The 5.7x28 was plastic coated brass (might still be) for extraction purposes. It wears of patchy after a few reloads - MEK peroxide with finish removing it. Chamber pressure are still high enough at unlocking and moves the shoulder forward, so you will be full length sizing the brass every time in autoloaders. I helped a guy on gunbroker figure out loading for it IIRC he had the FN pistol.
I'd bet money these will fit your magazine.
I'm near 100,000 rounds of 44 Mag shot in the last 30 years. Most all of it was my own cast bullet 245 gr Keith Lyman "# 429421". From a good handgun they will shoot threw an elk, often found under the hide on the off side