Barrel-Making Goes High-Tech

In gunsmithing circles, the name John Moses Browning is spoken with respect that borders on reverence. As well it should be. Even today, over 80 years after his death, Browning’s designs continue in production, and the influence of his genius can be seen in virtually all the automatics and semi-automatics that cross our benches. Unless you’re in the military or a recently discharged vet, however, it’s probably been awhile since you’ve seen a 50-caliber Browning machine gun.

Known affectionately as "Ma Deuce" and officially as "Browning Machine Gun, Cal. 50, M2, HB, Flexible," this Browning design has been in continuous military service on land, sea, and in the air for nearly 90 years. Although the modifications to Browning’s original design have been few and far between’ and exceedingly minor’re cent advances in barrel manufacturing have given the M2 a whole new reputation for straight shooting.

In the March 07 issue, Dennis Wood’s article on rebarreling gave a good look at barrel production from the past to the present. For this article, I toured Sabre Defense Industries’ Nashville, Tennessee, plant for a close look at barrel-making of the future. The .50-caliber machinegun barrels made by Sabre are reputed to be the best in the business.

Part of the reason, officials at the Nashville plant believe, is the superior accuracy and easy cleaning that result from the excellent bore geometry and surface finish imparted by a Sunnen HTB tube hone (Sunnen Products Co., St. Louis, MO; www.sunnen. com). Since installing the hone, test groups from sample barrels have tightened up to roughly half the size the Army’s specs require. U.

K-based Sabre acquired the Nashville facility in late 2002. "We’ve been making military .50-caliber barrels and guns since 1979, as well as commercial rifle barrels for various companies over the years, and the new owner, Guy Savage, planned to reposition our operation to be about 50/50, military and commercial, said Charles Shearon, general manager of the Nashville plant. "Our operation was to be a launch pad for U.

S. production of Guy’s XR15/16 rifles’a premium variant of the AR15/M16" for the civilian and police markets." The start of the Iraq war in early 2003 changed all that. "We received strong military orders for our .50-caliber machine-gun barrels, 7.62mm M60 machine gun barrels, M134 minigun barrels, and various other components. The demand for .50-cal barrels grew in the next few years from about 100 per month to 1,200 per month, and employment at the plant ramped up from 15 to 85.

Sabre makes two variations of the .50-caliber barrel: the heavy barrel for 50 caliber gun start as heat-treated bars of MIL-S-46047, a highvanadium steel alloy. The blanks weigh about 73 pounds, and the finished heavy barrel weighs approximately 25 pounds. Here, the gun-drilled blank is being loaded into the Sunnen HTB-2000 tubehone system.

0005 inch and produce a surface finish with a roughness average of approximately 20 micro-inches. Rifling is produced by pushing a carbide button (with the rifling form in high relief on it) through the bore. 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 percent cobalt and 25 percent chrome) withstands the intense heat and gas erosion of the initial discharge better than any ordnance steel. The heavy barrel starts as a 45inch piece of bar stock, 2-5/8 inches in diameter, weighing about 73 pounds. "For the heavy barrel, we use MIL-S-46047, a special alloy with extra vanadium to increase life," Shearon explained. "The aircraft barrel is MIL- S-11595, which we also use in our commercial guns. We have to buy these steels by mill run.

The heavybarrel material is cut by the mill and heat treated before we receive it. We do some preliminary operations to prep it, then gundrill the chamber end with a 0.75inch hole about 11 inches deep. A temporary liner is then installed and the rest of the barrel is gun-drilled with a 0.490inch hole. We ream after drilling, and have a hole-size tolerance of ±0.001 inch at that point. 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. "Sunnen’s senior field engineer in our area, Ron Williams, has always been helpful to us, so we consulted him on how current technology could improve our processes and output, said Shearon. The result was installation of 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 to 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 reduced our labor and helped improve output," explains Garry Hogan, Sabre’s plant manager. "More important to us is the automatic gauging system.

The machine gauges the bore after every stroke, allowing us to control hole size, roundness, and straightness to 0.0005 inch without operator intervention. Even after button rifling and plating we are able to stay well

50-caliber barrels are waiting to be air-gauged and fitted with a Stellite liner. Each liner is matchground to the individual barrel. Far below the Mil-Spec of ±0.004 inch on bore dimensions, which is quite a feat on a bore length of 33 inches. Tool life for the process varies with the amount of stock removal, which typically runs 0.002- 0.004 inch. Hogan says 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-inch carbide button is pushed through the bore, which is 0.503 inch 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 S. and our NATO allies. Both versions launch a 750grain slug at over 3,000 fps, delivering up to 10,000 to 13,000 foot-pounds of energy on the target.

For comparison purposes, that’s roughly equal to 12 M16 bullets of .223 caliber. 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 contribute to accuracy," Hogan adds. "We make our own buttons, too, which gives better control of quality.

The surface finish of the bore coming off the hone has a roughness average (Ra) of approximately 20 microinches (millionths of an inch) and drops into the low teens after button rifling, significantly exceeding the Mil-spec of 63 micro-inches.

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 precisionrifle shooting community, the sweet spot for surface finish is considered to be between 10 and 20 micro-inches," explains Hogan. "Though it seems contradictory, a smoother surface actually increases surface contact and friction with the bullet jacket, causing increased copper fouling. (continued on page 21)

After contouring the exterior of the barrel, the bore is chrome plated to a thickness of 0.0013-0.0020 inch. "One of the reasons we run such tight tolerances off the hone is because we lose some tolerance in the plating process," Hogan explains. "The platers are excellent and seldom vary more than 0.0005 inch, but if they do, we already have some margin for error to work with. The Stellite liner starts as a casting, which Sabre gun drills and reams, then sends to a third party for honing.

With the barrel induction heated and an alignment gauge in place, the liner is pressed into the barrel’s chamber so that its lands and grooves line up perfectly with those of the barrel. Every barrel is then fired with a high-pressure test round and examined with a Sunnen magnetic-particle inspection system. "We’re required to test fire a certain number of barrels from each lot for accuracy," Shearon says. "The Army’s spec calls for a 10round burst to hit within an 8-inch circle at 100 feet.

We’ve always been able to hit 7, but since we started honing we’re hitting 4 inches. According to Sunnen’s Ron Williams, Sabre’s experience is not unusual. "Honing is earning new respect from barrel makers, whether it’s pistol, rifle, shotgun, or even paintball," he says. "One leading maker of special-purpose rifle barrels both rough hones and finish hones.

They are perfectionists who hold ±0.000050 inch roundness the entire length of the barrel. "We’re lucky to be able to interact with users of our products," Shearon says. "At a recent armor-war symposium at Ft. Knox earlier this year, we had 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. I