Complex sentence: Due to concerns about stopping power, reliability and problems with using rimmed rimfire ammunition the MKR project was abandoned at the prototype stage and thus the 4.5mm MKR never saw service or any kind of widespread use.
Split into simpler sentences: Due to concerns about stopping power, reliability, and problems with using rimfire ammunition, the MKR project was abandoned at the prototype stage. The 4.5mm MKR never saw service or any kind of widespread use.