Re: Quarter bore and 6.5mm wildcat ???s
Just so ya know; the Scramjet case is completely different than the firebird/warbird case. The FB/WB case has 124grH2O capacity. The SJ has 95grH2O cap. Neither are based on, or derived from, a 416 case.
Given this, You'll gain nothing over the Scramjet using STW/300WBy cases, as they both have less capacity than the SJ.
With plenty of barrel, you could push a 90-100gr .257 bullet at ~3850, near max pressure. This with any case > 85gr cap. Beyond that, with light bullets anyway, there are no further gains with additional powder in this bore. Just heat and recoil.
With heavier bullets the extra capacity helps, but velocity drops.