Geissele 10.5" M-LOK MK4 Black Super Modular Rail 05-1656B

Geissele 10.5" M-LOK MK4 Black Super Modular Rail 05-1656B is basically the slightly-longer sibling to Geissele's Federal-contract 10" rail. Same guts, same engineering, just a half inch more room up front if you need it for a light or a bipod mount without crowding everything else.

It's machined from 7000-series aluminum, Type III hardcoat anodized, and locks onto the receiver with Geissele's proprietary 2.25" barrel nut - which sounds like a minor detail until you realize it's the reason these rails hold zero after you've pulled the handguard off for cleaning and put it back on. Standard nuts don't have nearly that much surface contact with the receiver, so they wobble more than people expect. A full-length Picatinny rail runs the top, and there are extra Picatinny sections up front at 3, 6, and 9 o'clock for anything that needs a hard mount instead of M-LOK.

Speaking of M-LOK - those slots sit at 3, 6, 9, and the upper 45° quadrants, and the lower sections are vented pretty aggressively, which does double duty: cuts weight and helps a hot barrel cool faster. All told it comes in around 10.6 ounces bare, closer to 12.7 with the barrel nut on. Not the lightest rail out there, but it's not trying to be - it's trying to not move.

Key Features:

• 10.5" free-float length, full-length Picatinny top rail
• M-LOK slots at 3, 6, 9 o'clock plus upper 45° quadrants
• 7000-series aluminum, Type III hardcoat anodized
• Proprietary 2.25" Geissele barrel nut
• Forward Picatinny segments at 3, 6, 9 o'clock
• Weight: 10.6 oz bare, 12.74 oz w/ barrel nut
• Inner diameter 1.35", outer diameter 2.11"
• Fits mil-spec M4/AR-15 upper receivers

If the 10" Federal rail is a hair too short for what you're mounting, this is the easy upgrade.