“Inherent accuracy” is to the ammunition world what bigfoot is to the natural sciences: elusive, controversial, and possibly (probably) a myth.
A tremendous amount of online ink has been spilled on the subject. In this thread on The High Road forum, gun owners rattle off a variety of cartridges that, in their opinion, are inherently accurate. The .243 Win., .222 Rem., 6mm PPC, 6mm BR, .300 WSM, and the 7.62x39mm Savage all make appearances.
Keyboard warriors aren’t the only ones who have weighed in. Writing for Field & Stream, gun writer David E. Petzal explains that cartridges with less powder, a steeper shoulder angle, and small rifle primers lend themselves to better accuracy. As for the most inherently accurate cartridges, Petzal doesn’t hesitate: the 6.5mm Creedmoor and the 6mm PPC.
Why do these attributes lend themselves to accuracy? It’s not entirely clear. That’s why we went to the experts to help us shed light on this elusive controversy.
Seth Swerczek is Hornady’s Communications Manager, but he worked for the ammo maker as a product technical representative and a ballistics engineer. He is also an active competitor on the precision rifle circuit.
Tommy Todd is the Chief Ballistician for Sierra. In the last 30 years, Todd has worked in every department at Sierra in everything from raw material acquisition to bullet forming to testing. He’s competed in rifle competitions such as the HiPower Silhouette, Across the Course (HiPower), Long Range Benchrest, and F-Class.
Cutting to the Chase
Based on our conversations with Swerczek and Todd, some cartridges do have certain attributes that produce better accuracy. But these attributes are far less important than most people think. The most important factor is how the cartridge interacts with the chamber. Well-designed chambers will be more accurate, no matter the attributes of the cartridge.
I should say at the outset that when I say, “inherent accuracy,” I’m referring specifically to short-range accuracy. Long-range accuracy is determined by the velocity of the projectile and the bullet design, and we all know that long, high-BC bullets fly flatter than short, stubby bullets. For this Fact Checker, I’m sticking to accuracy within 100 yards or so.
Here’s what I found.
When a manufacturer sends a cartridge design to SAAMI for approval, they send a chamber design as well. Ammo makers use the cartridge specs and gun makers use the chamber specs to ensure that the firearms produced can safely fire that specific cartridge. The relationship between the cartridge and the chamber is the most important contributor to “inherent accuracy” – or lack thereof.
“It doesn’t necessarily come down to the cartridge design,” Swerczek told me “There are some things that typical match-type cartridges have, but the most crucial thing is the chamber design. The cartridge-chamber interface is supremely important when we’re talking about ‘inherent accuracy.’”
Swerczek explained to achieve good accuracy, a bullet must enter the rifling of a barrel perfectly straight. Even the slightest variation will affect a shot’s point-of-impact. Whether a bullet enters the rifling straight depends on the measurements of the chamber and, more specifically, the free bore.
“The diameter and angle of the free bore is critical,” Swerczek said, referring to the unrifled portion of the barrel. “Because if it’s too large, as that bullet is coming out of the case mouth, gravity is working. It’s got more of a chance to enter that rifling at a slightly off angle, and you never get that back.”
Todd agreed. The relationship between the chamber throat configuration and the average bullet shape utilized in the cartridge is one of only two factors the Sierra ballistician mentioned as influencing inherent accuracy.
A cartridge/chamber design with lots of play between the free bore and the bullet will tend to shoot more inconsistently than a tight bullet/free bore configuration. The .300 Winchester Magnum, for example, has a free bore diameter of 0.315”, which allows for 0.007” between the bore and the 0.308” bullet.
“The .300 Win. Mag. is an atrociously designed chamber, per the SAAMI specs,” Swerczek said. “The tolerances that that chamber allows are laughable.”
Custom-built chambers can correct for this, of course. Swerczek said that the .300 Win. Mag. can shoot like a “house afire” in a rifle that was built with a match-grade reamer.
“It all comes down to the chamber and cartridge and bullet relationship,” he concluded.
This is why some cartridges earn a reputation for accuracy. It’s not really the cartridge—it’s the combination of the SAAMI-approved, factory production cartridges and chambers that either shoot well or don’t. A custom rifle shooting hand loads can be dead-nuts accurate in almost any shouldered cartridge.
(If you want to do your own digging, you can find new and old cartridge/chamber designs on the SAAMI website here.)
Of course, there are some cartridge attributes that lend themselves to accuracy. Todd said that, as a general rule, cartridges with shorter, larger-diameter powder columns shoot more accurately than longer, narrower powder columns.
“The reason for this seems to be more consistent burn of the powder which translates to better external performance of the shooting platform,” Todd said.
Swerczek reiterated this point and said that straight-wall cartridges can be especially difficult to ignite consistently.
“That tall powder column can be difficult to get ignition consistency. Typically, with a straight-wall cartridge, you see higher velocity extreme spreads, which usually occur because of inconsistent early peak pressure,” he said.
Swerczek also mentioned that balanced cartridges tend to perform better than overbore, super-fast cartridges. The .308 Win., for example, doesn’t boast a great chamber design, but its size and velocity are balanced nicely, and shooters have had great success with the cartridge.
Interestingly, and in contradiction to the accepted wisdom on this topic, neither expert mentioned the shoulder angle as having a consistent impact on accuracy. The height of the powder column, the presence of a shoulder, and the relationship between the cartridge and the chamber seem to be the most important factors.
There are some cartridge/chamber designs that shoot more accurately out of factory rifles, so it’s understandable why some cartridges earn a reputation for accuracy. But that accuracy has little to do with the cartridge itself. A well-designed cartridge/chamber combination will shoot almost any round accurately, which is why I’m calling the “inherent accuracy” claim partly true.