Hi Chris,
The Makarov was originally intended to shoot steel BBs and the factory claim to have undertaken long term testing with no ill effects.
However, I tend not to use steel BBs as I consider them to be less safe. This is because they have a tendency to bounce back from hard surfaces and they also damage steel targets. Both of the clubs I belong to ban their use on the range due to the increased risk of ricochets and I fully support this.
You can of course use a padded backstop such as a box filled with suitable soft materials but that kind of limits the targets you use to paper or similar soft materials. I have found the copper coated lead ball made by H&N and marketed under a number of different brands such as Excite or Accu BB work well in a Mak, along with GAMO ball. Some Maks prefer one over the other, so it is worth experimenting. I also tend not to fill magazines to capacity and load no more than 9-10 to avoid the bottom ball or balls deforming.
Kind regards,
John