Honing Scores with Machine Gunners
The classic Browning .50-caliber machine gun enjoys preferred status with American troops for many reasons, not the least of which that it is a straight shooter.
Part of the reason for that preference is the superior accuracy and easy cleaning that results from the bore geometry and surface finish of barrels made by UKbased Sabre Defence Industries. Since installing a Sunnen HTB tube hone from Sunnen Products Co. (St. Louis, MO) at its Nashville, TN plant, test fire groupings from sample barrels have tightened up nearly 100% over the Army’s requirement.
“We have been making military .50-caliber barrels and guns since 1979, as well as commercial rifle barrels for various companies over the years,” explains Charles Shearon, general manager of the Nashville plant, which was acquired by Sabre in late 2002. “The new owner, Guy Savage, planned to reposition our operation to be about 50/50 military and commercial,” says Shearon.
The Nashville plant was to be a launch pad for US production of Sabre’s XR15/16 rifles, a premium variant of the AR15/16 for the civilian and police markets. The start of the Iraq war in early 2003 changed that.
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“We received military orders for our .50-caliber machine-gun barrels, 7.62-mm M60 machine gun barrels, M134 minigun barrels, and M6 weapons mount. The demand for .50-caliber barrels grew in the next few years from about 100 per month to 1200 per month and employment at the plant was ramped up from 15 to 85,” says Shearon.
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-fit. 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″ (1143-mm) piece of bar stock 2.625″ (66.7 mm) in diam, that weighs about 73 lb (33 kg). “For the heavy barrel we use MIL-S46047, a special alloy with extra vanadium to increase life,” explains Shearon. The aircraft barrel is MIL-S11595, which is also used on Sabre’s commercial guns. Steel is bought by the mill run, cut by the mill, and heattreated when received.
“We do some preliminary operations to prep it, then gun-drill the chamber end with a 0.75″ [19-mm] diam hole about 11″ [279-mm] deep. A temporary liner is then installed and the rest of the barrel is gundrilled with a 0.490″ [12.45-mm] diam hole. We ream after drilling and have a hole size tolerance of ±0.001″ [0.03 mm] at that point, but the next step is to stress relieve and that often changes the bore.”
Honing allows Sabre to control final geometry and hole size of the bore to a fraction of the allowable MIL-Spec, which is helpful because of the small variations introduced later by burton 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. “We consulted with Ron Williams, Sunnen’s senior field engineer in our area, on how current technology could improve our processes,” says Shearon.
The result was the installation of a Sunnen HTB-2000 tube hone system in early 2006. Equipped with Borazon CBN stones, a traveling steadyrest, and whip guide bushings, the PLC-controlled machine has an output of 10-12 barrels an hour compared to about one an hour with the old process.
“The load-sensing system on the machine automatically adjusts the stone feed for optimum stock removal without tool crashes, which reduces our labor and helps improve output,” explains Garry Hogan, Sabre’s plant manager. “More important to us is the automatic gaging system. The machine gages the bore after every stroke, allowing us to control hole size, roundness, and straightness to 0.0005″ [0.013 mm] without operator intervention.
“Even after button rifling and plating, we are able to stay well below the MIL-Spec of ±0.004″ [0.10 mm] on bore dimensions, which is quite a feat on a bore length of 33″ [838 mm]. Tool life for the process varies with the amount of stock removal, which typically runs 0.002-0.004″ [0.05-0.10 mm],” Hogan explains.
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″ (13.1-mm) carbide button is pushed through the bore, which is 0.503″ (12.8 mm) at this stage. The button has the rifling form in high relief on it, and is rotated at the correct twist rate. 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 adds, “We’re unique in the business in that we make our own buttons, too, which gives better control of our quality.”