I use the same set of dies with a Model 1 Sales 300 Fireball barrel also. I tighten the die down until the press must just "bump over" to close all the way. If that does not fix the problem, I'll almost bet that you need to ream the inside of the neck to allow enough space. When you size the body of a .223 case down, the neck of the 300 Whisper gets thick, whch could be causing your case from chambering all the way. This can also cause some potentially dangerous pressure issues.
I use a Forster inside neck reaming bit in my case trimmer to buzz out the inside of the case necks. I've never had any problems. The bit costs around $28.00 and fits into the Forster case trimmer, and it fits into my old Pacific case trimmer. I think it will also fit the RCBS case trimmer. After removing the crank handle, I use a cordless drill on the shaft to speed things up.
Before I form my 300's, I cut the .223's down to length first, maybe a couple of thousandth's too long for the finished product. I use a drill press with a sanding pad, and a block of wood with the appropriate sized hole bored in it to hold the cartridge in place (don't use your fingers!) Adjust the stroke of the drill press to stop in the right place. If you don't have a drill press, I suppose you could rig up a jig to cut them down with a hacksaw or Dremel tool. I suppose a block of wood would work here too. Just bore the hole deep enough to allow you to use the top of the block of wood as a guide for the saw. Using the drill press seems to help in the forming process because I grind just fast enough to make the top of the cases turn red hot. Seems to anneal them quite nicely!
After that, I deburr the cases, then form the brass in my Redding sizer WITH the expander ball in place. This helps keep the neck from getting getting crushed. From there I ream and trim in one step, then deburr again and presto! you have 300 Whisper's. Make sure you get rid of any brass filings before you size the cases.
Sounds like a lot of trouble, but it's not too bad. I can have about 100 cases in around an hour or so.
You're other alternative is to purchase some .221 Firball cases and neck them UP to 300 Whisper. You won't have to do anything else to these other than perhaps trim them a little.
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