'over the top' field guns codex entry not working

I had the same issue until today (15/11/2016). The church gun didn't appear (the one to the left as you drive up on the church) but I carried on, and the achievement finally popped. Make sure you don't let one of the ones next to the depot (just before the bridge at the end) get destroyed by the massive collateral explosion of the depot (shoot the one on the right in front of the depot first, then the one on the left). I shot 35 guns in total