I've been dealing with the same issues.
 

Fired 2 fouling shots then a 5 shot group. First 3 shots went into 1/2", next two were 3/4" and 2" higher.
Velocities
1-1033 fps
2-1030
3-1033
4-1043
5-1049
When I got home I weighed cases and found that three weighed less than the other 2. I think it was only by about a half a grain. I'd have to check my notes.
You need to be careful when shortening seating depth. Pressure and velocities rise quite a bit.
So far I've seen the smallest Extreme spreads and Standard deviations with AA 5744. IMR 4227 was pretty good also.

More info
 

Checked notes this AM.
SSK upper 16.25" barrel gas port on Lo press. setting
220 grn Sierra Matchking
10 grn AA 5744 starting load from Western Powders
Rem 7 1/2 primer/ WP used WSR
OAL 2.100"
3- cases weighed 84 grns
2- cases weighed 83.5
I'll have to test further by weighing cases and loading to diff. weights and check vel. & group placement on target.

Brass weight is an indicator of case capacity, but since brass has a specific density of about 8.5 1/2 grain case differece is only equicvalent to 0.06 grains difference in case capacity.

From the above data I'd suggest dropping the loads to get under 1025 fps and weigh each load carefully. When I'm trying for precision loads in small cases I weigh each charged to +/- 0.02 grains on a lab scale. It's a bit tedious but the results are worth it. Consider that subsonics are 1/3 the speed of typical bechnrest bulelts so they have 9 times the drop and vertical stringing for the same percent velocty variation. . Also subsonics have to deal with transonic stability issues. You have to use benchrest techniques loading subsonic ammo if you want to achieve the accuray at 100 yards of typical commerical supersoinic ammo, like a 223.

Bullet and brass weight isn't as criical as charge weight by a factor of at least five, but I sort brass and bullets to 0.1 grain. Better bullet manufacturers and some brass manufacturers have already done that.

Don't think that it's easy to get good accuracy in subsonics just because they're small and slow. It's just the opposite.

If you are making bolt gun only ammo or AR ammo where you will just put a few rounds into a mag, sure - seat to 2.26 (for a BLACKOUT chamber only).

Which specific bullet is your 2.26" recommendation for? Are you assuming 220 Sierra Matchkings? If not, what? Do you have a recommendation for seating the 240 SMKs in a 300 BLK case usd in a Rem SA action with the AAC 700 barrel?

I meant for the Sierra 220. The 240 should be the same though.

RobRat, did you ever get your accuracy issue worked out?