Use #8 Magnum shot. Harder shot deforms less. The rounder pellet gives tighter patterns. Use the maximum weight/amount of shot allowed 1 1/8 oz. Magnum shot will weigh less than standard shot, the shot bushing can be adjusted to drop a full 1 1/8 oz, resulting in a higher pellet count.
If your load is 1200 fps, the pattern will get tighter with less velocity like 1145fps. A slower burn rate powder that gives the same velocity may deform pellets less. For 27yds, you will need 1200 fps to keep the same lead used at 16yds. Larger 7 1/2 shot will deform less then #8, but will leave holes in the pattern. If all else fails, plastic buffer may be place in with the shot. http://carneyro.dot5hosting.com/page9.html