Does a Rough Bore Mean Poor Accuracy?

July 08, 2017 By Brad Miller, Ph.D.

When I look down a barrel, I want to see a smooth, shiny bore. Polished lands and grooves are a beautiful thing. If I see machine marks or pitting, I cringe. They can't be good for accuracy. Can they? I had a chance to find out.

I recently acquired a more-or-less new ramped 9mm 1911 barrel for a mere pittance. It was too good a deal to pass up. I bought it sight unseen since it was via the Internet, but it was supposed to have been fired with less than a box of ammo.

When it arrived and I looked down the bore, I cringed a little. I did not see smooth, shiny lands and grooves. There was nothing shiny about it. It had what you would call a dark or rough bore.

The bore (barrel "A" in the figure) has two apparent features. The first is what appears to be pits in the steel. The second is "dark" surface areas. That said, the edges of the rifling are sharp. However, it is certainly not as nicely finished as other barrels I have, such as barrel "B" in the figure.

Sometimes, a dark or rough bore indicates a neglected, corroded or worn-out barrel. In these cases, accuracy can suffer, and sometimes accuracy is nonexistent. But you have to shoot them first to know if its appearance indicates a bad barrel or just cosmetic. This was a new barrel, but I had a nagging concern that it might not shoot very well because of the rough bore appearance. I could test it easy enough.

I have several Caspian 9mm/.38 Super slides for my 1911s, so I played mix-and-match with several bushings and slides until I came up with a combination that provided a nice fit. There was no discernible slop in the front.

In the rear, there was a slight bit of front-to-back movement, but it was only a couple of thousandths of an inch, and that's not bad. When this barrel and slide assembly was placed on a Para Ordnance high capacity frame and fully forward in battery, there was no obvious vertical movement of the barrel when it was linked up into position. Perfect.

The key to accuracy for most semi-automatic pistols is how precisely the barrel fits. A close fit that eliminates movement when the slide is fully in battery is ideal. Any slop in barrel fit means that it might not be in the same position every time the gun is fired, which means the barrel will be pointing at a slightly different spot on the target each time. That's what makes a "loose" barrel inaccurate. Repeatable precision requires that the barrel is pointing at the same spot on the target for every shot.

Now the question was, how would it shoot?

I assembled handloads for this test. I used handloads instead of factory ammunition because I could use bullets and powders with known accuracy potential. Now, just because one gun likes a particular load does not mean that another gun will. However, some bullet and powder combinations tend to be inherently accurate and can perform well in different pistols. In any case, I would find out how well the rough barrel liked these loads.

I slugged the barrel with a soft, oversized (.358-inch) cast bullet, and it measured .3563 of an inch. The usual jacketed 9mm bullets measure .355 of an inch. Some 9mm barrels are oversized like this one, and might benefit from larger diameter bullets. In addition to the accurate Nosler 115 and 124-grain .355-inch bullets, I used some larger diameter bullets.

I've always had good accuracy with Hornady Action Pistol (HAP) bullets, and I included their 125-grain .356-inch HAP bullet in this test. It's one of my favorites for target shooting. I also used a Hornady 110-grain .357-inch XTP revolver bullet, as it should fill the bore and provide a good gas seal.

Six different loads of 30 rounds each (180 rounds total) were loaded in Starline brass with CCI 500 or Federal GM100M primers. All 30 rounds were fired into a single group. A single group with a large number of rounds tells you how consistently the gun/ammunition shoots, and is a more accurate reflection of accuracy performance than traditional 5-shot groups.

The accuracy results show that even a rough bore can shoot extremely well. The smallest 30-shot group was produced with the Nosler 124-grain JHP at 2.42 inches. The largest group was the Nosler 115-grain JHP at 3.92 inches, but the majority of the hits are mostly clustered with two fliers expanding the group. The 110-grain .357 caliber bullet produced one of the most well-clustered groups with very few fliers.

There's nothing wrong with this barrel when it comes to accuracy. Most of these 30-shot groups are smaller than many guns can produce with 5-shot groups. Three of the loads kept all 30 shots under 2.50 inches. That's very impressive performance for any handgun, let alone a cobbled-together "Frankengun."

Good accuracy always depends on the ammunition, and that was true here as some loads produced larger groups, but a bad barrel won't shoot anything well. Clearly, this barrel is capable of excellent performance in spite of its cringe-worthy bore.

I was fortunate that it fit my slide and frame so well. The good fit allowed me to test the barrel's accuracy potential. A loose-fitting barrel will scatter shots and could have led me to falsely conclude that its lack of accuracy was due to the rough bore features.

I learned my lesson. You can't judge a barrel by appearance alone. This barrel's appearance is merely cosmetic and does not affect its ability to shoot straight. If you have a rough bore, take it to the range with some high-quality ammunition and see how it shoots. It might turn out, like this ugly duckling, to be a beautiful swan.

The author and publisher are not responsible for mishaps of any kind that might occur from the use of this data in developing your handloads. It is the user's responsibility to follow safe handloading guidelines to develop safe ammunition. You use this data at your own risk. No responsibility for the use or safety in use of this data is assumed or implied.