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.
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.
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.