Maybe he (Gallagher) was shooting at crabs, out of boredom. 22's are still relatively cheap by the 'brick'; I wouldn't assume that it would be regarded as spendthrift for him to have decided to enjoy a bit of plinking.
To have shot the plastron - that being the lower 'plate' of a turtle vs. the carapace, which is the upper 'dome', as it were - on a turtle normally-fixed upon the earth, implies either -
a) to have shot from above, i.e. from whence the turtle's innards once (but no longer) were, that the turtle was dismantled, e.g. carapace, etc. removed for harvest, etc. prior to the act, or
b) to have shot from 'below' that the creature was on it's proverbial back somehow (thus 'dispatch' might be possible - but this seems most unusual to the writer and unlikely to have happened) - or that the plastron was merely lying upon the earth belly-up, quite long removed from the dead creature.
Randomly shooting a dead turtle's partial remains seems the more likely scenario, somehow - and with 22's being lovely plinking instruments due to low cost and recoil, it was perhaps done for mere sport - by Gallagher.
The value in this to me seems to be some degree of affirmation of Gallagher having been where the shot critter was found, which adds weight to our supposition of the 7 site being 'the place' of the wretched castaway. The bird bones, shot turtle shell, etc. all seem to lean toward the 7 site being 'the place' of the Gallagher-found castaway, as the bullet hole and 22 casings seem to point to that gent - argue as one will just whom that hapless wretch of the thirteen bones was, if other than Earhart.