Grenade launching with a sniper rifle would be a "jarring experience" for the optics to say the least.
You have to be careful with M1Ds as they can be put together from parts quite easily. The giveaway on these is the unique M1D barrel which has the drawing number, including a "555", stamped on top of the barrel under the rear handguard. Any barrel can be made to work, but there is an authentic M1D barrel. Also, the stocks on the early Korean war vintage M1Ds will normally have an open box cartouche on the left side of the stock under the cheekpad. These will bear the letters "SA" along with another letter, like an "F" or an "M" below the cartouche. These are not unique M1D stocks, but they were used by Springfield Armory during the early 1950s. An M1D stock will normally also have 2 holes drilled to accomodate the cheekpad screws.
M1Cs are a different matter. They are very tough to fake as the receivers were made within known serial number ranges.