The Mariners Aren't A Sweeper Factory Anymore

Sweepers are so 2022 — now, it's all about the bullet slider.

When the sweeper swept across baseball during the 2022 season, the Seattle Mariners were some of its most visible practitioners. It wasn’t just Paul Sewald slinging backdoor filth to unsuspecting sluggers…

…it was also Matt Festa and Penn Murfee, ripping sweepers from seemingly identical low-slot release points. In 2022, it felt like the Mariners had found a machine that cloned Sewalds. Check out the similarity of the three movement charts of Sewald, Festa, and Murfee. The sweepers are on the left in gold, and the four-seam fastballs on the right in red:

But as the drawbacks of the sweeper have become more apparent — the dramatic horizontal movement makes it ineffective against opposite-handed hitters, not to mention difficult to command — the Mariners have quietly moved away from the trendy bender. Instead, Seattle’s pitching lab is making a notable move toward the bullet slider.

Gabe Speier might be the perfect case study. In 2022, Speier was an unremarkable lefty specialist type bouncing between the Kansas City major league and Triple A clubs. In his 19.1 major league innings, he posted a strikeout rate was 18%, well below league average. He simply did not have much of a swing-and-miss pitch in his profile; the slider, his primary pitch, had negative induced vertical break (IVB) — rare for a slider — but averaged just 82 mph with a bit of horizontal movement. Check out the yellow dots on the right of the plot:

Flash forward to 2023, after the Mariners acquired him off waivers from the Royals. Speier went from a pitch to contact guy to ranking first in all of baseball in getting hitters to swing at pitches outside the zone. How did he do it?

First, he emphasized the sinker over the four-seam fastball, creating more horizontal separation between his primary fastball and the slider. (Note the orange dots below — the sinker moves further to the arm-side than the four-seam fastball.) Second, he increased the velocity while decreasing the horizontal movement on his slider, amping up the average velocity to 85 mph. As a result, the whiff rate on the slider nearly doubled (!) between 2022 and 2023.

The Mariners vaunted pitching lab could have moved Speier in the opposite direction, asking him to sacrifice velocity in order to achieve more horizontal break on the pitch. After all, Speier is operating from a low 2/3rdish arm slot, favorable for sweeper development. Instead, they pushed Speier closer to the slider favored by Logan Gilbert and Luis Castillo: the bullet.

A theoretically “perfect” bullet slider — also known as the gyro slider — has no topspin or sidespin. Rather, it spins like a spiraling football, moving only as much as gravity allows. In contrast to a curveball, where the topspin makes the ball drop more than one would expect from gravity alone, or a four-seam fastball, where backspin makes the ball resist gravity, a bullet slider simply drops. The Castillo pitch plot provides a visual demonstration of what this looks like on a movement chart. Check out that big blob in yellow intersecting all four quadrants of the plot:

And here’s the pitch in visual form:

As you can see here, it does not move dramatically side-to-side like the Sewald frisbee at the start of this post; it just plops right at the knees of Correa for a called strike.

Unlike the sweeper, the bullet slider can be thrown to both righties and lefties, travels at much higher velocities, and is easier to command due to the relative lack of movement. These qualities, no doubt, have caused the Mariners to move away from the sweeper fad and toward pitchers who can master the bullet.

Gone are sweeper-heavy relievers such as Festa, Murfee, Sewald, and Justin Topa. Taking up prominent spots in the back of the bullpen for 2024 are Ryne Stanek and Gregory Santos, who both make frequent use of their low-movement, high-velocity slider offerings:

These two, alongside Speier, Andrés Munoz, and Matt Brash — whose hard slider defies categorization — will occupy the high-leverage spots in the bullpen once all involved are fully healthy. All five throw hard sliders with little horizontal break. On the player acquisition side, the preference for speed over movement in the slider profile seems clear.

Will this shift in slider philosophy filter down to the player development side of the team? That remains to be seen. Many of the arms on the fringes of the roster — Cody Bolton, Tyson Miller, Collin Snider, Austin Voth — featured frisbee-type sliders in 2023 with hardly anything in the middle of the pitch plot. Any one of those guys might be the next Mariners pitching lab reclamation project — and if this organizational trend is any indication, their breakout will rely on a heavy dose of bullet sliders.