If you're looking to mount a rail or a red dot, learning how to drill and tap a shotgun receiver is a skill that'll save you a lot of money and a long wait at the gunsmith. It's one of those projects that feels incredibly high-stakes because, let's be honest, you're literally poking permanent holes in a firearm. But if you have a steady hand, a decent drill press, and a bit of patience, it's something you can absolutely handle in your own garage.
Most older shotguns, like the classic Remington 870s or older Mossbergs, didn't always come from the factory ready for an optic. Back then, people were mostly just using the bead. Today, everyone wants a Holosun or a Vortex on their turkey or home defense gun. To get there, you need a solid base, and that means drilling and tapping.
Getting your workspace and tools ready
Before you even think about touching a drill bit to metal, you need the right gear. This isn't the kind of job where you can "wing it" with a handheld power drill. If you try that, I can almost guarantee the bit will walk, the hole will be crooked, and your red dot will be pointing three feet to the left.
You're going to need a drill press. It doesn't have to be a massive industrial model, but it needs to be square. You'll also need a centering punch, a set of high-quality drill bits (specifically the sizes meant for the taps you're using), the taps themselves, and plenty of cutting oil. Most shotgun rails use 6-48 or 8-40 screws. Don't just grab a random tap from the hardware store; gunsmithing threads are often finer than your standard bolt threads.
The most important thing, though, is a way to hold the receiver perfectly level. A drill press vice is a must, and some scrap leather or rubber pads will help you grip the receiver without scratching the finish.
Marking your spots
Once you've got the gun stripped down—and please, make sure it's unloaded and the trigger group is out—you need to figure out exactly where those holes are going. Usually, you'll have a rail or a base that you're planning to install.
I like to degrease the top of the receiver first with some brake cleaner or rubbing alcohol. Then, I apply a bit of layout fluid or even just a strip of masking tape. Place your rail where you want it, making sure it's centered and far enough back (or forward) to give you the right eye relief.
When you're happy with the position, use a transfer punch that fits the holes in the rail perfectly. Give it a light tap with a hammer. This marks the exact center of where you need to drill. If you don't use a transfer punch and just try to "eye it" with a scribe, you're asking for a headache later when the screws don't line up with the rail.
Setting up the drill press
This is the part where you spend twenty minutes measuring so that the actual drilling only takes ten seconds. Clamp the receiver in your vice. You need to make sure it's level in both directions—lengthwise and crosswise. If the receiver is tapered or rounded (which most are), this can be tricky. Use a small bubble level or a digital angle finder to get it as close to perfect as possible.
Once it's leveled and clamped tight, chuck up a center drill bit. These are short, stiff bits that won't flex. Use this to start a tiny pilot hole exactly on your punch marks. It keeps your main drill bit from wandering off-target when it first touches the steel.
The moment of truth: Drilling
Now it's time to actually drill. For a 6-48 tap, you'll usually use a #31 drill bit. For an 8-40, it's usually a #28. Double-check the chart that comes with your tap set just to be sure.
Set your drill press to a relatively slow speed. Steel doesn't like high-speed friction; it just creates heat and dulls your bit. Apply a drop of cutting oil to the spot and slowly lower the bit. You want to see nice, curly metal shavings coming out. If you're seeing smoke or the metal is turning blue, you're going too fast or pressing too hard.
Don't just drill straight through in one go. Lift the bit every couple of seconds to clear out the chips and add more oil. Since you're drilling through the top of the receiver into the empty space where the bolt goes, you'll feel the bit "pop" through. Stop immediately so you don't accidentally nick the bottom of the receiver.
Tapping the threads
This is the part where most people get nervous, and for good reason. Taps are made of very hard, brittle steel. If you put too much sideways pressure on one, it will snap off inside the hole. If that happens, you're in for a world of hurt trying to get it out.
Put your tap into a T-handle wrench. Some people like to start the tap while the receiver is still in the drill press by putting a center point in the drill chuck to keep the tap perfectly vertical. This is a great "pro tip" for keeping things straight.
Apply lots of oil. Turn the tap slowly clockwise until you feel it start to bite and get a bit heavy. The golden rule of tapping is: half a turn forward, then a quarter turn back. That backward turn breaks the metal chip off so it doesn't clog the flutes of the tap.
Take your time. If it feels like it's binding up, back it all the way out, clean the chips off the tap, add more oil, and start again. Once the tap spins freely all the way through, you're done with that hole.
Cleaning up and mounting the rail
Once all your holes are drilled and tapped, you'll probably have some burrs on the inside of the receiver. Take a small file or some fine-grit sandpaper and smooth those out. You don't want anything snagging on the bolt when you cycle the action.
Clean the holes thoroughly with a Q-tip and some degreaser. You want those threads bone-dry before you install the rail. When you finally go to mount your optic base, use a tiny drop of blue Loctite on each screw. Don't use the red stuff unless you want that rail to be a permanent part of the gun forever.
Tighten the screws down firmly, but don't go crazy. These are small screws, and you don't want to strip the very threads you just worked so hard to create.
A few final thoughts
It's completely normal to feel a bit of "pucker factor" when you start drilling into a perfectly good shotgun. But honestly, as long as you measure twice and drill once, it's a very manageable project.
The biggest mistakes I see are people rushing the layout or trying to use cheap, dull tools. If you invest in a good set of bits and take the time to level everything properly, the result will look just as good as if it came from the custom shop. Plus, there's a certain pride in knowing you did the work yourself every time you look through that red dot. Just remember: keep it slow, use plenty of oil, and don't force anything. Happy smithing!