What is paintball bore sizing? Bore sizing is the process of matching your barrel’s inner diameter to the outer diameter of your paintballs. A properly matched bore improves accuracy, reduces barrel breaks, and increases air efficiency. Most paintballs measure between .679 and .695 inches, and barrel bores are manufactured within that same range to allow precise matching.
| Bore Size | Best Paint Match | Common Use |
|---|---|---|
| .679 – .682 | Small or winter paint | Cold weather, brittle paint |
| .684 – .687 | Mid-range tournament paint | Most common match for quality paint |
| .689 – .691 | Standard recreational paint | Field-grade, rental setups |
| .693 – .695 | Large or summer paint | Hot weather, swollen paint |
Why Bore-to-Paint Match Matters
Three things change when you match your bore to your paint: accuracy, efficiency, and reliability.
Accuracy improves because a snug bore centers the paintball before it exits the barrel. A ball that rattles inside a loose bore bounces off the walls on its way out, leaving the muzzle on a slightly different angle with every shot. A matched bore eliminates that randomness.
Air efficiency increases because less air escapes around the ball. When the bore fits tightly, the burst of compressed air behind the paintball pushes it forward rather than leaking past it. Players running HPA tanks notice improved shot counts per fill when the bore is dialed in.
Reliability goes up because the right bore reduces barrel breaks. A bore that is too tight can pinch the shell and crack it inside the barrel. A bore that is too loose lets the ball roll forward and sit against the detents, where it can be clipped by the bolt on the next cycle. The correct fit holds the ball gently in place without crushing it.
Overbore vs Underbore
Players and manufacturers have debated overbore and underbore strategies for years. Both approaches work, but they solve different problems.
Overbore
Overboring means using a barrel bore that is slightly larger than the paintball. The ball does not contact the barrel walls on its way out, which virtually eliminates barrel breaks. Overboring wastes a small amount of air per shot because gas escapes around the ball, but the tradeoff is reliability with inconsistent paint.
Stock barrels on most paintball guns ship with a bore around .689 to .691, which overbores the majority of tournament-grade paint. This is intentional. Manufacturers prioritize avoiding breaks over squeezing out maximum efficiency, especially for players who may not own a bore kit.
Underbore
Underboring means using a barrel bore that is slightly smaller than the paintball. The ball sits on top of the bore opening and creates a seal, so nearly all the air pushes the ball rather than escaping around it. This method maximizes air efficiency and provides a consistent velocity shot to shot.
The risk is barrel breaks. If the bore is too tight for the paint, the shell can crack under pressure from the bolt or the air charge. Underboring works best with high-quality, round, thick-shelled paint. Players who underbore typically test multiple inserts before each session to find the tightest bore that still lets the ball pass through with a gentle blow of air.
The Sweet Spot
Most competitive players settle on a slight underbore. The ideal fit lets you hold the barrel vertically, drop a paintball into the breech end, and watch it stop partway down. A light puff of air from the muzzle end should push it back out. This “blow test” confirms the bore is snug enough to seal but not so tight that it will pinch the shell during firing.
How to Use a Bore Sizing Kit
A bore sizing kit is a set of barrel inserts or barrel backs, each machined to a specific bore diameter. Kits from brands like GOG Freak XL, Lurker Eigenbarrel, and Planet Eclipse come with inserts stepping in .001-inch increments. Here is the process for matching your bore on game day.
Step 1: Sample Your Paint
Open the bag of paintballs you plan to use. Grab five or six balls from different spots in the bag. Paint is not perfectly uniform, so sampling a handful gives you a better read on the average diameter.
Step 2: Roll Test
Start with a mid-range insert (around .685). Drop a paintball into the back of the insert. If the ball rolls straight through and falls out the other end, the bore is too large. Move down one size. If the ball sticks and will not budge, the bore is too tight. Move up one size.
