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Is it really worth going to the Dasher over the standard 6mm Br Lapua?
What velocity is being reached with a 107g projectile.
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Re the Dasher. You get 4-5gr more capacity. Max velocity increase will be about 150 fps, but it's not uncommon to see only 100-120 fps.
The negatives you already know about: lots of time spent running mandrels through cases and forming false shoulders, added barrel wear during fire-forming, increased cost of dies, and delays in getting dies. Short neck, so you can't seat short bullets at the lands.
That said, you can avoid the Die issue with the BRX version. Click on our 6BR Improved page for details. Personally, if I did an 6BR Improved, it would be a Dasher, with the 40-degree shoulder, but I might spec a new reamer so the neck was a little longer.
The key is to get the bullets to around 2950-3000 fps which seems to be an accuracy node for the VLDs.
If you want to get up to 3050 with the 107s, you should also be looking at the 6XC or the 6x47 Swiss Match (but SM brass is almost impossible to get outside of Europe). That said, Richard Schatz is getting 3025 fps with Berger 105s and RL15 powder from a 26" Hart barrel.
It really comes down to this. If you're going to shoot 100-107gr bullets ALL the time, I'd go the Dasher route. It is a very accurate cartridge, and once the cases are formed, if the chamber is cut right, it is VERY stable with the 40-degree shoulder. Richard gets about 8 firings off a case before he has to full-length resize. As for accuracy, Richard's Dasher holds a world record at 1000 yards, and it shoots in the teens at 100 yards.
Personally, I have a rifle in progress, and I'm going standard case. This is because half the time the gun will be shot at 600 yards, and the other half of the time at 100-200 yards. With the long neck, I can seat everything from 75gr to 107gr at the lands and still keep the long bullets away from the neck-shoulder junction where doughnuts cause seating issues.
BTW, keep posting on the NEW BOARD. Eventually forum members will see the light. Gunners tend to get stuck in their ways...
One last thought. Why don't you finish your barrel at 29" and shoot a standard case for a while. You may be perfectly happy. Then you can always set it back an inch and recut the chamber for a Dasher.