In essence, the M4 should make plenty enough noise to show up on the DPD dictabelt recording, while delivering a low enough level of energy on target to arguably match the known ballistic effects.
The M4 also should have a fairly high ballistic arc, allowing for the difficulties in range estimation that I suspect are reflected in the four misses out of the five shots fired with the low powered small caliber weapon.
For a couple of days I've been ticked off because the National Firearms Act of 1934 placed such onerous restrictions on legal ownership and transfer of the M4, due to its short barrel, that I am effectively locked out of the weapon's price range. A member of the International Ammunition Association then suggested I could buy a Thompson/Center single shot pistol in .22 Hornet with a 14 inch barrel -- and that's a great idea -- but now I'm ticked off because I never wanted to own such a weapon and I'll have to pay top dollar for a new one because it's a bit unusual.
This has brought me to a realization regarding one of the realities of running Keg Island Research:
I don't need the guns I want. I want the guns I need.
Okay. Done with the pity party.
The good news is that with the T/C I'll be able to properly test the acoustics of 1960s - era ammunition from a 14 inch barrel, both for its terminal ballistics and acoustic properties.
Summer break for the kids is on the way, so I've got to get my gear together and prep for this test session in the mountains.