Bullet impacting mortar shell/grenade?!

[Caradhras]

In my campaign some 'bad guys' were in cover (behind wall) and my player (sniper with Barrett) dropped one that made a run for it. He asked me if he could see any grenades on the body through his sights as he had fallen not far from the corner. I knew the NPCs had a 60mm mortar and rolled a chance this guy was carrying shells..he was. The player makes his roll to spot one, aimed shot @ 1/2 normal to hit it...and boom.

Now, I dont know how unrealistic this was. But my sniper player has a taste for it now so I would like to know before I compund my error - if any - for allowing it.