How to Choose the Right Gun Drill Bit for Every Caliber – A Step‑by‑Step Guide

If you’ve ever tried to bore a barrel with a bit that was too small, you know the frustration of a jammed drill, a ruined workpiece, and a sore wrist. Picking the right gun drill bit isn’t just a convenience—it’s the difference between a clean, repeatable hole and a costly mistake. At Precision Drilling Hub we’ve seen every size from .22 to .50, and the right bit makes the job feel like a walk in the shop.

Know Your Caliber

Start with the bullet

The first thing you need to know is the exact diameter of the projectile you’ll be loading. A .223 Remington, for example, measures about 0.224 inches, while a .45 ACP is roughly 0.452 inches. Write that number down. It’s the baseline for everything that follows.

Allow for clearance

You don’t want the bullet to scrape the bore. A typical clearance is 0.001 to 0.002 inches for most pistol calibers, a bit more for high‑pressure rifle rounds. So for a .308 Winchester (0.308 inches) you’d look for a bit around 0.310 inches. This tiny gap gives the bullet room to travel without binding, and it also helps keep pressure spikes down.

Match the Bit Geometry

Twist rate matters

Gun drill bits have a “twist per inch” rating. A tighter twist (more turns per inch) is better for harder metals like stainless steel, while a looser twist works well in softer alloys. In my early days I once used a high‑twist bit on a 7075‑T6 aluminum barrel and spent an hour cleaning up a mess of chips. Lesson learned: match the twist to the material.

Point angle

Most gun bits come with a 118‑degree point angle, which is a good all‑round choice. If you’re drilling into hardened steel, a 135‑degree angle can reduce chatter. For brass or copper, stick with the standard 118 degrees. The angle determines how the bit cuts; too sharp and you’ll get a lot of heat, too blunt and you’ll push the material aside.

Pick the Right Material

High‑speed steel (HSS)

HSS bits are cheap, durable, and work fine for most handgun calibers in mild steel. They’re the go‑to for hobbyists who don’t want to break the bank. If you’re drilling a lot of .38 Special or .45 ACP barrels, an HSS bit will see plenty of use before you need a replacement.

Cobalt (M42)

When you move up to .308 or .30‑06 rifle barrels, the steel gets tougher. Cobalt bits retain hardness at higher temperatures, so they stay sharp longer under heavy cuts. They’re pricier, but the extra life is worth it when you’re pulling 55,000‑plus rounds through the same bore.

Carbide

Carbide bits are the heavy hitters. They’re virtually unbreakable and stay sharp even in the toughest alloys. The downside? They’re brittle and can snap if you force them. Use carbide only when you’re drilling hardened steel or exotic alloys, and always run a light feed.

Consider Length and Shank Type

Length

A longer bit gives you more stability when you’re drilling deep holes, but it also adds flex. For most pistol barrels (about 4‑5 inches long) a 6‑inch bit is plenty. For rifle barrels, especially when you’re drilling a gas port or a threaded barrel, a 12‑inch or even 16‑inch bit may be needed.

Shank

Most gun bits have a 1/4‑inch shank that fits standard drill presses. If you’re using a CNC machine, you might need a 5/8‑inch or a custom collet. I keep a few 1/4‑inch and a couple of 5/8‑inch shanks in my toolbox so I’m never caught short.

Test Before You Trust

Dry run

Before you mount the bit on a live barrel, do a dry run on a piece of scrap metal of the same hardness. This lets you check the feed rate, see if the bit is wobbling, and verify that the point angle is cutting cleanly.

Coolant

Never underestimate the power of a good coolant. A simple mix of water and a drop of dish soap works fine for most steel, but for high‑speed runs I use a petroleum‑based cutting oil. It reduces heat, extends bit life, and gives a smoother finish.

Check the hole

After the test cut, measure the hole with a set of calipers. It should be within the clearance range you calculated earlier. If it’s too tight, step up to the next size. If it’s too loose, you may have a worn bit or a mis‑set drill press.

Final Checklist

Know the exact bullet diameter – write it down.
Add proper clearance – 0.001‑0.002 inches for most rounds.
Select twist rate – tighter for hard steel, looser for soft alloys.
Pick material – HSS for pistols, cobalt for rifles, carbide for hardened steel.
Choose length and shank – match the barrel depth and your machine.
Run a dry test – verify feed, coolant, and hole size.
Inspect the finished hole – measure, clean, and be satisfied.

When you follow these steps, you’ll find that picking the right gun drill bit is less guesswork and more a simple, repeatable process. I’ve saved countless hours – and a few barrels – by sticking to this routine. The next time you pull out your drill press, you’ll know exactly which bit to grab, and you’ll get that perfect, straight‑through hole every single time.