Is it safe to use steel shot in my 10-bore shotguns?

Is it safe to use steel shot in my 10-bore shotguns?

Steel shot: I have two 10-bores. One is a Gunmark Kestrel with 3.5in chambers rated at 1,200kg/cm2. The right bore is machined to 0.751in and the left bore 0.729in. The second gun is a Zabala LP71, which also has 3.5in chambers rated at 1,000kg/cm2.

Why are some shotguns coloured blue and brown?

The receivers on most over-unders shotguns are given a plain silver finish but every now and again I see pictures of some with lots of blue and brown marks on the surface. What is this, and why are they not polished smooth like all the others?

Why are some shotguns coloured blue and brown?

The receivers on most over-unders guns are given a plain silver finish but every now and again I see pictures of some with lots of blue and brown marks on the surface. What is this, and why are they not polished smooth like all the others?