There is a myth that's developed about micro-grooved rifling. They can accurately shoot cast bullets. The trick is to shoot bullets 0.001" over jacketed bullet size. Regardless, the question was is it safe. Assuming the rifle is Ballard cut it will shoot safely and well with nominal diameter cast bullets. If microgrooved, yes it's safe but accuracy may be disappointing unless you are shooting slighly larger diameter bullets.
The rule for shooting lead in a micro-groove is "Big and Hard" I shoot laser cast bullets out of my Marlin 30/30 those bullets are very hard. If you keep em under 1300 you don't even need gas checks. The point is that I sluged my barrel at .308 and shoot .310 bullets out of it and the guy that taught me sluged at .3085 and he shoots .311 bullets out of his. We both found these rounds to be very accurate at 100 yards with no leading of the bore. So if you want to shoot lead out of a micro-grooved barrel the bullets must be hard and the bullets have to fit tight.