I like to look at how high a ratio of shots a player has got on target and forced a save to judge how accurate a striker is.
According to the Football League Stats Centre this season (league only):
Barnes: 28 on target. 28 off target. Therefore 50% of his shots hit the target. Overall scoring to shot rate is eight goals from 56 shots, or a goal from roughly 14% of his overall efforts.
Mackail-Smith: 28 on target. 20 off target. Around 58% of shots therefore force a save, block etc. Overall scoring to shot rate is 11 goals from 48 shots, or a goal from 22% of his attempts.
CMS also only played in 29 league games this last season. That means he scores a goal roughly every two and a half games. Barnes' ratio is around a goal every four games.
Of course, he has not played in the central striking slot anywhere near as often as CMS and Barnes' shots are probably coming from further out and from wider areas so more likely to miss the target.
I'm quite cynical of my own shots stats (which were league and cup), because I take them from espnsoccernet after the game. The get them from the AP, which can make mistakes (a couple of assists given to number 4 hammond instead of number 14 calderon last season), yet, when I go back to espensoccernet at a later date they seem to have changed them, or at leas their totals under player profiles don't tend to match what you get if you tally them match by match.