[daveinsvaz]

Living near a large military base, I can go to the traing areas. Generally can find unfired 223 blanks.
I take an unfired blank, run it into a Forester trim die. Cut off the exposed part about 1/8 hign. Remove, dump blank powder (wife puts it in her garden...phosperous and nitrogen. Use a modified Lee case trimer to trim length. Run into the de-prime die. Presto, a primed 300 Whisper case.
I know, some one will say the cases are defective (it's got a canleture...so does m118LR!), or too thick. Have a neck trimmer, but haven't had to use it yet.

Has worked well, no splits, no signs of pressure. Use 14 grains of H110 and 147 grain FMJ military bullets.