6BRX Starting Load

[Dubya]

Need starting load for 6BRX using 105-107 Lapua-Sierra bullets.
Fire formed brass water weight is 41.1 grains to top of neck.
Fire formed the brass with 31.0 gr. of Varget w/Hornady 105s
with jam lenght OAL, brass looks good, but need a starting point
using ; Varget-RL15-IMR4895 powders, or any other suggestions.
Thanks.

 

[Forum Boss]

Dubya, read the Gun of the Week article about Scott Parker's BRX Barnard-Mastin. He fire-formed with 30.5 Varget and is using about 33.5gr Varget now in matches, with MOLY 107s. If you're using naked bullets, that may be too hot.

http://www.6mmbr.com/gunweek020.html

"I ran a ladder test at 100 yards with 11 shots covering from 32.5 grains to 35.0 grains of Varget. This was fruitless since all 11 shots went into a group less than .5". I re-tested charge weights with 3-shot groups and evaluated them for size and shape. Two sweet spots appeared with groups in the mid 'Ones'. The lower charge weight was 33.5 grains at 3020 fps. The upper spot was at 34.5 grains and 3104 fps. Both loads used CCI BR4 primers, about .0015" neck tension, and moly-coated 107s seated .015" off the lands. I Loaded 10 rounds at both charge weights and did 5-shot groups. Both loads grouped less than .170" center to center as shown in the above left target."

But FYI, the 105 Amax has a long bearing surface relative to the 107 SMK so you may encounter pressure earlier. Small changes in seating depths can make big differences in pressure so there really is not alternative to working up a ladder. Based on my experience with IMR 4895 in std case, I bet you'd end up within 0.2 grains,more) of your preferred Varget charge.

 

[Dubya]

Thanks Mod for pointing that out for me, appreciate it.

 

[sparker]

Anything in the 33.5-34.5 grain range of Varget with molied 105-107s shoots very tight groups in my rifle. CCI BR4s and Rem 7.5s give the same results. Good luck.

Scott

 

[Changeling]

Scott, have you tried the 115 DTAC bullets yet? Opinion in the 6BRX?
Changeling

 

[sparker]

I have not tried the Tubb 115gr. bullet yet. I have some. I ran the following #s and this is what I came up with. the 115 at 2950 and the 105 at 3050 are ballistcally equivalent to 1000yds. I think a 8" twist is at best marginal for the 115. I spoke with Bill Davis, the designer of the Berger 115. His calculations show that the Berger 115 needs a 6.7" twist for a proper stability factor of 1.5. that bullet is 5.5 calibers long, just like the JLK 90gr .22 bullet that needs a 6.5" twist. I do not think that anyone has shot the Berger with enough twist yet to get it to group well. It may be that the 6BRX can do a little better than 2950fps. However, the chamber would need to be throated accordingly. I purchased the bullets for measurement purposes only. I think the benefit of shooting that bullet lies in some very narrow margins.

I can shoot the 105s to 3100fps. This of course is a bit warm. However, no signs of pressure with this load. Whatever I might be giving up in drift to the 115,probably none),I am gaining in inherent accuracy of the cartridge/load.

Scott

 

[Changeling]

Scott, always a pleasure! I have talked to quite a few guys on the 115 Sierra/Tubb and am hearing a lot of conflicting findings. It appears an accuracy node is very thin but there! I don't want to mention names because I forget just who said what. Take a look at the "115 posts" on benchrest central. It was recently discussed.
My belief is it is starting to appear that it may be a might to heavy and something better is on the way. Then again maybe one of the larger capacity cases will find a node that is in tune with it's size, but then other factors are introduced, time will tell, very interesting.
Take care, Changeling