 40 years....

That said, since I have no idea of your budget or goals, I will make a simple suggestion and hope for the best without turning this thread into a trimmer review.

I started my rookies with a live demo of a Wilson and a Forster, both with and without a three-way cutter.

Then I showed them the Giraud and the Henderson, and we went through those set-ups and their costs. For some students, I demonstrated the use of the Dillon 650/750 and how multi-stage bulk prep works on a progressive.

In the very end, I bought the ones who were my personal problems a Frankford Arsenal Platinum Case Trim and Prep Center and got them rolling. The cutter is a replaceable standard tool which can be run with or without a center pilot and even upgraded to a carbide if you choose. It does not do a three-way cut, but it has stations for the ID and OD chamfer tools and is a very flexible unit in terms of adapting to multiple calibers without extra spending.

(BTW, I also recommend a borescope so they can learn several things including cleaning and being able to directly observe the position of the case neck in the chamber. Teslong NTG500 before you ask.)
Good Luck. YMMV