The MPAR is held together by a pivot pin and a breakdown pin, and can open up fairly realistically. (Although the pivot pin is oversized for durability)
The bolt carrier group is a compromise between fidelity, buildability, and sturdiness. It's integrated with the charging handle because otherwise there would be no way to push it forward (except a spring, and I wouldn't require that). It's cut short almost like an LR300's just so it wouldn't get caught on anything, and also because the cylinder is stronger when the front and back walls are closer together.