To signal for help with a whistle you sound three blasts. Do this in a consistent interval until help comes. The plus side to this whistle is you’ll have a conversation starter when help finally comes.
Step 1: File out a notch
Grip the shell in a clamp. Make sure to clamp the primer side of the shell or it will collapse. Also if you don’t have a wooded clamp you can improvise to protect from scratching up the brass with something like an old leather belt.
Using a ¼” triangle file, file a notch ¼” down from the taper line on the shell. Make sure to keep the leading edge of the file at 90 degrees. See the pictures. Keep filing until the trail edge of the file is flush with the shell wall.
Remove any burs.
Step 2: Prep the .22 shell
Use metal shears to cut off the excess. Smooth out any rough edges with a hobby file.