Sabre Defence Scores a Hit
Sunnen's HTB Tube Hone

If the United States' warfighters are to be believed, the .50-caliber machine gun barrels made by U.K.-based Sabre Defence Industries are the best in the business. Part of the reason for that preference, officials at Sabre's Nashville plant believe, is the superior accuracy and easy cleaning that result from the excellent bore geometry and surface finish imparted by a HTB tube hone from Sunnen Products Co. of St. Louis. Since installing the hone, test groups from sample barrels have tightened up nearly 100 percent over the Army's requirement.

Sabre makes two variations of the .50-caliber barrel: the heavy barrel for the M2 Browning gun, and a lighter, shorter version for the M3 aircraft machine gun. Both have a Stellite liner for the chamber throat and first few inches of rifling. The liner and a retainer for attaching the barrel to the receiver are both shrink-fitted. The Stellite liner (75% cobalt and 25% chrome) withstands the intense heat and gas erosion of the initial discharge better than any ordnance steel.

The heavy barrel starts as a 45-in. piece of bar stock, 2.625 in. in diameter, weighing about 73 pounds. "For the heavy barrel we use MIL-S-46047, a special alloy with extra vanadium to increase life," says Charles Shearon, general manager of the Nashville plant. "We have to buy these steels by mill run. The heavy-barrel material is cut by the mill and heat- treated when we receive it. We do some preliminary operations to prep it, then gun-drill the chamber end with a 0.75-in. hole about 11-in. deep. A temporary liner is then installed, and the rest of the barrel is gun-drilled with a 0.490-in. hole. We ream after drilling, and have a hole-size tolerance of ± 0.001 in. at that point, but the next step is to stress-relieve, and that often changes the bore. Honing allows us to control the bore's final geometry and hole size to a fraction of the allowable MIL-Spec, which is helpful because of the small variations introduced later with button rifling and chrome plating."

Sabre had been using a manual lapping machine to finish bores, but the increase in military orders resulted in a bottleneck of work at that operation. To answer this challenge, the company installed a Sunnen HTB-2000 tube hone system in early 2006. Equipped with Borazon CBN stones, a traveling steady rest and whip-guide bushings, the PLC-controlled machine has an output of 10–12 barrels per hour, compared to about one an hour with the old process. "Most important to us is the automatic gauging system," says Garry Hogan, Sabre's plant manager. "The machine gauges the bore after every stroke, allowing us to control hole size, roundness, and straightness to 0.0005 in. without operator intervention."

The crosshatch pattern that honing leaves on the bore surface aids in rifling the barrel by maintaining a consistent lubricant film. To create the rifling, a 0.517-in. carbide button is pushed through the bore, which is 0.503 in. at this stage. The button has the rifling form in high relief on it and is rotated at the correct twist rate. The lands on the button engrave the grooves in the bore. "The very round hole we get with the hone helps prevent high and low spots in the rifling, and keeps the grooves concentric with the bore, all of which aid accuracy," Hogan says. "We're unique in the business in that we make our own buttons, too, which gives better control of our quality."

Surface finish of the bore coming off the hone is approximately 20 µin. R a , and drops into the low teens after button r i fling, significantly exceeding the MIL-Spec of 63 µin. This helps to maximize muzzle velocity and makes it harder for metal and powder fouling to get a toehold, so cleaning is easier. "In the precision rifle shooting community, the sweet spot for surface finish is considered 10–20 µin.," says Hogan. "Though it seems contradictory, a smoother surface actually increases surface contact and friction with the bullet jacket, causing increased copper fouling."

"We're lucky to be able to interact with users of our products," Shearon says. "At a recent armor-warfighting symposium at Fort Knox earlier this year, we had several tank gunners tell us they knew as soon as they shot a .50 if it was a Sabre barrel or not. If it wasn't, they found one of ours and installed it. We're sure our commercial rifles will earn the same loyalty, too, as we ramp up production and get the word out."

Sunnen's HTB Tube Hone

Benefits:

• A PLC control and digital display allows complete control and monitoring of machine conditions.
• A built-in load meter indicates high and low spots in the tube bore so the operator can adjust the stroke as required for optimum cycle time.
• The Sunnen Modular Honing Tool System continually compensates for stock removal and stone wear to keep the machine running at peak efficiency.
• Available in two-meter, four-meter, six-meter, and eight-meter models

www.sunnen.com