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What size expander mandrel 6br

I doubt it...
Some of us are in the camp that the bullet is a mandrel.
You can just as easily control interference fit with your bushings and only move the brass once.
But in order to get consistent neck tension, you will likely need to neck turn (so that the neck thickness in consistent) - I hate neck turning. Which is the main reason I've gone to finish sizing with a mandrel.
 
Just to add another way mandrels can be used...

I'm using a 30-06 and 7x57 Forster Benchrest full length resizing die. Was not happy with the neck tension I was getting. I could tell the tension was a too tight when I seated bullets. I was thinking about getting the dies honed, but I had no idea what size to get them honed to. So I added a step into my process where I run the cases in a 0.307 and a 0.283 porter's mandrel (minus tolerance). Made a big difference when seating bullets as well as the SD/ES of my handloads. Did it tighten my groups? I'm not sure, I was in the middle of developing a load. I have a load I like with the 30-06, max extreme spread of 10 with N150. I think it helped my groups but I really have no interest in going back to test.
 
I did a small sample size (10 rounds each ) of mandrel vs no mandrel in 6BRA with .272 no turn neck chamber using .266” bushing and a .241” mandrel. Not a huge difference on target, both grouped about 0.4” for the 10 shots but the SD was lower (4.5 vs 8 )in the mandrel group. Velocity was a tic higher in the non mandrel set. (5fps) Again small sample size so if we do 30 of each, the difference may disappear.
 
Can't say anything about group size as I was shooting steel. However after full length bushing sizing, and then using 21st century mandrel for final neck sizing. Moly dipped bullet base, I was extremely happy with the results.!
This included temperature change from mid morning to early afternoon.

20240616_112920.jpg
 

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