USMC M40A5 Build- Part 5: Bedding and final assembly

Welcome to the final installment of’s USMC M40A5 build series, USMC M40A5 Build- Part 5: Bedding and final assembly.  To read the rest of the series, see:

To complete the M40A5 project, I ordered the following tools and materials from Brownells:

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Modifications to a firearm may result in personal injury or death, cause the firearm to not function properly, or malfunction, and cause the firearm to become unsafe.

torquing action on barrel

With the metal work complete, the barrel needs to be installed on the action one last time.  The recoil lug is aligned with a lug alignment tool and an action wrench is used to tighten the action while the barrel is secured in a vise.

checking head space one last time

The headpsace is checked one more time.  The bolt should easily close on the go gauge, and should not close on the no go gauge.

A great reference for this is TM 05539-IN, which is the Technical Manual for the M40A3 and M40A5.  You can find a copy of it here.  TM 05539-IN covers the bedding process used by the Marines in depth.  As a matter of personal preference, I don’t like to bed bottom metal or use a barrel pad on rifles I build, so I’ll bed this rifle differently than the Marine Corps.

taped lug m40a5

The metal surfaces are degreased and the barreled action is seated in the stock to ensure everything clears.  Sometimes minor fitting is required, however, in this case it was not.  While the surfaces are degreased I wrap the front and sides of the recoil lug in masking tape (oil or release agent will prevent the tape from sticking).  The metal is sprayed with release agent (I prefer Acra-Release), and the voids in the action are packed with modeling clay.

M40a5 prep for bedding

Note the rear of the recoil lug doesn’t have tape on it (above), this is the only surface of the lug that should contact the bedding.  Once the excess clay is cleaned up, I apply a second coat of release agent. Far better to have it and not need it than need it and not have it.

bedding compound in stock

Some painters tape on the stock helps make clean up easier.  I apply some Marine-Tex epoxy to the mating surfaces.

action in bedding compound

The action is placed into the stock and the bottom metal is screwed into the action.  I prefer this method over using alignment pins, since everything is oriented the way it will be on the finished rifle.

bedding compound removed from stock

Using cotton swabs and razor blades, the excess bedding compound is removed from the stock and surrounding metalwork.  The epoxy is allowed to cure for 24 hours.

dried bedding compound on stock

To remove the action from the stock, the action is gently tapped with a nylon mallet.  Note the excess epoxy inside the stock.  This will be removed with the mill.

leveling stock in mill

The stock is leveled in both directions (I’m using a Starrett cross test level) and secured in the vise with brass jaws and tape.

cleaning up bedding compound

An endmill makes short work of the excess epoxy.

finished stock

The finished bedding looks slick.

uns mount glued into stock

To install the night vision mount, it is centered on the barrel and epoxied in place.  After the mount has hardened, a lip was left on the front of the barrel channel.

opening up forend

Wrapping a section of barrel with abrasive cloth makes short work of this raised area.

UNS is flush with forend

The surfaces are now flush.

The barreled action was sent off to C&H Precision Weapons Shop for the correct black oxide finish.  Dave Clark did an excellent job and promptly returned the metal parts.  A quick note on the barrel finish, while the metal was at  C&H Precision Weapons Shop, Dave mentioned that when he ran the PWS for the Marines, he started grinding the tooling marks off the barrel prior to finishing, so both barrel finishes (tool mark and polished) are in fact correct.

Upon final assembly, the rifle looks great!

M40A5 action close up

IMG_6455 USMC M40a5 right side PWS-P stamp on barrel

A special thank you to Dave Clark, former Staff Non-commissioned Officer In Charge (SNCOIC) of the USMC Precision Weapons Section (PWS) in Quantico, VA.  Dave 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