Impact MkII, 700 mm, PP, .22. Shooting H&N 23 gr slugs, which were previously doing very well: 1/2" 5-shot groups at 70 yd, often better. For about a month I've been getting random fliers, with three shots touching and two an inch or more away, not always in the same spot.
Scope? Swapped it out, sent it back to mfg to check. Backup scope-still fliers.
Rings/mounts? all tight, done with torque wrench.
Bore leading? Don't have a bore scope, can't see much lead at either end, thorough cleaning didn't help.
Crown? Looks perfect.
Irregular velocity? Tried various tunes from 880 to 945 fps, usually with extreme spreads 14 or less, often below 10. Adjusted regulator as needed, lower for slow tunes and higher for fast ones, from 95 to 120 bar. Still fliers.
Ammo? I sort by weight, and over 95% of these slugs are either 23.2 or 23.4 gr. Don't have a good way to measure them otherwise, and anyway they were working before, and it's the same lot.
Tried pellets again: JSB 18.13, FX 18.1, both of which were good in the past, at 880 fps: problem still there.
Liner is indexed, tried changing that, no better.
Clipping? can see no lead streaks on either end of moderator, none on the end of the shroud. Tried the issued FX shroud. Louder but still fliers, no sign of clipping.
Too fast? Too slow? Tried the above velocity ranges with no improvement.
Liner is a Superior standard, which was working well before.
C1 is tight.
Hold sensitive? Wasn't before, but I tried loose and tighter holds without improvement. Shooting from a good steady bench with a good rear bag and a bipod.
Breech O-ring? Changed it . No better.
Probe? Tight, and adjusted to fit the port in the barrel.
Rail is firmly tightened but not overly so.
Barrel grub screw firmly but not excessively tight. Jam nut holding liner to barrel assembly tight but not too much. Barrel screwed into shroud firmly but not too hard.
What am I missing?
Scope? Swapped it out, sent it back to mfg to check. Backup scope-still fliers.
Rings/mounts? all tight, done with torque wrench.
Bore leading? Don't have a bore scope, can't see much lead at either end, thorough cleaning didn't help.
Crown? Looks perfect.
Irregular velocity? Tried various tunes from 880 to 945 fps, usually with extreme spreads 14 or less, often below 10. Adjusted regulator as needed, lower for slow tunes and higher for fast ones, from 95 to 120 bar. Still fliers.
Ammo? I sort by weight, and over 95% of these slugs are either 23.2 or 23.4 gr. Don't have a good way to measure them otherwise, and anyway they were working before, and it's the same lot.
Tried pellets again: JSB 18.13, FX 18.1, both of which were good in the past, at 880 fps: problem still there.
Liner is indexed, tried changing that, no better.
Clipping? can see no lead streaks on either end of moderator, none on the end of the shroud. Tried the issued FX shroud. Louder but still fliers, no sign of clipping.
Too fast? Too slow? Tried the above velocity ranges with no improvement.
Liner is a Superior standard, which was working well before.
C1 is tight.
Hold sensitive? Wasn't before, but I tried loose and tighter holds without improvement. Shooting from a good steady bench with a good rear bag and a bipod.
Breech O-ring? Changed it . No better.
Probe? Tight, and adjusted to fit the port in the barrel.
Rail is firmly tightened but not overly so.
Barrel grub screw firmly but not excessively tight. Jam nut holding liner to barrel assembly tight but not too much. Barrel screwed into shroud firmly but not too hard.
What am I missing?
And your list of tried fixes is awesome. 
