The McBros 50 BMG Action

By: Daniel Lilja

To those not familiar with the 50 caliber BMG round, their first impression upon seeing a loaded round is; hey, that thing is BIG. Indeed, a 50 caliber round is big, even compared to our largest magnum rounds. And so it is with the McBros 50 BMG bolt action, designed for shoulder fired, sporting type rifles and full-blown benchrest rifles based on the 50 BMG round. The McBros action alone weighs as much as many sporter rifles, a full 7.5 pounds.

Here is a Fifty BMG built on a McBros single shot action. The muzzle brake is a McBros too and the stock is from McMillan. This rifle has the Night Force 5.5x22x56mm NXS scope.  It is nicknamed Colonel Mustard.
Here is a Fifty BMG built on a McBros single shot action. The muzzle brake is a McBros too and the stock is from McMillan. This rifle has the Night Force 5.5x22x56mm NXS scope. It is nicknamed Colonel Mustard.

There are actually two versions of this action. The more common is the single shot variation, comprising about two thirds of McBros’ orders. A box fed repeater is also offered. The only difference between the two is the addition of the 5-round magazine.

Before we begin discussing the McBros action in detail however, perhaps we should take a brief look at who McBros Rifle Company is, and isn’t. Some understandable confusion exists about the various firearms related companies using the McMillan name.

In the beginning, two brothers from Arizona, named Pat and Gale McMillan started their businesses, catering to the gunsmithing needs of benchrest shooters. Both were active shooters and made some real contributions to the accuracy game early on. They made some quality bullet-making dies and single shot benchrest actions. Later, Pat built a company with an
excellent reputation for making high quality match grade barrels. Gale did some early development work with fiberglass stocks and started making them for his benchrest shooting friends. When I first met Gale, in about 1977, he was making fiberglass stocks and building target rifles.

Pat later sold his business to Bill Wiseman, who still manufactures barrels under the Wiseman/McMillan name. Gale turned over the stock manufacturing business to his son Kelly. Gale, along with another son, Rock, continued with their gunsmithing business which had now grown into a fairly good size manufacturing concern making complete custom rifles, known as G. McMillan & Co. Gale later sold his interest in this business to some investors. Today that business is known as Harris/McMillan Gunworks. Gale has since started another company that manufactures rifle scopes, known as McMillan Optical Gunsight Company.

In the meantime, Rock started his own business, manufacturing parts for G. McMillan Company. Today he markets custom rifles and the 50 BMG action and 50 caliber muzzle brakes under the McBros name. As indicated by the name, his brother Kelly has an interest in McBros too. Incidentally, Kelly’s stock shop is just a few blocks from Rock’s machine shop.

Rock’s background includes an engineering degree from Arizona State University and 10 years of CNC machine shop supervision in other industries.

Gale McMillan has commented that this 50 BMG action is just a scaled-up version of their early benchrest action, which was designed for the little .222 Remington case. The overall length of the action is 12″ and the diameter is 1.950″. The massive bolt is 1.25″ in diameter, as big as the breech end of most sporter barrels. Without a doubt, it is a big action designed for a big round.

I’ve had a chance to look at a few early McMillan benchrest actions and the similarity is quite noticeable. One distinguishing feature is the bolt design. The bolt is a “full diameter bolt”. This means that the locking lugs and the bolt body are the same diameter. Also, the two lugs are in the horizontal position when the bolt is closed. This is opposite most bolts, whose lugs are vertical when locked up. An advantage to McMillan’s system lies in the positioning of the extractor. It isn’t necessary to make any machining cuts in or near the port side locking lug, which can weaken the lug to some extent. In the case of the McBros 50 BMG action, the extractor is a big Sako-type hook extractor. It is 5/16″ wide and made from heat-treated 4130 steel.

