Im new to bowhunting and am going to use nap spitfire mechanicals on my easton epic 340s. When I screwed the heads on they seem to be much bigger around at the base than my arrow. I havent gotten a chance to shoot them yet but was wondering if this is normal and if it will cause bad flight?

No it's not a problem and you should be fine. Spitfires are very good mechanical heads, just be sure you have enough Kinetic energy to push them through.
www.broadheadtests.com

youll be fine itll actually help because the broadhead will open a larger then shaft hole and that mean less friction with the shaft. my dad uses those heads and loves them.

they are good heads and the size differense is ok hey wv hunter where in wv u from im in fayette co.