Grizzly Mill

This is the Grizzly milling machine used for the port slotting function. I use a rotary table, with a 3 jaw chuck to hold the 303 stainless bar stock.

Rock River Varmint 24" EOP

This is the rifle, with the first brake removed, and the cap in place. My Grizzly 13X40 lathe is in the background.

First Brake Design

This was my first try at a brake. The large internal bore diameter caused too much perturbation of the bullet, and the groups were terrible.

Threaded Muzzle

This is the threaded muzzle and crown. The thread is 11/16" X 24 TPI, about 0.350" deep. It was turned on the lathe after indicating in the barrel bore to the axis of rotation.

Muzzle Cap

I made this cap along with the first brake design. It protects the threads when you are not using a brake.

Stripped Upper

Here is the upper stripped of all hardware, prior to removing the barrel. The QRW rings make the scope removal very easy. The barrel will be completely stripped, so that it can be mounted in the lathe to turn the brake to final OD.

Spotting for Drilling Procedure

Here is where we start the machining of the brake. In order to not abuse the end mill, I found it easier if holes were drilled through before the slot is cut. This gives the chips somewhere to go, and is much easier on the end mill. I used a standard center drill to place starting spots. I used a 0.125" drill to make the five holes, placed approprately for the three slots.

End Mill

This was the 5 flute, high helix, coated carbide end mill used. It has a flute length of 5/8", making the slotting operation easy. Mill down halfway through the part, rotate the table 180 degrees, and finish the back side of the slot. The slots were 3/16" (end mill diameter), with a length of about 0.700" I wanted 0.150" thickness on the top and bottom after the part was turned to final OD.

Finished Slots

Here is the final result from the milling operation. Unfortunately, I did not take a picture of the part after the hole drilling, or during the milling.

Chucking the Brake

Here is the part mounted in the 3 jaw chuck. The rotational alignment was set such that the top of the part was exactly opposite one jaw. This was necessary so that the rotational alignment of the brake to the barrel can be accurately set during the final fitting stage.

Brake with Rough Bore Hole

A series of drills, from 0.250" up to 0.5", were used to bore this initial hole. The hole goes all the way through to the back of the wall formed by the slot closest to the muzzle.

Bore to Final ID

Here, a boring bar was used to enlarge the hole to 0.645" for the 11/16" X 24 TPI, Class 3 thread.

Threading

This internal threading tool was used to cut the thread. A very close fit is desired, so thread dimension for a class 3 thread were used. Many light cuts are used, checking for fit after each cut. After each tool position change, at least two more passes are done to correct for as much tool deflection as possible.

Thread Gage

When I made the first brake, I cut this thread gage from 303 stainless to the precise dimension for this 11/16" X 24 TPI, Class 3 thread. I used the three wire method to ensure the the pitch diameter was perfect. The same measuring technique was used on the muzzle thread, making the gage and the muzzle identical. This gage is used to check for fit when threading the ID of the brake.

Finished Thread

This picture shows the finished internal thread. The rough burr shown is removed in subsequent machining operations.

Test Fitting the Barrel

This shows the barrel threaded into the part. Check for a tight, but not binding fit. It should go on easily, and not wobble.

Final Fit to the Barrel

In this step, we are trying to ensure that the brake is aligned with the top of the rifle when screwed on firmly. We want the ports to be exactly out the sides. Note the small gas port about halfway up the barrel. This is used to indicate the top of the barrel. Since the brake was set in the chuck with one jaw going straight down, it is easy to tell if there is any angle error. For a 24TPI thread, one degree of rotation corresponds to about 0.0001". Estimate the number of degrees in error, and take off the apropriate amount from the annular mating surface until the alignment is satisfactory.

Reaming the Final Bore

After rough drilling to about 0.25", a series of reaming operations, in steps of 1/64", is done to get to the final size of 9/32" (0.281"). This provides adequate, but not excessive, clearance for the 0.224" bullet. After this operation, the part can be removed from the lathe.

Bored Part

This is the top view of the part after final boring.

Final Part

Here is the threaded end of the final part. Note the slight counter bore used to protect the thread end, and the very smooth finish on the mating surface. Also note how the threaded bore goes completely through to the first baffle.

Indicating in the Barrel

In order to get a perfect fit to the barrel when turning down the brake OD, it is neccessary to use an indicator rod, two dial indicators, a four jaw chuck, and a spider on the other end of the lathe spindle to precisely align the barrel bore to the axis of rotation of the lathe. The indicator rod uses a precision bushing and taper to place the rod precisely in the bore center. By adjusting the four jaw and the spider, you can reduce the runout at both points on the rod to under 0.0001". Note the thick copper wire around the barrel at the four jaw, protecting the barrel.

Before Turning

This shot shows the brake on the indicated barrel. Note the excess material, which will be removed in the next operation.

Turning the Brake to Size

Here is the turning operation. Coolant is used to get the best finish, and longer tool life to boot. Successive light passes are performed until the part is at exactly the same OD as the barrel.

Final Fit

After turning, the fit is perfect. You cannot feel the transition with your finger.

Cutting the Brake Crown

Here, the compound, set at an 11 degree angle, is used to cut a recessed crown in the brake. While this is not really important, it is a nice esthetic touch.

Cutting the Brake Crown

A view of the tool used to turn this dished crown. It is the same process used to turn a muzzle crown. You cut from the inside out, leaving no burr at the center. A very sharp high speed steel cutter works better for this than a carbide cutter.

Finished Part

Here is the part on the barrel. The next step is to bead blast the part on the barrel, getting that seamless look.

Protecting the Muzzle

You do not want beads to hit the crown when blasting, so protect the crown with masking tape. You should also tape up the gas port area, as well as the chamber end. Be sure to remove the tape and clean the bore before assembling the barrel back into the upper.

Final Finish

The finished brake. The joint is visible here, primarily due to the color difference between the 416 stainless of the barrel and the 303 stainless fo the brake, and some small oil seepage. I always use some light oil (Kroil) on the brake threads after each cleaning. You cannot feel the joint, even with a fingernail.

Finished Rifle

Here is the finished, reassembled rifle. The brake worked perfectly, not affecting accuracy a bit.