FX Bobcat MkII .30 cal. with side fill port question.

 If your taking about a regular fill "probe" with two orings and a tine hole between them it is the air pressure trapped between the two orings that holds it in place, it would be very hard ( and NOT safe) to remove the fill probe before bleeding. 
Same sort of idea on the single shot big bores with a sliding loading port ( think 909 bigbore or Fire 201) the brass section slides easily for loading, has an oring front and rear of the loading port, as it slides with figure pressure you might think it wouldn't seal well but for the brief moment while psi is inside the oring do indeed seal 100%.


John