I forgot to measure mine when I got home last night but will tonight, that seems to be very close to mine though if memory serves. As far as length, mine is too short (that's what she said) but works. If I were doing it again I would measure from the top of the die to the inside base of a case with the ram fully extended and add 1/4 inch. Use that length for the smaller diameter machined portion then a slightly larger head to hit with the deadblow hammer, obviously stop hitting before the punch bottoms out in the case. Mine seem to be formed after 2 solid whacks but I did 4 just for extra assurance.
Just as a caution, make sure to go .001" under your measurement for the neck ID, don't just use mine, different size dies measure slightly different and brass thickness at the neck comes into play here too.
Edit/Addition: I just got home and took a measurement on my setup. My die was leaving an ID in the case neck of .306" in my Federal pull-down brass so my punch is sized .305". With this I still get a small mist/spray of water with every hit of the hammer but I think this is necessary to maintain at least a little clearance and not get the punch stuck. YMMV
Last Edit: Aug 16, 2015 1:54:13 GMT -8 by mosigdude
Mine id necks measure .303 sized. .306 sounds like a fired case maybe its just the difference. Ive been shooting some formed cases and am up to 29.8 grs of RL 7 with accuracy holding and no really pressure signs. A little primer flattening but nothing to stop at. So the formed brass seems to give this round a boost. Anyways im going to try your method out soon as I can get a plunger made. I think 90% would be interesting to try. Sounds like your getting closer to 100% since your water weight is the same.