This is a discussion on UNITIZED GAS CYLINDERS within the Ted Brown forums, part of the Gun Professionals category; I get asked a lot about unitized gas cylinders, welded or screwed together, and why the big difference in cost. I just finished unitizing a ...
I get asked a lot about unitized gas cylinders, welded or screwed together, and why the big difference in cost. I just finished unitizing a half dozen and thought it would be a good time to answer some of these questions.
First off, either type will give the results we want to achieve enhanced accuracy. However, unitizing by itself is not going to make a rifle shoot sub minute of angle. It ties in with glass bedding. Bedding firmly secures the action into the stock and the unitized gas cylinder secures the front end, stabilizing the barrel and it's relation to the stock. Bedding and unitizing, together, are the single most effective accurizing enhancements one can do.
The Navy, Marines, and Air Force use welded together gas systems. It works well enough and it's easy (and cheap) to do. However, welds sometimes fail and a cracked weld can be very detrimental to accuracy. It's also almost impossible to fix in the field or in the middle of a rifle match. Most of the time the failed welded parts must get tossed and replaced with new.
The Army MTU and National Guard used screwed together unitized gas systems. They are much harder to fabricate, labor intensive, and cost more. The reason for the higher cost is the complexity of the unitizing operation. Spindle valves and barrel bands must be annealed before they can be drilled. Drilling the screw holes is a four step operation doubled because there are two holes to each gas cylinder. That's eight operations for each gas cylinder after which the spindle valve must be tapped to accept the screws. Special drilling jigs are required and a high degree of skill since all the parts are a very close fit and it's really easy to screw up (no pun intended).
The screws must be epoxied and staked in place to prevent them from shooting loose. It's not required to refinish the gas cylinder, but the barrel band looks a lot better if refinished after annealing. By the way, you'll need an annealing furnace for the valves. They must be heated to 1200 F. for an hour and allowed to cool down until they can be handled, not quenched. The barrel bands can be spot annealed with a torch.
I don't bend up the tabs on the barrel band until the hand guard is installed. It just works better for me. I haven't had any reports of my unitized cylinders coming loose. The nice thing is that if they do ever get loose screws it only takes a screw driver to repair and that can be done even at a match.
Keep in mind that many things done by the military are done for expediency. Welding is easy and fast even if it doesn't look pretty the way the military did it. The MTU method is labor intensive, but has some definite advantages. They also suggested (or required) that the holes in the barrel band be enlarged for clearance. The big reason for that is, again, efficiency. It allows a degree of slop on workmanship while preventing miss-alignment issues. Barrel band contact with the barrel is not the issue since all the parts are in contact anyway. The goal in unitizing is to prevent the barrel band from moving and relocating under recoil and torque forces when the rifle is fired. The military may have had to modify a hundred units at a time so shortcuts were taken. I'm sure they didn't take the time and effort to do these modifications as precisely as I try to do them.
Can you give us an idea of the cost differences - just the percentage maybe. Guess I'm wondering if buying two welded (1 to install, 1 carried as spare) would be more cost effective over a single screwed-type that would be field repairable. Understanding that the replacement welded unit would likely mandate re-zeroing or am I all wet on that point?
Changing the unitized gas cylinder may cause a change in the rifles zeros. It depends on tolerances between the barrel band and the center line of the barrel. Two different gas cylinder assemblies may have attributes that effect the zero.
Costs vary depending on who does the unitizing and what parts are used. the cheapest being factory Springfield units using commercial parts, welded together. Professional armorers using NOS GI parts are going to charge more for their work and parts, not to mention the type of unitizing used. The cost of a unitized gas cylinder will vary from approximately $125 to $285.