Building a custom 300 Blackout (BLK) Precision Rifle

Since its approval by SAMMI in 2011, the 300 AAC Blackout (abbreviated 300 BLK) has been a popular choice with shooters. Often recognized as the go to choice for suppressed applications with heavy subsonic loads; the lighter, supersonic offerings exhibit similar performance to the 7.62x39mm Russian.

While the 300 BLK is commonly encountered in AR-15/M16 M4 type rifles, bolt actions rifles are offered from a number of manufactures. In this post, I’ll be building a custom precision rifle chambered in 300 BLK.

The rifle will be based on a Remington Model Seven action. The Model Seven (above, left) is a compact version of the 700 (above, right). To read more about the similarities and differences, see Remington Model Seven (Model 7) Versus The Model 700.

Selecting a barrel was a little more complex. From what I have researched, it is hard to get a 300 BLK to shoot well with both supersonic and subsonic loads. The heavy subsonic loads prefer the fast twist rates, with the slower twist is favored by the lighter bullets. I really wanted a rifle that could handle both, so a fast twist would be used. After talking extensively to Frank Green at Bartlein Barrels, I ordered a 34″ long, 30 caliber, 1:7 twist blank. I ordered it long because I only wanted a 16″ finished barrel on the rifle. With 34″ of barrel, I could cut it in half and get two barrels out of it.

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For use in this project, the following items were ordered from Brownells:

All lathe work is conducted on a Grizzly gunsmith’s lathe.

I’ll by truing or blueprinting the action. For this I will use a Manson Receiver Accurizing System. The action is secured in an action wrench to hold it in place.

The Manson kit (above) includes a reamer, tap, and tapered bushings.

The reamer and the tap are aligned with the bolt hole raceway by two tapered bushings.

The receiver reamer is used to cut the minor diameter of the threads and the receiver lugs.

The receiver tap is used to recut the receiver threads.

To true the front of the receiver, a receiver ring facing tool is guided by the tap.

After inspection of the bolt lugs, I decided that they do not need to be cut on the lathe, but make enough contact to be lapped in. This is done with the aid of a bolt lapping tool (above). The tool screws into the front of the receiver, the spring loaded plunger on the front of the tool pushes the bolt to the rear of the receiver, so the lugs can be lapped.

A view of the lapping tool installed. 600 grit abrasive is placed on the rear surface of the bolt lugs and the handle is lifted up and down to lap the lugs into place.

With the lugs lapped, the bolt face can be trued. The cut is made with a Manson bolt face truing tool, the tool consists of a carbide burr and tooling block. The burr (above, bottom right) driven by a drill. The tooling block screwed into the front of the receiver guides the burr. At this point, the action is trued.

This build will use a Holland’s recoil lug. This tapered lug is typically pinned in place to make alignment easier. The receiver is mounted into a fixture and secured in the mill. A plug is used to retain the recoil lug and a blind hole is drilled to receiver the pin that will keep it aligned.

The receiver is measured with a depth micrometer to determine the dimensions of the barrel tenon and headspace.

A view of the Bartlein barrel blank.

Another view of the barrel blank– a lot of steel here.

The ends of the barrel are dialing in on the lathe and the faces cut true.

A piloted 60 degree center drill is used to machine a recess for the lathe’s centers.

Since the barrel arrived as a straight blank, it needs to be tapered. This process has been covered in detail in previous posts. On this particular project I’ll be using the tapering fixture installed on a lathe, rather than offsetting the tail stock.

This is a view of the tapering attachment on the lathe. It can be adjusted to automatically cut different tapers.

The tapering fixture does a good job, however, it does have a size limitation. This fixture will only cut tapers 12″ long. Since the tapered section of the barrel is longer, the taper will be cut in a few different set ups.

The finished taper. I cut the taper towards the tail stock, this prevented the barrel from sliding into the chuck. Sometimes I’ll use a dead center in the headstock and drive the barrel with a dog. The problem with this method is the dog introduces vibration into the machine since it is off balance.

