3 year old thread, you're lucky I seen it.

I made a bushing two times as long as the button and the same OD as
the barrel. Bore it so the button just slides easily. Clamped barrel
and bushing in a long "V" block so it was right on center under the
hydraulic cylinder. There was a small divot in the center of the
cylinder top locate the top of the push rod. I used six inch long HSS
rods just smaller than the bore. ground a small taper on the ends so
they would not catch and score the rifling.

Push to bushing, retract, remove bushing, push to barrel, retract,
repeat.