We remember the first MUR in early 2000s, it was, and remains one of our favorite upper receivers, from one of our favorite companies, Vltor. We're proud to offer MUR in our own store, things have a funny way of working out.
This early 2000 era MUR was one of the first MURs released, and we still have it :)

The MUR is a rigid Mil-Spec flat-top upper made to fit all AR15/M16
M4 rifles and carbines. The MUR was developed to give the customer a
Mil-Spec correct upper receiver with a thicker wall, making the MUR
upper receiver a stronger platform. This more rigid upper receiver will
increase accuracy and reliability, while extending the life of the
interfacing components. The MUR-1A model comes equipped with a shell
deflector and forward assist. The MUR-1S comes with a one-piece machined
shell deflector only.
FEATURES:
- Aircraft-grade aluminum construction
- Machined from 7075 Forging
- Stress relieved, heat treated, aged twice and cryogenic treated
- Hard coat anodized, black per MIL-A-8625 TYPE III Class 2
- Precision machined and finished by hand
- Bore coated with dry film lube over the anodized surface
- Compatible with all Mil-Spec AR-15 parts and accessories
- Available with (MUR-1A) or without forward assist (MUR-1S)
- M4 feed ramps