To remove tool marks from the freshly tapered barrel, I use a 4″x24″ belt sander with a 180 grit aluminum oxide belt. I find this is the best method. Extreme care must be taken to ensure the cord of the sander doesn’t go anyway near the moving parts of the lathe.

Finally, a short tenon is cut on the muzzle end of the barrel. This will be used to drive the barrel during the chambering process.

The barrel is reversed in the lathe. The a steady rest is used to help stop deflection of the barrel during the machining process. The tenon is cut to accept the recoil lug. I like using a 35 degree high-speed steel insert tool for this. I find it gives great results with stainless steel barrels.

The tenon is coated in Dykem and the threading tool is installed.

The threads are cut.

The action is test fit with the lug. The action and lug should all fit tight against the barrel tenon’s shoulder.

The Remington Model Seven, like the 700 requires a bolt nose recess. This can be cut with either a boring bar or form tool. I find the form tool (or counterbore) is easier to use. This is a piloted .708″ counter bore. The tailstock is used to drive it into the cut.

A view of the completed bolt nose recess. Note the steady rest had been moved closer to the chamber.

The bolt, action and lug are test fitted. So far, so good.

The chamber is cut with a piloted Manson reamer. The reamer is held with a set of locking pliers and driven by the tail stock of the lathe. The tailstock is equipped with a MT3 blank, this has a flat front face which allows the reamer to float. A PTG micrometer adjustable reamer stop if used to limit the depth of cut.

I make initial headspace measurements with a go gauge and depth micrometer.

As the headspace gets closer to the final dimension, feeler gauges are used to measure the space between the recoil lug and barrel tenon shoulder while the go gauge is in the rifle.

When the bolt closes on the go gauge, and doesn’t on the no go (above), the chamber is cut to the proper depth.

The edge of the chamber, and outside edge of the bolt nose recess are eased. This helps in feeding and prevents brass from being scratched.

The barrel is reversed in the lathe and a tenon is turned and thread for the muzzle brake. In this case, the tenon is about a half inch too long. This material will be removed.

The final crown will be cut with a crowing tool.

The finished crown.

The barrel is secured in a barrel vise and the action is tightened with an action wrench.

This rifle will have an oversized bolt knob installed. To cut the handle to receive the knob, the bolt is held in a fixture and aligned with the tailstock.

At the end of the bolt handle is drilled with a center drill.

A live center is placed against the handle and the factory knob is removed. The cut is made towards the tailstock to prevent too much stress from being placed on the handle, potentially snapping it off.

When the handle is the correct diameter, a 5/16″-24 die is held in the tail stock to cut the threads.

The die works fast and does a great job.

A quick test fit shows the handle fits well.

Many feel the original rear action screw of the Model Seven is undersized (above, bottom), enlarging this screw to 1/4″-28 (above, top), enhances the rifle.

I mount the action in the milling machine’s vise and located the center of the rear action screw with a gauge pin.

A #3 drill is used to open up the hole.

And a 1/4-28 high-speed steel tap is used to cut the new threads. This particular tap has a guide that attaches to the drill chuck.

The action no accepts a 1/4-28 screw.

The metal parts of the rifle will be coated with Cerakote. To begin, they are heated in the curing oven.

The heat helps trapped oil come out of the parts. In the image above, note the residual cutting oil on the recoil lug. The part was dry when it was placed in the oven.

The parts are hung and degreased.

Once degreased they are placed into the blast cabinet.

I make sure the chamber and muzzle are plugged.

This is what the barreled action looks like after it comes out of the cabinet.

The parts are the coated with Cerakote and cured in the oven.

I had originally planned on bedding the stock for this rifle myself. When I received the stock from the manufacturer, Manners Composite Stocks, it had some issues. When I called Manners, they offered to address them and bed the action to the stock when they fixed the stock. Manners only beds the front and rear of the action. This is a view of the front bedding area.