Answer
The rifle with the shorter barrel
Work Step by Step
a = $\frac{v-u}{t}$
For both rifles, u=0 thus a = $\frac{v}{t}$
Let $a_{short}$ = $\frac{v_{short}}{t_{short}}$
Let $a_{long}$ = $\frac{v_{long}}{t_{long}}$
Since $v_{short}$ > $v_{long}$ and $t_{short}$ < $t_{slong}$,
$a_{short}$ > $a_{long}$
