I used to shoot NRA Hunter's Pistol Silhouette matches at the old K.I. Sawyer AFB outside Marquette, MI many years ago. The gate guards had what looked like M15s from my truck. They were all long-barreled, never saw an M56.

Most looked in pretty good shape but one I used to notice was visibly rusted.

When the base closed the range we shot on was destroyed. This despite the fact that the local police academy was setting up shop
on the old base. The rationale was lead contamination.

I figure it was just a bunch of bureaucratic types wasting our tax
money.