Advice for a custom 6mm Dasher

[J@50n]

I'm planning to build a 6mm Dasher to compete in PRS style matches and for long range plinking.
There's a lot of information out there about loads for the Berger 105gr hybrid, so I was planning to start with that bullet and Varget.

I'm looking at getting a Benchmark barrel on a Defiance Deviant action.
For their 6mm barrel, they have a lot of twist options:
3 grooves with 1:7, 7.5, 8, 9, 10, and 14
5 grooves with 1:7.75, 8, 9, 13, and 14
and 6 grooves with 1:7, 8, 9, 10, 12, and 14

What twist and groove count would people suggest for the 105 hybrid? I know generally the tighter twists are better for stabilizing the heavier bullets, but 105 is pretty light for a 6mm. And I'm not sure how the hybrid affects the math.

Also, how much, if at all, does it matter which reamer they use? I've seen .104, .105, and .134 options for freebore and .272 and .273 as available neck sizes. I suspect 1 thou won't make an appreciable difference in my use case, but could 30 thou? And could i just account for that by seating the bullet differently?

 

[Sheldon N]

IMO I'd say 5 groove 7.5 twist but all of them should shoot fine. 104 freebore is most common but again all of them should shoot good. The 134 freebore probably gets the bullet more into an optimum location with bearing surface being clear of neck/shoulder. 273 neck would be my choice.

Bit of a rabbit trail on specs, but I'd ask about base diameter of the reamer at the .200 line as well. If the spec is smaller (like .4708) then it's harder to find a sizing die small enough. If the spec is larger (like .4715) then you've got more options for sizing dies.

I'd suggest Hydro brass, or if you can try out some of the new Alpha Dasher brass that's an option too.

 

[spife7980]

I too would go with a 5 groove 7.5 twist (or more specific to your listing, the 7.75) if youre set on the 105.
If you were going 115 I would want a 7 twist just for good measure which would dictate you take a 6 groove.
You wont notice any practical difference downrange from the number of grooves.

The .104/.105 freebore are just about perfect for the 105 grain bullets imo, it gives you enough room out front of the brass to keep the bullet up out of the case body and from robbing powder space but is short enough to put the base very even with the neck shoulder junction. And with the short neck on a dasher that allows you the maximum room to chase the lands as they erode without ever having to worry about the bullet falling out of the end of the neck because its eroded too far forward.
If you were going to try a 115 class bullet I would go with the .134 just to get a bit of that longer bullet base up out of the case.
Both will work, ones just optimized for the other, with a .104/.105 the heavier bullets are deeper into the case, with a .134 the light bullets will have less neck material gripping them. Just the slight tradeoffs one has to make.

My loaded dashers using ff lapua brass has a neck diameter of .267" which means either the .272 or .273 necks would be fine. My criterion barrel has a .272 neck which gives me .005 which is more than adequate but not what I would call excessive for hassle free operation. Too tight and fouling and grime has no where to go creating a mechanical interference.
Some have said their ff lapua brass has a neck of .269" and again either the .272 or .273 would be fine. Personally I like the larger option.
Beware of reamers for brass with turned necks, stay above .272 and youll be gravy. Typically people want .003 of difference between their rounds loaded neck diameter and the chamber diameter so that the brass has ample room to let go.



Id pay headed to Sheldons remarks on base diameters. Here is my chamber print.

Here is what my fired bras measures. Notice that my .4708 base barely gets changed and that half thou difference I noted was more of an exaggeration. It changed but not consistently enough for each one to change with my mitutoyo calipers that read in .0005 increments. I was too lazy to get out the micrometer. Hasnt caused me issues yet but it was the one thing I noticed on sizing and said "Shit, Ill need to watch this in future loads."

 

[lte82]

Can’t say it better than Sheldons post.

 

[J@50n]

Wow. Thanks for the detailed answers. I'll get the 1:7.75 5 groove then.

 

[J@50n]

I've been researching brass (and other components) and there seem to be three main sources of brass: fireforming Lapua 6BR (which I'd prefer to avoid as it adds the cost of a bullet, a primer, and some powder to the cost of the brass, plus the time), Alpha (which I can't find in stock anywhere), and Norma (which I can find easily). I've read here and on other sites that the Norma brass is too long for some chambers. I went with the reamer with a .272 neck and .104 throat.
Will the Norma feed into that properly?

 

[Bender830]

I went with the reamer with a .272 neck and .104 throat.
Will the Norma feed into that properly?

Those specs don't address the length of the neck. You need to know the length to know if the longer necked brass will fit within that length.

If it were cut for lapua brass then you will need to trim that norma back to fit. If it were cut for norma brass then you should be good. I doubt its cut for norma brass otherwise you would probably have been made aware of that fact.

fireforming Lapua 6BR (which I'd prefer to avoid as it adds the cost of a bullet, a primer, and some powder to the cost of the brass, plus the time),

Only by the same logic that the first shots with anything else don't count either. Fireforming is no big issue, is accurate and get you the best resultant brass I think.

 

[Sheldon N]

Will the Norma feed into that properly?

No, unless you specifically asked for a "Norma brass" chamber. The long neck of the Norma brass will not fit into the shorter standard Dasher chamber.

 

[jre0807]

It’s worth the wait for Alpha to get dasher back in stock. Last time it happened it took a couple weeks to sell out so you’ll have a chance to grab some when it comes back in stock. Plus you can get on their waiting list to avoid checking if it’s back in stock all the time if you wanted to do that.

Hydroforming is a good option as well. Pretty easy once you get used to it and goes pretty quick. Whidden helped me out quite a bit to accelerate the learning curve as well.

 

[J@50n]

I'm on Alpha's waitlist, and A-Team's waitlist for Alpha 6 Dasher brass.
My barrel should be on the action and back in my hands by the end of February, so I'd like to be working up loads and testing them out in March.