From d20srd.org:
Right now, at apprentice skill level, the spell is granting me about 18 spell resistance, which means about 50% of the time I would be unaffected by a level 8 character's spells. Against any character over level 17, Antimagic Shell would be useless.To affect a creature that has spell resistance, a spellcaster must make a caster level check (1d20 + caster level) at least equal to the creature’s spell resistance.
Antimagic Shell seems to be the equivalent of Antimagic Field (http://www.d20srd.org/srd/spells/antimagicField.htm), but I'm assuming it confers straight-up spell resistance due to the game mechanics. (?) In that case, since it is a higher-level spell than Spell Resistance, it seems like it should confer at least the same level of spell resistance as the spell:
Thoughts, or am I misunderstanding how this is intended to work?d20srd.org - Spell Resistance wrote:The creature gains spell resistance equal to 12 + your caster level.