USMC M40A5 Build- Part 3: Receiver Truing, is the third part of Rifleshooter.com’s series, USMC M40A5 Build. With a goal of cloning the USMC M40A5 (M40A3) sniper rifle, USMC M40A5 Build- Part 1: Gathering the Parts, discussed the parts necessary to build a M40A3 or M40A5, and USMC M40A5 Build-Part 2: Lug slotting the receiver showed how the receiver was lug slotted for the optical mounting platform. In Part 3 we will true the receiver.
To read Rifleshooter.com’s M40A5 Build Series, see:
- Remington 700 (USMC M40A1, M40A3, M40A5) Q&A: What is a clip slot? Lug slot? Lugged base?
- USMC M40A5 Build- Part 1: Gathering the Parts
- USMC M40A5 Build-Part 2: Lug slotting the receiver
- USMC M40A5 Build- Part 3: Receiver Truing
- USMC M40A5 Build- Part 4: Threading and chambering the barrel and brake installation
- USMC M40A5 Build- Part 5: Bedding and final assembly
Receiver truing, sometimes referred to as blueprinting, ensures the receiver threads, lugs, and face, as well as the bolt lugs and face are square with each other. Blueprinting is a term borrowed from custom car makers. It refers to a process in which a machinist would rebuilt an engine to the dimensions specified in the original factory blueprints. For firearm work, I prefer the term truing over blueprinting, since the receiver isn’t being cut to a specific set of prints.
If your Google skills are strong, you can find quite a bit of information about the techniques the Precision Weapons Section (PWS) uses to build the M40A3 and M40A5 online. Based on what I have read and been told in person by various Marine 2112 Precision Weapons Repairer/Technicians, the Marines true receivers using either a Manson or PTG receiver truing kit.
The process shown here is tooling specific, the tools I am using were ordered from Brownells:
- Manson Receiver Accruing System
- Bolt face facing burr, 308
- Bolt face tooling block
- Receiver ring facing tool
- Bolt lapping tool
- Lapping compound
- Do-Drill cutting oil
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Any modifications made to a firearm should be made by a licensed gunsmith. Failure to do so may void warranties and result in an unsafe firearm and may cause injury or death.
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I’ll be truing my action with a Manson Receiver Accurizing System. I realize some precision rifle builders prefer single pointing the receiver on the lathe, however, I have had great success with the Manson kit. It is available in two sizes 1 1/16″ + .010″ x 16 (oversized) and 1 1/16″x 16 (factory Remington 700 thread specification). The kit shown above is the same one the USMC uses, 1 1/16″ + .010 x 16. The kit consists of a reamer that cleans up the minor diameter of the lugs (above, top), two tapered bushings to align the reamer and tap (above, center left), a recoil lug reamer (above, center right), and an oversized receiver tap (above, bottom).
The two tapered bushings are inserted into the bolt hole.
The oversized tap is turned with a tap handle to cut the minor diameter of the threads and square the receiver lugs. I use plenty of Do-Drill cutting oil during this operation.
The reamer is removed and the threads are recut with the oversized tap. The tap is coated with a generous amount of cutting oil before making the cut.
The end of the tap acts as a guide for the receiver ring facing tool. This tool ensures the front edge of the receiver is perpendicular to the bolt hole.
The facing tool uses three carbide blades to remove material from the front edge of the receiver. It works extremely well. Alternatively, the tap could be used to mount the receiver between centers on a metal lathe to square the front of the receiver ring.
This is what the front edge of the action looks like so far. Note the fresh cut surfaces on the front receiver, receiver lugs, and threads.
With the receiver lugs cut, now is a good time to check how well the bolt lugs mate to it. I coat the receiver lug surfaces with a black magic marker, insert the bolt, then raise and lower the the handle a few times. The areas that contact the bolt lugs will have the marker removed. On many Remington 700s, only one bolt lug will contact the receiver lugs. In these cases, the rear faces of the bolt lugs need to be squared on the lathe. See, Truing the bolt on a Remington 700, for more information on how this is done.
On this receiver, both bolt lugs contacted the receiver lugs so the bolt lugs can be lapped without cutting them first.
Lapping the bolt lugs against the receiver will ensure they are properly mated when the bolt is closed. The spring-loaded bolt lapping tool shown above screws into the front of the action and applies rearward pressure to the bolt. A thin layer of lapping compound is applied to the bolt lugs and the handle is lifted and closed repeatedly.
When the lugs are properly mated, the rear surfaces of the bolt lugs will be shiny where the factory bluing has been removed by the lapping compound.
Finally, the bolt face can be cut square. This operation can be performed on the lathe or with a specialized tool. I like the Manson bolt face truing tool and tooling block. The tooling block (top right) screws into the front of the receiver and the carbide burr (bottom) snaps under the extractor.
With the burr and block in place, the bolt handle is held firmly against the receiver lugs and the burr is driven with a hand drill. This cuts the bolt face. A series of lights cuts is made until the entire bolt face is squared.
The action is now trued and ready for a barrel!
Read more about Rifleshooter.com’s M40A5 Build below!
Dave Clark, former Staff Non-commissioned Officer In Charge (SNCOIC) of the USMC Precision Weapons Section (PWS) in Quantico, VA, is the co-owner of C&H Precision Weapons Shop located in Labelle, FL. C&H Precision Weapons Shop offers a wide variety of custom rifle building services, including clip and lug slotting. If you are looking to build a USMC rifle clone, he also builds complete M40A1, M40A3, and M40A5 rifles.
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