Different types of rifling
by B.B. Pelletier
This will be limited to the common types of rifling found in modern airguns. I can't possibly cover all the types of rifling in this posting - or even in this entire blog! There are several books written on this subject, so if you are interested in pursuing the subject further, I recommend you do a search in the used book stores online. I'm doing this post because several people have asked why it's okay to shoot both BBs and pellets in some guns but not in others.
Lands and grooves
Lands and grooves are a common type of rifling. The lands are square-cornered ridges that raise up from the grooves. They are cut in a gentle spiral, so they impart a spin to the projectile. The twist rate is how many inches of bore it takes for the projectile to make one complete rotation, so one turn in 10 inches of twist (1:10) means the rifling (and anything that passes through it) rotates once every ten inches. A one-in-sixteen twist rate (1:16) that is standard for .22 long rifle is also very common in airguns of all calibers. Because the diabolo shape also helps stabilize the pellet, it doesn't rely entirely on the rifling, so the twist rate isn't as critical.
Some big bore air rifles have traditional land-and-groove rifling, which makes loading difficult if you try to seat the bullet into the bore (because the lands are taller and offer greater resistance). Makers compensate for that by tapering the lands at the breech.
Traditional land-and-groove rifling looks like this in cross section.
Microgroove lands and grooves
Microgroove is a trade name of Marlin. It was trademarked in the early 1950s, but the type of rifling has been known since at least the late 19th century. If he didn't invent the idea, Harry Pope perfected it. Microgroove rifling refers to the conventional land-and-groove pattern where the height of the lands is lower, and the width is narrower. While this isn't all that Pope created, it is an important part. Shallower, thinner rifling means less deformation of the bullet/pellet, which improves accuracy.
Airguns are perfect for microgrooves, because they also offer less resistance when the pellet is engraved by the rifling and help seal the bore better because there is less depth and width to seal. The simplest kind of microgroove rifling is just shallower and thinner lands of the land-and-groove type. Twelve lands are common, but sometimes six are used, especially in .22 caliber. Where normal rifling lands are 0.005" high, microgrooves might only be 0.0015" high.
Barrels that shoot both BBs and pellets often have polygonal rifling. This is an old type that dates back to the 19th century, but it is also used by some very modern makers of firearms. Instead of the bore being perfectly round, it is a polygon. It still twists just like the other kinds of rifling, only the entire polygonal shape twists.
This drawing is exaggerated to better show the concept of polygonal rifling.
While the BB is still loose in any barrel that also accommodates a .177 pellet, the air blowby does no real harm. There are no lands to damage in polygonal rifling. However, accuracy is generally not as good as in a dedicated microgrooved barrel shooting lead pellets only. The dual ammunition is a good sales point in lower-priced airguns, and the much lighter steel BBs fly faster regardless of the amount of air that blows by. To compare this type of rifling to what is possible with a BB in a smoothbore barrel, Daisy's Avanti Cyampion 499 will group 10 shots in 0.20" at 16 feet, while a polygonal barrel might group BBs in one inch or larger at the same range.
For this reason, I call all such dual ammunition barrels a compromise. Like the .357 Magnum revolvers that also have cylinders for 9MM, neither one will be as accurate as a gun that is dedicated to just one type.