I have resealed a 310 and...

Yup that nut was on mine crazy tight. I suppose it was age and what not but that nut didnt want to budge and I had to get rough with it. Clamp the action in a vice (carefully as to not mar anything) and then use a larger leverage thing on the rear of the special tool and then give it a sharp wack and keep your fingers crossed. I was going to use some heat but I managed to get mine off ok. Now putting that back together isnt any fun either as that model has to have the air transfer hole line up just right and also those lead seals are kind of a B to work with as you dont have much threads to start that nut back on.

Best of luck to you.