The M4 flattop upper is essentially the same as the A4 upper. It has a Picatinny rail that supports pretty much every optic and accessory on the market. This makes it the most popular choice for most factory and custom AR-15s. Complete vs Stripped Upper Receivers Complete upper assemblies a...
The XPS2-300 uses the standard mounting system that quickly and easily mounts to any standard 1-inch Weaver or Picatinny rail and will re-zero within 2 MOA if removed and remounted — not perfect, but not bad. This is close enough to get you back on target and make re-zeroing simple ...