It must be a difficult object visually too with that bright mag 4.4+6.0 (dist 0.36") double star mu Ori next to it.
Abell 12, mag 12.4 and 37" across, is less than 3 degrees from Betelgeuse.
I found this description by Steve Coe at:
http://www.saguaroastro.org/content/obsnotes/ORION.htm
"PK198- 6.1 Abell 12 Sentinel S=7 T=9 13" 220X Pretty faint, pretty small, round, little brighter middle. Just seen at 220X in the glow of Mu ORI. 330X helps the contrast, the UHC filter helps a lot, the glow is much larger and more contrasty with the filter."
He used a 13" scope.
Details on mu Ori are here:
http://adsabs.harvard.edu/abs/2002AJ....123.1723F