It has to be remembered that, due to MTCR convention, we are not supposed to sell any missiles with ranges more than 300km. So there is no point in declaring that Atmaca-UM has a range of more than that.
280+km covers what it needs to cover.
With the KTJ-3700 powering it where specific fuel consumption is lower than the engine Roketsan was using for its sea attack brethren; the overall range of this new missile is extended considerably. But they will not let it come out in to the open so that it will please a few of us defence industry enthusiasts, unless they want to make a point of it to an adversary, like in the case of Tayfun missile. Even then we don’t really know what the final exact range of that missile is.
We know that the Atmaca missile has a very good seeker head when it was tested without GPS sometime ago ; and was very accurate. The engine the UM version carries is supposed to be usable at 10K metre altitude. That means it has no thrust issues and doesn’t strain under less oxygen.