Soot on Case Shoulder

[fourpower]

Necks are .248 loaded and chamber is supposedly .251. I'm getting blowback down past the neck shoulder junction onto about 1/16" the shoulder of the case before it seals off. If I use a bushing die I don't see it as often as a fl die used in conjunction with an expander but accuracy is not as good with the bushing die. I loaded 30 up with the bushing die yesterday and fired 3 strings of 10 at 600 at a F class target and the shots that dropped low were ones that didn't seal at the case neck and soot ended up on the shoulder, those shots were slower across the chronograph as well. How much damage is it doing to the chamber when cases are not sealing completely on the neck and with a low powder capacity cartridge case (24 gr) is it ok to run tighter neck clearances than you would with 2x the case capacity?

if I only got 3to 4 reloads on my brass I would be looking hard at my headspace! as far as soot usally you need more powder or try different one.