The receivers are made from 4340 chrome-moly material, and heat treated to 45-48 Rockwell “C”. The heat treatment of this receiver is a little unusual, in that it’s done after the machining is completed. Many accuracy-minded action makers machine their actions after heat treatment in order to prevent the inevitable warping that occurs during the quenching stage. It’s Rock’s desire, however, to offer an extremely strong receiver, and machining heat treated material as hard as Rockwell “C” 48 would be both difficult and expensive. His solution is to have the receivers heat treated using a process called austempering. In practice, the part is brought up to temperature in molten salt and then quenched in another, cooler, salt pot. The only thing cool about this process is the near elimination of warpage. This heat treating is performed by a professional heat treater.

The only other operation not performed in-house is the initial deep-hole drilling of the bolt hole. This job is done by a company that specializes in that type of drilling.

After receiving the action blanks from the deep-hole driller, the bolt hole is honed to an internal diameter of 1.250/1.251″ with a 32 micro finish. After honing, the receiver is mounted on an arbor and the outside of the action is turned to the 1.950″ diameter and concentric to the inside bolt hole, to within .002″. The front end of the receiver is then threaded in a slant bed CNC lathe made by Nakamura-Tome. The barrel shank thread size is 1.500″ x 12 TPI with a 1.5″ long thread shank. Most other operations on the receiver are performed on one of two Matsuura CNC milling machines.

Rock seems to especially like his milling machines. Some work that would normally be assigned to a lathe, is performed on the mill in his shop. For example, the receiver face is cut true with an endmill, as are the locking lug abutments inside the receiver. Another example of using the mill, where other manufacturers have alternate processes, is machining the lug raceways. Often this job is done on a shaper or broach, set up just for this one operation. Rock, however, cuts the raceways from the end using a small diameter end mill. I suspect that the reason for this is twofold. With the generous proportions of his big action, an end mill can get into these normally hard-to-reach areas. The second has to do with technology improvements. Today’s multi-axis, computer-controlled machinery allows savvy machinists to do things that were just not practical before. The fewer times that a part–in this case the receiver–is handled, the cheaper it is to make. Also, the greater the number of operations that can be performed in one machine setup, the greater the accuracy of the part should be. For example, machining both the receiver face and the lug abutments in the same setup means that they should be completely parallel to each other.

Rock takes full advantage of the abilities of his 4-axis CNC mill. In the photograph, you will notice the neatly milled helical flutes in the bolt body. The bolt opening cam is another helix milled with the fourth axis. Even the lettering and serial number on the outside of the receiver are milled with the aid of the computer and a lettering software package. The cocking cam on the back end of the bolt is one more example of a helix being milled. And although I’ll discuss its virtues later, the bolt closing cam is milled onto the bolt lugs in the mill.

The bolt is made from aircraft quality 9310 steel. Here again we find another example of wise material selection and excellent heat treating. This type of steel is referred to as a carburizing-type, lending itself to case hardening. It is similar to 8620, which has been widely used in making gears. Rock specifies that the bolts are to be gas carburized for a case hardness of 60 on the Rockwell “C” scale, and for a depth of .015″-.020″. This results in a core hardness of about 38 Rockwell “C”. One plus in using 9310 is the high hardenability of the core. This steel possesses a narrow hardness range between thick and thin sections too. The hard case results in a super-slick operating bolt, and one that will probably never gall. This material and heat treatment has a tensile strength of about 171,000 PSI and a yield strength of about 143,000 PSI. These two figures are derived from only the core hardness of this material. A metallurgist that I contacted regarding these strength values doubted that a thin case (.020″ thick maximum) would add measurably to the strength of this part.

I wondered about the strength of this big bolt when compared to the amount of thrust generated by the 50 BMG cartridge. (The details of these calculations were discussed in an earlier article in PRECISION SHOOTING.) The McBros bolt has 2 lugs that are .600″ wide and .76″ long. The minor diameter of the bolt is .925″. From these measurements, the total area in shear is .992 square inches. If we plug a peak chamber pressure of 65,000 PSI into the formula to simulate a fairly stiff handload, we find that the amount of bolt thrust generated is about 23,610 pounds. This thrust load is offset by the lug shear strength, which is approximately 70,950 pounds. This is a healthy margin, as it should be with a big cartridge like this. I also found that the bolt will flex about .0021″ under this load, demonstrating again why it becomes necessary to bump case shoulders back between loadings.

