Making and Using a Receiver-Facing Mandrel

Guns are often tested for accuracy without knowing fully what contributes or detracts from the magic formula. A shooter may get ever so close to a desired level of accuracy, only to be stopped just short by some unknown factor. It can be so perplexing, even the most experienced shooter may not know where to turn. But one fact remains: If everything about the rifle and load, the sighting equipment, shooting technique, and shooting conditions were perfectly uniform, one-hole accuracy could be attained.

Of course, this arrangement of circumstances is virtually impossible to achieve. Yet the search goes on. In this article, I’m going to touch on a single factor or condition that is absolutely vital to rifle accuracy, and yet is often overlooked. I’m referring to the joint or union between the rifle-barrel shoulder and the receiver. This juncture is formed by the threaded barrel and receiver, Right top: Set up in the lathe here is the completed receiver-facing mandrel made for a Remington 700-Series receiver.

Right bottom: The important part of the receiver mandrel is the precision threading and its relationship to the section of turned shaft that abuts the locking-lug race on the Remington Model 700 receiver. as the barrel shoulder mates with the forward surface of the receiver. Factory-made parts are made in large numbers and assembled with varying degree of precision, and it isn’t until the shooter suspects or discovers problems that anything is done to correct it.

Discovering barrel/receiver discrepancies is sometimes fortuitous. Not many rifles are going to be blue printed and accurized by the average hunter/shooter, but discovering a correctable problem is always a big plus. So it was with one of my Remington 700 rifles. I purchased the rifle in .223 caliber, with plans to bed it into a Bell & Carlson synthetic stock. As a means of improving accuracy, I filled in the magazine cutout with an aluminum bedding block/cartridge ramp. As a single shot, I felt

Above top: This is the completed mandrel showing the portion of the threaded section and varied diameters. (See Figure 1 for the exact specs used for Remington 700 actions). Above bottom: The Remington 700 receiver is screwed onto the mandrel while still in the lathe.

A perfect degree of precision for inspection and possible facing is attained in this way. this rifle would be a performer as a switch-barrel using a cartridge of my own design, with a Shilen 1:14 twist barrel, and a couple others—including a .223 1:7 twist also chambered for a Shilen barrel. 0.925" Initial accuracy with the barrels was borderline acceptable, with some good bullets in the half-minute category.

Then one day, as I removed one of the barrels, I noticed there was poor contact at the receiver face evidenced by intermittent metal pressure contact. Locking-lug contact, as determined with Prussian blue and a snugly chambered cartridge, appeared perfect however. Making a Receiver-Facing Mandrel I pondered awhile before performing a few tests to determine the squareness of the receiver face.

I eventually decided to make a mandrel threaded to the receiver with a shoulder to 1.054" Figure 1. .474" 0.693" 0.995" 0.681" 1.155" contact the inner locking-lug race. This was made with a steel shaft centered in the lathe and was not removed until all tests and receiver facing were completed. I began with a shaft just over 1.6 inches in diameter, with enough extra length for 11 inches beyond the lathe chuck. The tail end of the shaft was drilled for alive center.

The tail end of the shaft was turned to 0.693 inch for 7-1/8 inches (see Figure 1) to a square shoulder to form a square contact with the locking-lug race when the receiver is screwed in place. The diameter of the shoulder was turned to 0.995 inch, just under the minor thread diameter for passage through the receiver threads. The shaft was then turned to the major thread diameter of 1.054 inches for a length of 0.681 inch.

The remaining portion of the shaft was turned to 0.925 inch, allowing for the tool-bit clearance as the receiver face is tested for runout and faced for squareness with the threads. With the shaft prepared thus far, the threads were cut as required for the Remington receiver. Backing-off the live center, I tried the threads for fit and shoulder contact at the recoil-lug race. All was well as I used firm hand torque to install the receiver to the facing mandrel.

