M1A Accuracy Issues: How to Unitize an M-14 Gas System
September 21, 2012
When an M-14 type rifle is optimized for accuracy, the stock fit will be adjusted (possibly bedded) for consistent downward tension on the front band and the handguard will not touch the stock. This barrel tension is critical to match grade accuracy. Unlike bolt guns, M-14s do not shoot well with free floated barrels.
Why? No idea. To paraphrase Dr. McCoy from the original Star Trek TV show, "Dammit Jim, I'm just a simple country gunsmith, not an engineer." It works. Trust me.
The front band is sandwiched between the gas cylinder and the barrel shoulder, and it will move around a bit on a standard rifle, creating inconsistent barrel tension. Many years ago, military rifle teams developed methods of "unitizing" the gas system and front band. This means the front band is attached solidly to the gas cylinder resulting in the same barrel tension shot after shot. There are two common methods of unitizing: welding, or drill and tap.
Welding the band to the cylinder must be done carefully to avoid creating tight spots in the cylinder bore. The USMC attached front bands to the rear gas cylinder mounting ring with three small TIG welds. The cylinders from Quantico were so cleanly done on the inside of the ring that the welds could not be seen when the rifle was assembled. On some commercially welded assemblies, the front band is welded directly to the cylinder with a heavy bead risking distortion of the cylinder bore.
I am not a welder and don't pretend to be one, so I use the old AMU/NGMTU "screw and glue" method of unitizing. The front band, gas cylinder and spindle valve are assembled in a fixture, and two holes are drilled through through all three parts. Spindle valves are very hard and must be annealed before drilling. The parts are then removed from the fixture, and the holes in the front band and cylinder are clearance drilled slightly larger so 6-32 machine screws will pass through them. The holes in the spindle valve are tapped and the band is countersunk so the flat head screws will seat flush. The screws are permanently epoxied in place during assembly. Correctly done, this should result in a permanent assembly with no cylinder bore distortion.
Countersink the Screw Heads
The front band is countersunk for the screw heads.
Tap the Valve
The spindle valve is tapped for 6-32 screws.
Install the Screws
The screws are installed with epoxy on final assembly.
Accurate rifles must maintain consistent tension between the front band and stock ferrule.
Assembly and Drilling
The front band, gas cylinder and spindle valve are assembled in a fixture and drilled for screws.