USMC M40A5 Build- Part 4: Threading and chambering the barrel and brake installation

Scout snipers perform machine gun, sniper rifle exercise — Marine Corps Base Camp Lejeune, North Carolina - Corporal Brandon Luke, a mortarman with Scout Sniper Platoon, 1st Battalion, 2nd Marine Regiment, fires an M40A5 Bolt-Action Sniper Rifle during an unknown distance range aboard Camp Lejeune, April 6, 2015. The platoon is currently working up to their deployment, which is slated to be a Unit Deployment Program to Okinawa, Japan.

This is Part 4 of Rifleshooter.com’s USMC M40A5 build.

So far, we’ve gathered the parts, lug slotted and trued the receiver for Rifleshooter.com’s USMC M40A5 build. In this post I’ll be fitting the Schneider United State Marine Corps (USMC) contract barrel to the receiver. In Part 3 of the series, the receiver was trued and lug slotted.

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I ordered the following tools and materials from Brownells:

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The USMC Precision Weapons Section (PWS) normally chambers barrels between centers. While some rifle builders dislike this technique, I don’t see any problems with it. However, since my lathe is fairly small, 12×36, I don’t have a lot of room to work on a 25″ long barrel, so I’ll be chambering the barrel through the headstock of my lathe.

Proper measurements are needed to appropriately fit a barrel. I take these with a depth micrometer, the proper tool for the job. A few measurements are needed, the distance from the front of the receiver ring to (A) the bolt nose, (B) the bolt face, and (C) the front edge of the bolt lugs.

The barrel tenon length is dimension C, plus the thickness of the recoil lug (.312″), minus .010″. Headspace is dimension B, plus the thickness of the recoil lug. The bolt nose recess (or counter bore) depth is C minus A.

I cut 1/4″ off the chamber end of the blank. The barrel is secured in two spiders through the headstock of the lathe. I use a range rod with fitted bushings to dial in the bore centric with the lathe on both ends of the barrel.

This is the muzzle end of the barrel secured in the lathes outboard spider.

The end of the barrel is faced and the tenon cut to the finished length. I had been using high-speed steel insert tooling, however, for this project I switched to carbide.

The lug should slide onto the tenon with a light drag. You don’t want it to be too loose.

I make a groove in the tenon to stop the threads where the lug begins and coat the tenon in Dykem.

I am using a full profile 16 TPI Carmex carbide insert to thread the tenon. As mentioned earlier, I typically use high-speed steel insert tooling, but I am experimenting with carbide on this project.

The carbide cuts fairly well. I threaded this barrel with a spindle speed of 220 RPM. I think the threads would have looked a little nicer with a higher spindle speed, but I don’t have a carriage stop on this lathe- threading to a shoulder at such a high speed is a little nerve racking. The chamfer on the end of the tenon is a little deeper than normal, I was a little over zealous cutting it.

Another view of the completed tenon.

Now is a good time to test fit the action and lug. The action should thread on smoothly, not too loose, not too tight. The recoil lug should fit tightly against the barrel tenon’s shoulder.

The bolt nose recess is cut with a .705 #3 pilot form tool (counterbore). This cut can also be made with a boring bar, however, using a counter bore is faster and easier. The tool is secured with a pair of vise-grip pliers, it is driven with a dead center in the lathe’s tailstock, and the depth of cut is monitored with the dial indicator on top of the tail stock. I run the lathe at 70 RPM.

The action, lug and bolt are test fitted. The bolt handle should easily close at this point.

The chamber is cut with a Manson live pilot 308 Match reamer. The reamer is held with a small pair of vise grips. It is driven by slowly applying pressure to its rear with a dead center in the lathe’s tail stock. The collar on the reamer is a PTG micro adjustable reamer stop. Since I am not using the lathe’s chamber flush system, the reamer is thoroughly coated in Viper’s Venom cutting oil prior to starting the cut. The lathe’s spindle speed is 70 RPM for this operation.

Another view of the chambering set up. I’ve tried a number of ways to hold chamber reamers and have decided I like this set up the best. Cutting chambers is a time consuming process, a light cut is made, the lathe is stopped, reamer retracted, cleaned, lubricated, reinserted, and another cut is started.
Initial headspace measurements can be made in a couple of different ways, I like to use a depth micrometer for the initial readings and a feeler gauge for the final readings. The depth micrometer is held against the go gauge to determine how much deeper the chamber needs to be. Since the lug on this action is .312″ thick, and the headspace dimension against the tenon shoulder is over 1.000″, I am measuring against the recoil lug. This allows me to use the depth micrometer with the 1″ bar.

As the chamber gets closer to the proper headspace measurement, a feeler gauge can be used. The action, bolt and recoil lug are screwed onto the barrel with the go gauge in the chamber. The go gauge will prevent the action with bolt from snugging up against the tenon shoulder. Feeler gauges can be inserted into this area to determine how much deeper the chamber needs to be cut.

When the chamber is the proper depth, the bolt handle will close on the go gauge…

And remain open on the no-go gauge.

The final step is top cut a small radius around the counter bore and on the edge of the chamber. These cuts aid in feeding and prevent scratches in brass.

At this point, the chamber work is complete on the rifle. The barrel can be reversed to facilitate muzzle brake installation.

The barrel is cut about 1/4″ over finished length, in this case 24″ from the front edge of the lug or 25″ overall. A used recoil lug is used to protect the tenon shoulder from the lathe’s 3 jaw chuck. A steady rest is set up on the muzzle end of the barrel and the barrel is faced.

A #3 pilot 60 degree center drill is used to cut the end of the barrel. This cut will allow the lathe’s live center to be inserted into the end of the barrel.

I tried turning the tenon with carbide tooling and wasn’t happy with the finished surface.

I cleaned up what I could with a high-speed steel insert tool.

I used a high-speed steel insert tool to cut the 5/8″-24 threads.Test fitting the brake shows it fits well.

A .420 dish target crown form tool is used to clean up the muzzle and remove the tapered area the center drill cut. I believe the Marines just square the face of the muzzle, without a dish.

The finished muzzle. Again, I am unhappy with the finish the carbide tooling left on the tenon, however, it will function perfectly. I’ve had great success with the high-speed steel tooling, but the only way you can see how well something will work is to give it a try.

The barrel is now chambered and the muzzle is cut for the Surefire brake.

The next post in this series will cover bedding, finishing and final assembly.

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.