USMC M40A5 Build-Part 2: Lug slotting the receiver

Marines from 3rd Battalion 1st marine Division and 1st Battalion 7th Marines all attend the pre-scout sniper course snap in on targets before engaging. 1st Marine Division schools recently began the second week of training for Marines attending the pre-scout sniper course aboard Camp Pendleton California, Oct., 27 2014.

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:

First, a quick recap about the optical platform.

M40A5 A3 badger lugged baseM40A5 A3 badger lugged base bottom

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.

PWS M40A3 slot

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:

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For the record, I spent a lot of time measuring prior to machining.

m40A3 a5 lug slot locating hole

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.

m40A3 a5 lug slot 7:16 plunge

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.

m40A3 a5 lug slot roughing

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.

m40A3 a5 lug slot after 7 16 end mill

This the what the rear receiver lug opening looks like after the 7/16″ end mill is used to rough in the opening.

U.S. Marine Corps Cpl. Matthew D. Harris, a scout sniper team leader with Weapons Company, Battalion Landing Team 2nd Battalion, 1st Marines, 11th Marine Expeditionary Unit (MEU), and native of Butler, Indiana, provides precision fire during a live-fire and maneuver exercise as part of sustainment training at D’Arta Plage, Djibouti, Nov. 3. The 11th MEU is deployed to maintain regional security in the U.S. 5th Fleet area of responsibility. (U.S. Marine Corps photos by Gunnery Sgt. Rome M. Lazarus/Released)

U.S. Marine Corps Cpl. Matthew D. Harris, a scout sniper team leader with Weapons Company, Battalion Landing Team 2nd Battalion, 1st Marines, 11th Marine Expeditionary Unit (MEU), and native of Butler, Indiana, provides precision fire during a live-fire and maneuver exercise as part of sustainment training at D’Arta Plage, Djibouti, Nov. 3. The 11th MEU is deployed to maintain regional security in the U.S. 5th Fleet area of responsibility. (U.S. Marine Corps photos by Gunnery Sgt. Rome M. Lazarus/Released)

m40A3 a5 lug slot 1 8 end mill

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.

cleaned up corner m40A3 a5 lug slotThe rear lug cut.

m40A3 a5 lug slot drilling holes

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.

m40A3 a5 lug slot 8 40 tap

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.

m40A3 a5 lug slot test fit after milled

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.

m40A3 a5 lug slot milling bridge

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.

YAMATO, Kumamoto, Japan - U.S. Marine Lance Cpl. Jonas G. Dewald demonstrates the effectiveness of a ghillie suit Dec. 2 in the Oyanohara Training Area in Yamato, Kumamoto prefecture, Japan. The concealment training was part of Forest Light 15-1, a semi-annual, bilateral exercise consisting of a command post exercise and field training events conducted by elements of III Marine Expeditionary Force and the Japan Ground Self-Defense Force to enhance the U.S. and Japan military partnership, solidify regional security agreements and improve individual and unit-level skills. Dewald, from Wilson, North Carolina, is a machine gunner with Weapons Company, 2nd Battalion, 9th Marine Regiment, currently attached to 4th Marine Regiment, 3rd Marine Division, III MEF, under the unit deployment program.

YAMATO, Kumamoto, Japan – U.S. Marine Lance Cpl. Jonas G. Dewald demonstrates the effectiveness of a ghillie suit Dec. 2 in the Oyanohara Training Area in Yamato, Kumamoto prefecture, Japan. The concealment training was part of Forest Light 15-1, a semi-annual, bilateral exercise consisting of a command post exercise and field training events conducted by elements of III Marine Expeditionary Force and the Japan Ground Self-Defense Force to enhance the U.S. and Japan military partnership, solidify regional security agreements and improve individual and unit-level skills. Dewald, from Wilson, North Carolina, is a machine gunner with Weapons Company, 2nd Battalion, 9th Marine Regiment, currently attached to 4th Marine Regiment, 3rd Marine Division, III MEF, under the unit deployment program.

 

m40A3 a5 lug slot opening ejection port

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.

m40A3 a5 lug slot final fit

Final fit, looks good!

m40A3 a5 lug slot done

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.