Porting a shotgun barrel with a milling machine

Porting a shotgun barrel can help reduce felt recoil and muzzle rise for the shooter. Previously, I showed how a shotgun barrel can be ported with a simple fixture and a hand drill, see: Porting a shotgun barrel with a drill. This method worked well, however, you are limited to the pattern and drill sizes established by the fixture. When available, using a milling machine and an indexing set up (in this case a horizontal rotary table with chuck) allows a far greater range of porting sizes and configurations.

In this post I will be porting a Remington 870 Police 18.5″ barrel.

I ordered the following items from Brownells:

18.5″ 870 Police barrel (767-000-719)
.001″ Dial indicator (749-007-761)
Starrett dial calipers (827-559-101)

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Planning and layout

The design I am using in this write up is safe for my barrel. Use caution planning your own porting job to make sure you do not weaken the barrel. Consult a professional if you have an doubts about the hole size and pattern you plan on trying with your gun.

Since the barrel is basically a giant lever, moving the ports closer to the ends will help it work better. Any choke tubes and front sight assemblies need to be accounted for, or you can end up cutting through a changeable choke system or heavily fouling your front sight assembly with carbon.

I ended up starting my ports 35 degrees from the top on each side. This 70 degree included angle provides clearance for the front sight and should prevent fouling. Since I am running three rows of ports, I calculated that the next rows would each be another 12 degrees away, with the center row being offset. If you need help with calculating how to index your ports, google “circle calculator”. There are plenty of online resources available to help you with the math.

To index the barrel I will be using a 10" rotary table equipped with an 8" three jaw chuck and tail stock. The chuck will secure the chamber end of the barrel. I wrapped the area the chuck will touch with a layer of tape to protect its finish. — To index (or turn) the barrel I will be using a 10″ rotary table equipped with an 8″ three jaw chuck and tail stock. The chuck will secure the chamber end of the barrel. The tailstock secures the muzzle end of the barrel. I wrapped the area the chuck will touch with a layer of tape to protect its finish.

I aligned the barrel both vertically and horizontally in the mill using a dial indicator. By indicating along the barrel I can make sure it is properly aligned. Note: On this barrel, the diameter forward of the lug are the same diameter.

Here is a picture of the dial indicator verifying the barrel is parallel to the mills bed.

A view of the indexing set up. The rotary table indexes the barrel against the tailstock. I left my vise installed since it wasn’t in the way.

I begin with the rotary table on 0 degrees.

I will make the first series of holes 2.000" from the muzzle. This places the porting behind a Scattergun Technologies front sight. The barrel is roatated to 35 degrees and I drill a series of porting holes 3/32" in diameter on .188" centers. I drill 7 holes, skip a space, and drill one more. — I will make the first series of holes 2.000″ from the muzzle. This places the porting behind a Scattergun Technologies front sight. The barrel is rotated to 35 degrees and I drill a series of porting holes 3/32″ in diameter on .188″ centers. I drill 7 holes, skip a space, and drill one more.

I selected a double ended 3/32″ center cutting solid carbide 4 flute end mill. Secured in an R8 collet, this set up doesn’t walk and allows precise placement of the holes. I used the “line bore” feature on my digital readout (DRO) to calculate the hole placement, however, a dial indicator would have worked just as well.

Here is my first series of holes at 35 degrees.

Rotating the barrel another 24 degrees, to 59 degrees, I drill another series of 8 holes. I start the first hole aligned with the second hole of the first row.

I rotate the barrel over to the other side and drill the same series of holes. The first at 325 degrees (360-35) and the second at 301 degrees (360-59).

This is where it gets cool. I reset my X axis an additional 0 .094" in from the muzzle and make the same series of holes, .188" on center at 47 and 313 degrees. — This is where it gets cool. I reset my X axis an additional 0 .094″ in from the muzzle and make the same series of holes, .188″ on center at 47 and 313 degrees.

The final pattern- evenly spaced 3/32" holes. — The final pattern- evenly spaced 3/32″ holes.

Close up of the 24 3/32" ports on the side of the barrel. — Close up of the 24 3/32″ ports on the side of the barrel.

View of the 48 3/32" ports from the top of the barrel. — View of the 48 3/32″ ports from the top of the barrel.

Here is another porting design I machined on the same set up. — Two ported barrels. On the top is another porting design I machined on the same set up.

Since I ran a sharp endmill at high speed a quick pass with the bore brush removed any burrs. Alternatively, the inside of the barrel could have been honed smooth.