neck tension?

[rockhound78]

Shooting a round based on the 6mm GT case that I call the 6mm BRZ, and just wondering how much neck tension is needed to properly grip the bullet. I just made a bushing to fit an unused Lee expander die and sized case necks come out around .266"-.267" O.D. and fired cases are .269" O.D. Don't know if .002" is enough or should I get a .265" bushing, for .003"?

The case , from the shoulder forward , is 6mm BR. 30 degree shoulder and 6mm Lapua length neck. I use an inside neck reamer on fireformed cases to get rid of the "donut" and give bullet clearance as the GT case necks are a bit thick.

34gr Varget seems to be the ticket for the 70gr Sierra bthp match and the 85gr bthp bullets

 

[rardoin]

I have shot loads with as low as .0005” NT with graphite lube before seating with 6 Creedmoor and 108gr Bergers in a 10rd mag for a Tac class match. I’ve measured the CBTO on round 10 and it was no different that round 1. As long as you can’t move the bullet with a very firm finger grip it is enough. At 0.002” you’ll have plenty of bullet grip.

 

[Rob01]

.002" of neck tension will be fine with that set up.

 

[rockhound78]

Thanks for the info. Thought I would be OK, but wasn't sure

 

[Danley]

I see where your sized cases are .266-.267” and fired cases are .269”

What is the neck diameter of the loaded round?

 

[rockhound78]

.268"

 

[Rob01]

Loaded is .268 then subtract .002” off that and go with it.

 

[JMayo]

Testing at the different sizes will tell you. My 6 bra liked 267.
Using Varget 30.6
It's 268 loaded.
266 were bigger groups.

 

[BoydAllen]

Shooting a round based on the 6mm GT case that I call the 6mm BRZ, and just wondering how much neck tension is needed to properly grip the bullet. I just made a bushing to fit an unused Lee expander die and sized case necks come out around .266"-.267" O.D. and fired cases are .269" O.D. Don't know if .002" is enough or should I get a .265" bushing, for .003"?

The case , from the shoulder forward , is 6mm BR. 30 degree shoulder and 6mm Lapua length neck. I use an inside neck reamer on fireformed cases to get rid of the "donut" and give bullet clearance as the GT case necks are a bit thick.

34gr Varget seems to be the ticket for the 70gr Sierra bthp match and the 85gr bthp bullets

The numbers that you gave will give you loaded round neck to chamber neck clearance but not neck tension, which is calculated by comparing loaded round neck diameter over the bullet with sized case neck diameter, measured at the same place on the neck. It is the amount that the neck diameter is increased by the insertion of the bullet. Collet dies can make slight ribs on the ODs of necks, depending on how they are set up and used, so I would take care to measure in the areas between them.

 

[Ringostar]

Just a curious question. If your loaded round is .268, and a fired case measures .269. What is your chamber neck diameter? It sounds like a tight neck, and you might run into carbon issues.