17 fireball

[Rapier]

Bob,
This is a note from Handloader Magazine about the 17s.

"Still the list goes on. The .17 Squirrel is simply another version of the .22 Hornet case with the shoulder pushed back slightly; the .17-221 is an early version of the .17 Mach IV with its shoulder angle moved back. Later, an improved .17 Mach IV somewhat overpowered the original version by incorporating a sharper shoulder angle."

If I read the article correctly, the .17 M4 has a sharper shoulder angle, thus it should have a longer body, in order to achieve more capacity, than the .17x221. You should be able to chamber the .17x221 (.17 Fireball) in a .17 M4 chamber. If chambered, fireforming the .17x221 to .17 M4 should be exactly the same as creating an improved case. It appears that the .17 M4 is simply an improved .17x221. Once the case is fireformed it will require an M4 set of dies to reload.

The only caution is that there may be two versions of the .17 M4.
Best,
Ed