USMC M40A5 Build-Part 2: Lug slotting the receiver
USMC M40A5 Build- Part 1: Gathering the Parts provided an overview of the parts required to complete an M40A5 (or M40A3). In Part 2 of this series, the receiver will be lug slotted for the M40A3/A5 Optical Platform. I decided to slot the receiver prior to fitting the barrel because I was new to this operation.
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
First, a quick recap about the optical platform.
The M40A3 and A5 both use a lugged, 30 MOA scope base, or optical mounting platform. The original contract base was made by D.D. Ross, however, he will not sell them. When I spoke to Ross, he told me that his contract does not allow him too. Badger Ordnance also makes a M40A3/A5 mount that is almost identical to the Ross mount. I’ve been told by a couple of sources, that the Marines have used this mount on some of the current M40A5s. Since you can’t buy a D.D. Ross mount, I went with the next best thing and purchased a Badger mount. The Badger mount is part number 306-06-A3 and is available from a number of retailers online.
This base requires the receiver to be inletted for the optical platform’s mounting lugs. These cuts are known as “lug slotting”. If you plan on doing your own build, you’ll need a receiver cut for a lugged base. If you don’t have a milling machine to do it yourself, Retired Master Gunnery Sergeant Dave Clark offers this service. Clark is the former Staff Non-commissioned Officer In Charge (SNCOIC) of the USMC Precision Weapons Section (PWS) in Quantico, VA, and co-owner of C&H Precision Weapons Shop located in Labelle, FL.
Both Ross and Clark recommended measuring the base first, then cutting the lug slot to match it. Both suggested avoiding the use of a drawing to make the cuts due to variations in the parts as well as the receivers.
I ordered the following items from Brownells for this project:
- Remington 700 short-action receiver
- 1/8″ solid carbide end mill
- 7/16″ solid carbide end mill
- Dial calipers
- 8-40 high-speed steel tap
- Tap guide
- Tapping paste
- Tap handle
- India stone
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For the record, I spent a lot of time measuring prior to machining.
The receiver is secured in a v-block and held in the milling machine’s vise. The center of the receiver is located using an edge finder. A .119″ gauge pin in a chuck is used to locate the second hole on the receiver. The receiver’s second hole will be the datum point (0) for all cuts.
A 7/16″ solid carbide center cutting end mill is secured in a collet and plunged into the receiver, behind the second hole. This notch will engage the front lug of the optical mounting platform.
The 7/16″ end mill is used to rough out the rear lug opening. I made my rough cuts a few thousandths shy of the final dimensions.
This the what the rear receiver lug opening looks like after the 7/16″ end mill is used to rough in the opening.
A 1/8″ solid carbide end mill is used to clean up the corners left by the 7/16″ end mill. I make my cuts to the final dimensions with the 1/8″ end mill.
At this point, I decided to open up the receiver holes for 8-40 screws. I’ve noticed in PWS build threads, they don’t do this until later. I thought it made sense to do it now.
A 9/64″ solid carbide end mill is used to cut the new holes. I use an end mill instead of a drill because it is more rigid and less likely to follow a existing hole drilled in the wrong location. Using hole #2 as a datum, hole #1 is drilled .860″ towards the front of the receiver, hole #3, 3.630″ towards the rear, and hole #4, another .605″ towards the rear.
Each hole is tapped with a tap guide in the mill. This ensures the hole is threaded square. I prefer high-speed steel taps. I use tapping paste as a lubricant.
A quick test fit to make sure everything lines up. Note the space under the rear of the base. The rounded rear bridge of the Remington action needs to be flattened with an end mill.
Back to the 7/16″ end mill. Ross and Clark had different methods of determining the depth of cut. I used Clark’s method. He told me to “ignore what everyone says, zero your x-axis off the front of the receiver ring and come up .135″ to make your cut”. That’s what I did and it worked.
Time to open up the ejection port. I used a 7/16″ solid carbide ball nose end mill to make this cut. I marked the front edge of the rear lug of the base when it was mounted so I would know how far I’d have to open it. I zeroed my z-axis with the bottom of the end mill contacting the top of the receiver ring and raised the table .675″. I made a series of light cuts along the y-axis.
Final fit, looks good!
I have a few tool marks I need to stone out, but overall I am pleased with how it came out so far.
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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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