Then I tested the receiver face for squareness with the threads. A dial indicator clearly indicated a 0.003inch runout for nearly a third of the receiver face. Thus, receiver facing was in order. Facing the Receiver Total Length – 11.5"

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With a properly shaped and sharpened tool bit, facing the receiver is a straightforward procedure. It comes down to precisely how much metal is removed from the receiver face. In my case, I elected to use a shim and leave the barrel shoulder exactly as it was. I already had two other barrels headspaced to this action as switch-barrels, so I didn’t want to toy with barrel-shoulder turning and re-chambering. Worthy of mention here: Brownells steel of 0.050-inch thickness.

I avoided a shim that would be too thin, and I wanted to stay just forward of the protruding scope base by a few This steel shim is 0.050 inch in thickness—the exact amount removed from the receiver as it is trued. A portion of the receiver ring extends just beyond the threaded mandrel to avoid tool-bit contact with the shaft while facing the receiver. Montezuma, IA 50171) offers oversize recoil lugs in 0.250 and 0.300 inch thicknesses. The standard thickness of recoil lugs is 0.185 inch.

At a price of $40, Brownells also offers receiver-facing mandrels for Winchester M70, ’03 Springfield, largeand small-ring Mausers, and Enfield rifles. So, electing to make a shim, I used stainless thousandths. With the micrometer carriage stop on the lathe set at 0.040 inch, I made my first facing and tried the action for headspace. After a few follow-up cuts, I arrived close enough to try all three barrels for headspace with about 125 foot-pounds of receiver torque. I found them to be perfect.

Replacing the receiver back on the lathe mandrel, I followed up with a careful test for runout. This also was right on the money, with absolutely zero movement on the dial indicator. This told me the receiver threads were square with the face of the receiver at optimal axial preload. Both feel and visual inspection of the barrel and receiver face with Prussian blue indicated a totally uniform contact.

Followup Accuracy Tests For a pretty demanding accuracy test, I used the Shilen 1:14-twist varmint barrel and my 5.6×41 SW-NEJ cartridge. The load was 24 grains of 2015 BR powder, along with 52-grain Shilen benchrest bullets, Lapua cases, and Federal 205M primers. (continued on page 16)

050-inch steel shim is sandwiched between the receiver face and recoil lug on this Remington action in the process of truing up the receiver. 050-inch shim used to maintain exact headspace following the receiver-facing procedure. Note by the serial numbers that this barrel is headspaced for another rifle as well—a Remington Model 40X. This is another reason to maintain exact barrel-to-receiver spacing by means of a shim. The first test involved removing and replacing the barrel after each shot at 100 yards.

A special lubricant was used on the barrel shoulder and threads along with 100 pounds of applied torque. Accuracy was under one minute of angle for five shots in this test. For anyone who has not tried such a test, the outcome will prove most interesting. Where barrels are switched frequently—as is common with switch-barrel applications—my experience has shown that a good lubricant on the threads and barrel/ receiver shoulder is vital to attaining uniform axial preload as torque is applied.

Lubricant also helps prevent induced bare-metal wear at this juncture. Whereas 150 pounds of torque may be required with a dry-thread application, as little as 110 pounds of torque to a lubricated thread/shoulder will do the same job. In fact, I’ve performed tests on well-fitted barrels using as little as 70 pounds of torque on lubricated threads—with perfect accuracy results. Even barrel/receiver joint loading produces thousands of pounds of thrust (axial preload).

There is an equation to determine this, but I won’t go into it here. Subsequent accuracy tests indicated a definite improvement in performance with all three barrels. Headspace remained unchanged. The 5.6×41 SW-NEJ shot one outstanding bughole at 100 yards and a nice round 0.750-inch group at 300 yards—a first for this rifle and cartridge. Conclusions While barrel/receiver squareness may not matter to most shooters, it represents an important area of accuracy for target and varmint shooters.

And a precision-made, receiver-facing mandrel is the place to start for developing the ultimate accuracy. ■