Egovro, I tried some NSA slugs in my BSA Lonestar, and they shot really lousy. A guy over on a Brit forum told me my rifle was actually a .243 bore, and that might explain why the NSA slugs were very tight in my bore and VERY hard to get seated. You might want to compare slugs to pellets in your Impact. Might be better to make pellet dies.
My Lonestar was only made for export, and not available for sale in the UK. Maybe that is why it is supposedly .243 bore. A pellet that I removed from the chamber miked out at .245", which is quite a bit smaller than .250". So definitely not exactly a true "quarterbore". Maybe BSA hammer forged the barrel with a standard .243 mandrel since it was headed for the USA? Anyway, the tight bore shoots very accurately, 3/8" 5-shot groups at 50 yards. Is your Impact bore bigger than .243, or is it a true .250 bore?