Rifling Profile

While rifling profile is only really a concern for precision shooting, it is good to understand what the profiles are and why you would care.

Below is an illustration of the most common rifling profiles.

There are two classes of rifling: grooved and polygonal.

Grooved Rifling Profiles

Standard Rifling

Standard rifling has a rectangular land profile (the legs of a land are parallel to each other). 9 barrels out of 10 will have standard rifling.

The lands of standard rifling have sharp corners. This profile gives the most “bite” into the bullet. Because the edges cut into the bullet, it is more persuasive in imparting spin on the bullet.

The grooves have the most “dead” space out of any profile. This can allow gas to escape around the bullet, which can have a miniscule effect on chamber pressure…at least until this dead space becomes fouled up. Fouling effectively seals this dead space as more rounds are passed downrange after cleaning the bore.

Because of the dead space formed in the groove, cleaning the fouling out can be more challenging than other profiles. In addition, cleaning patches are more likely to snag on the sharper edges of the lands.

The lands of standard rifling have the most severe corner angles. As a result, the edge of the lands in standard rifling tends to wear faster than other profiles.

Radial Rifling

Radial rifling has a trapezoidal profile with the legs aligned with the radius of the bore.

The legs of radial rifling have a more balanced angle, which reduces the dead space in the corners of the groove while still retaining a sharp corner on the land. The result is good engagement of the bullet while having better gas sealing, less fouling, and easier cleaning when compared to standard rifling.

Radial rifling is ideal for long range precision shooting.

Hybrid Rifling

Hybrid rifling has a trapezoidal profile, like radial rifling, but the angle of the legs is more extreme (the projection intersects between the land and the center of the bore).

Hybrid rifling is less persuasive than standard and radial rifling, but not as subtle as polygonal rifling. The corners of the lands are nowhere near as sharp as standard rifling. There is no dead space in the corners of the groove. The result is moderate engagement of the bullet while having excellent gas sealing, less fouling, and easier cleaning than standard rifling.

A Comparison of Grooved Rifling

The primary difference between grooved rifling profiles is the angle of the legs of the lands.

If we extend the legs into the bore, we can see the exagertion of the angles:

Sharper lands leads to more persuasive engagement of the bullet.

The sharper edges cut into the jacket of the bullet and force it to spin earlier and more consistently. The earlier a bullet’s intended rotational speed can be achieved in the barrel, the longer it is spinning “at speed”, which contributes to a more stable spin once the bullet leaves the muzzle. The more consistently the bullet spins as it moves down the bore, the more consistently it spins when it leaves the barrel.

It should be noted that sharper edges do wear faster.

If we look a little closer at the grooves (in red in the diagram below), we can see the effect of the land profile on the groove profile:

Upon close examination of the groove profile, you can see how the standard rifling is more likely to have “dead” space, which is an opportunity both for gas to escape around the bullet and for fouling to deposit (which will actually fix the gas issue between cleanings). On the other hand, the hybrid profile has virtually no dead space. This contributes to a better gas seal and less fouling deposit.

The difference between grooved rifling profiles on ballistic performance will not be noticed by the vast majority of shooters. That said, precision shooters will notice the differences.

Inconsistencies in bullet behavior become amplified over a longer distance, so we want to focus on the characteristics of the rifling that contribute to consistency of the bullet in flight. If you have the option, we recommend a radial rifling profile for long range precision guns. The corners of the lands are not as sharp as standard rifling, which reduces some of the deformation of the bullet jacket. There is less dead space in the grooves, which means less variation in the gas seal between cleanings.

We recommend the 5R profile for long range precision shooting. This profile has an added benefit of not having diametrically opposed lands (i.e. lands are opposite grooves). This reduces the stress on the bullet, compared to standard, even-numbered rifling.

Polygonal Rifling Profiles

Unlike the other types of rifling, polygonal rifling does not have any edges. Instead of lands and grooves, polygonal rifling forms “hills” and “valleys”.

The theorical benefit of polygonal rifling is that the lack of sharp edges eliminates the cutting deformation of the jacket of the bullet and there is no dead space to allow gas to escape around the bullet or for fouling to build up in.

It is true that the lack of sharp edges eliminates much of the visible damage to the jacket of the bullet. However, polygonal rifling still causes plastic deformation of the bullet. As the bullet passes through the bore, it assumes a polygonal cross section (reference). This is not ideal for applications that require long range precision, as the deformed bullet will experience additional rotational drag, which will slow the spin (decreasing its gyroscopic stability) and make the bullet more susceptible to the Magnus effect (a.k.a. spin drift). This is one reason polygonal rifling is not ideal for precision shooting.

The lack of sharp edges also means that the bullet is not engaged as robustly. The bullet still spins in a polygonal barrel, but it will slip more as it begins to move down the bore and may skip over hills as it moves toward the muzzle. As discussed previously, the sharper the edges of the lands, the more consistent and stable the spin of the bullet. The lack of sharp edges in polygonal rifling allow the bullet to slip and skip over hills, leading to inconsistent spin stability after the bullet leaves the barrel. This inconsistency can have a dramatic effect over long distances. This is another reason to not use polygonal rifling for precision shooting.

It is true that the lack of groove corners improves the gas seal. However, it is worth noting that there is no noticeable effect on bore pressure or bullet velocity (reference).

It is true that the lack of sharp edges and dead space reduces fouling…usually. However, this only applies to copper jacketed bullets. Lead bullets have the opposite effect. Because polygonal rifling lacks a sharp edge, bullets are more likely to slip and skip over the hills (as previously mentioned). For unjacketed lead bullets, this causes streaking and accumulation of lead in the valleys with each shot. Beyond the implications for cleaning, this accumulation will cause constriction of the bore, which increases friction to the bullet, which can lead to dangerous bore pressures.

It is true that polygonal rifling is easier to clean…sort of. A patch will pass down a polygonal rifled bore without snagging. However, the ability to pass a patch down the bore and the need to do so are not the same thing. Depending on the bullets that you fire, you may have more fouling to deal with.

Because polygonal rifling lacks sharp edges, it does not wear as fast as grooved rifling. The hills are the first part of the rifling to wear, but they do wear much slower than the lands of grooved rifling.