The big 1.25″ diameter bolt offers quite a bit of camming power. On the action that I checked, the cam retracted the bolt about .170″. That’s a lot of cam, more than any other bolt action that I’m familiar with.

A large loading port is milled into the action. It measures a full 5.5″ by 1.25″. With the bolt fully to the rear, I can extract an unfired TCCI or military loaded round. Pulling fired brass out is even easier.

As is common with full diameter bolts, the bolt stop is an externally pivoted pin that rides in a milled slot in the bolt body. This not only serves as a bolt stop, but also keeps the bolt lugs aligned with the raceways.

The McBros action is machined to accept a Remington type trigger. The trigger is held in place by cross pins. I like trigger carriers, but the pins will perform their job just as well.

With the single shot version, two other milling cuts become noticeable. They are two flats milled at 45 degrees from the vertical centerline, a little smaller than the size of the loading port. Because the action is round, there is a tendency for the action to torque in the stock under fire. These flats help control this action torquing. Without them, the torquing would be offset only by the 5/16-24 guard screws.

The bolt shroud is another product of the milling machine. It has nice lines, but the feature I like best is the ease with which it can be removed from the bolt. While most bolt shrouds are screwed into the back end of the bolt, the McBros has two ears milled on it. These ears fit into corresponding slots in the bolt, and with about an eighth of a turn, the entire firing pin assembly is either removed or reinstalled. No tools are required, just a reasonably strong grip.

The bolt handle appears to be rather firmly attached. A dovetail is milled into the handle where it attaches to the bolt, and slid over a matching dovetail milled onto the bolt body. The two are then silver-soldered together. The joint looks like it would stand up to quite a bit of abuse.

An ejector is furnished with the bolt, but probably not used often with the more popular single shot version. The recoil lug is big, like everything else associated with this action. It is a Remington style, and it’s 3/8″ thick. The lug is also made with a small pin that fits into a milled slot in the receiver face. This keeps the lug in alignment when barrels are screwed up tight.

A nice piece of standard equipment with the McBros action is the one-piece scope base. It’s machined for a Weaver-style scope ring, which includes the Leupold MK 4 ring. I suspect that many of these actions will either start out with a Leupold MK 4 scope on top or end up with one later, after lesser scopes have taken their licks. So, it makes good sense to make a base that will accept the MK 4 ring. It also makes good sense to machine the base with a built in slope so the maximum amount of vertical scope adjustment can be utilized. Rock offers the bases with either a 1/2 or 1 degree forward cant. For use with the MK 4 scope, I would suggest the 1 degree version. This will work out just about right for a 100 yard zero, leaving well over 100 minutes of vertical adjustment remaining, for reaching way out there where the 50 BMG was meant to play. The base is firmly held in place by six #10 screws.

Rock also offers the McBros 50 BMG action with a 378 Weatherby/416 Rigby bolt face. I recently barreled one of these actions for the 338/378 Weatherby case and it worked out well. This is more action than needed for this case, but it’s not a bad choice, especially with a long, large diameter barrel. The barrel we used on that rifle was 1.850″ diameter and 40″ long. A big barrel on a big action.

There are just a few negatives that I can see with this nicely made action. In the closed position, the bolt handle doesn’t protrude very much from the side of the action. This is fine if the rifle uses one of the two stocks that Kelly McMillan makes for this action, the Big Mac or Dual 50. But if it’s put into a wider stock, like a Six Enterprises Unlimited stock, or a big laminated wood stock, the bolt handle is going to get lost. I had to re-forge one of these handles to get it to work in a Six stock.

