Sniper rifles don't count for the achievement. That's been the case for awhile and isn't considered a bug. While it may be disappointing, Batman and Scripted Cheater are meant to be hard even for highly skilled players. Compared to other weapon types sniper rifles are very easy to use to get long killstreaks, and those achievements would be much easier to acquire if they were allowed. It's the same reason Dasanfall doesn't count sniper rifles in their infantry versus infantry scores.