Step 3: Blow Test
Once the ball stops partway through the insert on its own, hold the insert horizontally and blow gently into the back. The ball should pop out with light air pressure. If you have to blow hard, the bore is too tight and you risk breaks during play. If the ball rolls out without any air at all, the bore is too loose to seal properly.
Step 4: Confirm With Multiple Balls
Repeat the blow test with three or four more paintballs from the same bag. Paint varies slightly ball to ball, and you want a bore that works for the full range, not just one lucky sample. If most balls pass the blow test cleanly, you have found your match.
Step 5: Assemble and Chrono
Drop the matched insert into your barrel, thread it onto your marker, and chronograph your first few shots. A good bore match produces consistent velocity readings with minimal spread. If your chrono shows a spread of more than 10 fps across ten shots, the issue is likely paint quality rather than bore fit.
Factors That Change Paint Size
Paintball diameter is not fixed. Several factors cause paint to swell or shrink during a day of play.
Temperature is the biggest variable. Heat causes the gelatin shell and fill to expand, pushing the outer diameter up. Cold weather shrinks paint. A bore that fits perfectly in the staging area at 8 AM may be too tight by noon when the sun heats up your pod pack.
Humidity softens gelatin shells and can cause swelling. On muggy summer days, paint pulled from a sealed bag may grow measurably within an hour of exposure. Store your paint in a cool, shaded spot and avoid opening bags until you need them.
Brand and grade determine the baseline diameter. Tournament-grade paint tends to be smaller and more consistent (often .681 to .685). Field-grade and recreational paint runs larger and less uniform (.688 to .693). Knowing your paint’s typical range helps you grab the right insert before you even start testing.
Do You Need a Bore Kit?
Not every player needs a full bore sizing kit. Here is a breakdown by player type.
Tournament and competitive players benefit the most. Paint quality varies between events, weather shifts throughout the day, and every bit of accuracy and efficiency matters in a race-to format. A bore kit pays for itself quickly.
Regular recreational players who visit the same field and buy the same paint every weekend can get away with a single aftermarket barrel in a bore that matches their usual paint. A .685 bore covers most mid-grade paint comfortably.
New players should focus on fundamentals before investing in bore matching. A stock barrel with a .689 bore handles the widest range of paint without fuss. Spend the money on a quality marker and good paint first.
Common Bore Sizing Mistakes
Using one insert all day. Conditions change. Check your bore match after lunch, especially on hot or humid days, and swap inserts if the paint has shifted.
Ignoring paint quality. Bore sizing cannot fix bad paint. Dimpled, out-of-round, or thin-shelled paintballs will break regardless of bore fit. Start with the best paint you can afford.
Going too tight. Players chasing maximum efficiency sometimes underbore aggressively. A bore that is .002 inches smaller than the paint may seal well on the blow test but crack under the force of a bolt cycling at 10+ balls per second. Leave a small margin.
Skipping barrel maintenance. Paint residue builds up inside the barrel and effectively shrinks the bore. A dirty barrel that fit perfectly in the morning can start breaking paint by the afternoon. Swab your barrel between games and clean your marker at the end of every session.
Barrel Length and Bore Size
Barrel length and bore size are separate variables, but they interact. A longer barrel gives the air charge more time to accelerate the paintball, which can smooth out velocity in an overbore setup where some air escapes around the ball. A shorter barrel paired with an underbore is efficient because the tight seal means the ball reaches full velocity quickly.
Most players run barrels between 12 and 16 inches. Length beyond 16 inches adds weight and bulk without meaningful accuracy gains. The bore match matters far more than the length for shot-to-shot consistency.
Final Recommendations
Bore sizing is one of the simplest upgrades you can make to your setup. A barrel kit with inserts from .679 to .695 covers every paint and weather scenario you will encounter. Test your bore match before every session, recheck it when conditions shift, and keep your barrel clean. Pair a good bore match with quality paint and a reliable marker, and you will see tighter groupings and fewer breaks from the first game of the day to the last.