I mentioned earlier that the closing cam is machined onto the bolt lugs. This is an acceptable and easy way to achieve this cam, but I feel that it’s better to put the cam on the inside of the receiver. That is, as part of the locking lug abutment. It’s much more difficult to cut there, requiring either a special machine designed just for that one operation or a sinker-type EDM machine. The disadvantage to having all of the cam on the lug is decreased contact area on the lug abutment. Although this doesn’t decrease the bolt lug shear strength, it does put all of the back thrust onto a smaller area. Lug setback shouldn’t be a problem with the McBros action because of its heat treatment, but that decreased contact area increases the potential for it.

I also feel that with any custom action, the locking lugs should be lapped into place. McBros will lap the lugs on these actions on request but its not standard procedure. If the lugs are not lapped into place, the machine marks from the end mill are visible on the internal abutments. With the hardness of these actions, it takes quite a bit of lapping to achieve near 100% contact and remove the mill marks.

One other quibble, which I’ve mentioned about other custom actions, is the lack of a serial number on the bolt. Sooner or later a mix-up is going to occur. The McBros action is furnished in the white with a heavy bead blasted finish. A few machine marks do show through though.

So you ask, who needs or wants a rifle chambered for the big fifty? Well, Rocks says that business is brisk on the BMG action. There are probably two types of shooters drawn to the fifty. One type includes those who want the biggest of the big. They like it because of its sheer size, power and energy. These fellows probably use it mostly for plinking. Like shooting at rocks 1/2 mile away and other fun targets.

The second group is drawn to the fifty not so much because of its size, but rather in spite of it. They’re looking for the ultimate in long-range performance and precision. Bullets with a ballistic coefficent of over 1.0 are available! Its flat trajectory and tremendous downrange energy is remarkable. As one fifty shooter I know commented, “I can see the fifty hit at 1000 yards with my bare eyes!”.

The fifty has been used rather successfully for hunting at long-range too. A big game animal hit with a fifty way out there dies just as quickly as one tipped over at close range with smaller arms. Others use it for benchrest shooting at 1000 yards. The Fifty Caliber Shooters Association was established, in part, to promote competition with the fifty and sponsors 1000 yard competitions around the country. A National match is held at the NRA Whitington Center near Raton, New
Mexico each July.

It is here, in the competitive 1000 yard benchrest matches, that the McBros action has really shown its worth. At the 1994 Nationals the winners of both the light and heavy gun classes, Skip Talbot and Buddy Clifton, used rifles built on the McBros single shot action. Both Rock and Gale McMillan attended that match, too. Gale placed a respectable 8th in the heavy gun class. The McBros actions have been proven in other matches across the country, too.

An obvious concern about the 50 BMG has to do with recoil. Shooters not familiar with the fifties are certain that the recoil, if survivable, must be severe. The simple answer is: it’s not. The recoil brakes available today are very effective at diminishing recoil. Considering that most of these rifles are quite heavy too, recoil is highly manageable. In the two classes supported by the 50 Caliber Shooters Association, the light gun class allows for a rifle weight up to 32 pounds and the heavy class, up to 50 pounds. Admitting that recoil comparisons are highly subjective, I would say that firing a 50 pound fifty with a good brake is similar to shooting a .308 Winchester loaded with heavy bullets.

My wife has fired a fifty while out spring bear hunting and not been bothered by the recoil in the least. There’s a big blast, but recoil is moderate.

In general, I’m really impressed with the McBros 50 BMG action. The material selections for the bolt and receiver are good, and the heat treatment is outstanding. The firing pin retention system is first rate, too. The overall appearance and lines are eye pleasing and tastefully done. Providing a scope base as well as the oversized, milled aluminum trigger guard, are nice extras not often furnished with custom actions. The actions are well-made, with everything square and parallel that should be.

For more information about the Fifty Caliber Shooters Association, they may be contacted at:

web page: