these muffs fit 100% under ACH - mine is large sizs and it works fine with both types of chin straps. i was unsure of geting this because i was unsure it it would fit so i rolled the dice and won. it fits and works great. don't wory about the batt. replace ment its not hard pop off cup and replade and put back its not hard so many say it is but ? i think they r just scaird to break them. it does take some med force but u will be fine thay are quite strong and were mabe to pop off and back on this way