Testing the Claim: Does Bore Fouling from Other Bullets Hurt Copper-Bullet Accuracy?

# Background

# Why sample size matters Small group tests (three to five shots) can vary widely—often by 50% or more—so they can create false impressions that a cleaning or switch improved accuracy. Aggregating four five-shot groups into a 20-shot sample provides a steadier measure of true precision. The author and team use 20-shot samples to see real trends instead of noise caused by small samples.

# Test setup and procedure Two rifles were used: a 20-inch 22 ARC with Hornady 70-grain CX copper bullets, and a 20-inch.223 Rem with Barnes 55-grain TSX copper bullets. Both barrels had about 300 rounds of mixed ammo on them at the start and were not cleaned.

• Fire one five-shot fouling group after the barrel's preexisting use.
• Fire four five-shot groups (20 shots total) and record for the "dirty" condition, allowing barrels to cool between groups.
• Clean barrels thoroughly and inspect with a borescope to remove most copper fouling.
• Fire one five-shot fouling group after cleaning.
• Fire another four five-shot groups (20 shots) and record for the "clean" condition.
• Aggregate dirty and clean data into two 20-shot groups each and compare group size and mean radius. The threshold for a meaningful change was set at greater than 20% difference.

# Results

• Dirty 20-shot group: 1.53 inches. Clean: 1.66 inches.
• Mean radius dirty:.41 in. Clean:.40 in.

• Dirty 20-shot group: 1.97 inches. Clean: 1.59 inches.
• Mean radius dirty:.55 in. Clean:.45 in.
• Net: 19.2% smaller group and 18% better mean radius after cleaning—close to the 20% threshold but still within typical noise.

# Interpretation The 22 ARC showed virtually no meaningful change after cleaning. The.223 showed a moderate improvement after cleaning, but not large enough to be conclusive given normal variability in 20-shot samples. The modest change could indicate a real effect for that specific rifle/ammo pairing, or it could be normal noise.

The testers also note a repeatable short-term effect: the first one to three rounds after switching ammo or starting with a clean bore sometimes print differently. That initial settle-down period is unpredictable and not reliably indicative of long-term performance.

# Practical takeaway for shooters If you want a reliable read on how copper bullets perform in a rifle, use larger sample sizes (20 shots aggregated) rather than a few quick groups. In this test, deep-cleaning to remove prior fouling did not produce a clear, consistent improvement in copper-bullet accuracy across both rifles. Cleaning may help in some individual rifle/ammo combinations, but expect only modest or inconsistent gains rather than a guaranteed accuracy fix.

# Final thought Proving the fouling theory false across all rifles would require broader testing, but these results indicate that routine deep cleaning to remove previous bullet fouling is unlikely to universally transform copper-bullet accuracy. Aggregated samples give a clearer, steadier picture of how a rifle will actually shoot with a particular bullet.