I went with a 0.255" neck to avoid turning necks. Both Alpha and Lapua brass measure 0.251" loaded from memory. So the 0.255" neck works with either and avoids the need to turn necks.
I've used Lapua before and the current brass I'm shooting is Alpha. With either brand I start with a 6 BR case and fire form it with a false shoulder and jammed round to my BRA chamber. For 22 Dasher I'm not sure which route I'd go. I've read someone say they only necked down Alpha 6 Dasher brass and then fired it and shot it like normal. However I'm not sure if that would work well with every 22 Dasher reamer. If your working with Manson ask the person designing your reamer what brass they recommend to start with. I'd guess Ray is probably designing the reamer and I expect he could give you a better starting point with brass than me for that specific reamer.
However if I had to speculate, I'd guess necking down 6 Dasher brass should work fine because the shoulder angle isn't changing so I'd expect the current 6 Dasher shoulder remains in place. My BRA is a whole different situation since the shoulder angle changes while the neck diameter also changes compared to the parent case which results in the shoulder location moving and requiring more jamming to fire form.
Also I should add:
22 BRA with an 85.5gr Berger LRHT, Alpha Brass, and Varget can achieve around 3150 in a 26" barrel and I start to get cratered primers and some heavy bolt lift pushing past 3200 FPS. I also think there's powders out there than could push the speed even further. N540 shot extremely fast for me the one time I tried it, XBR was also reported to have high velocity. I tend to run mine more in the 3000 - 3080 range since that's plenty performance for me and I'd rather extend barrel life. Although I haven't tried a 22 Dasher yet it seems to have a few more grains of capacity than I need for a 22 cal. What's your velocity goal with the Dasher or is there any specific reason to go with a Dasher for a 22 over a BR/BRA?
