DO a search on COW (cream of wheat) or corn meal method.
Generally its a: non magnum primer, a little Unique (I can't say how much for the 7 Mag.) a case full of corn meal, a wad of one square of TP.
Don't let the barrel get too hot.
However, I don't think this method will be all that much of an improvement over what you are doing. Just shoot the bullets at something important.
Even the COW method still needs a "real" round to get the sharp corners (finish the job, so to speak). But the fire forming loads are usually just as accurate as the fully formed loads.
FWIW.....