Or otherwise, just say that whatever was used at Dagorlad (3440 S.A.) was the best of pre-gunpowder technology. And that in 3018 T.A., gunpowder research was at its crudest stage. I thought that from Dagorlad, they might have developed gunpowder weapons to such an extent that they might have M1s or bazookas to throw at Mordor Orcs or B-2s to bomb Barad-Dur or the Black Gate.
There is no blasters in the scenario I laid out. The counterattack should have been of rail guns (still small enough to be carried over by infantry) and the rail guns could be dismissed as being magic implements, as would be all gunpowder-based weapons in Middle-Earth.
From what I know, the Mordor Orcs would have only have two occasions to capture the technology: either by capturing Minas ithil, or Osgiliath.