503rd Parachute Infantry Regiment.

[painter777]

GI-Bob,
Is it POP QUIZ TIME ?
Gotta be 20 or 30 rd Thompson.... or maybe M3 30 rd Grease gun mags.... Or both.... because they look like round nose .45's.
I don't think the bullets look long enough to be .30 cal for the BAR.
And I don't know much about the Johnsons, but I think they had a Rotary mag...... not sure about the caliber without looking it up.

Read where the Carbine was a concern when fired:

From page 95 in the Book linked below-
They found the report of the Carbine closely resembled that of the Jap .256 rifle.
They had contemplated replacing the Carbine with the M1 because of the confusion.

Blossoming Silk Against the Rising Sun: U.S. and Japanese Paratroopers at ... - Gene Eric Salecker - Google Books