Recently started slinging baseballs and they fly straight. Checking the video shows the baseballs have a “rifle spin” which is similar to a four-seam fastball spin…..which is known as a pitch that flies straight with no movement.

I pitched thru high school and am finding the parallels between baseball and slinging fascinating.

Video is a little poor quality but the spin is visible. https://youtube.com/shorts/bkojiYW22Nw?is=G1sVKYwLec78ytBM

I have seen irongoober’s video inducing tennis curve balls with wrist orientation. Surely that would work with a baseball too. I never had much movement on my pitches tho so might be a challenge to curve a baseball with a sling for me. I relied on pitch location, a decent fastball, and a super-deceptive change up. A limited arsenal which explains why I never made it to MLB!  ;D

Yep! A "neutral" sling release results in rifle spin, which is ideal for a straight accurate throw, especially at higher distances.

Archaic Arms has a great demonstration of it in his video: https://youtu.be/WHyK6r1Jbng?t=208

Like you mentioned, you can pronate or supinate at release to get right or left spin(respectively), and curve the same direction if desired for certain scenarios, or to adjust your throw closer to neutral if you aren't quite there.

One thing I do as habit now is load the tennis ball into the pouch with the logo facing the back of the pouch(at release) so that you'll see the logo after your throw. This way you can get a good gauge of your spin control with every throw and adjust over time.

At 15-20m for target slinging, the spin on a baseball won't have much curve effect due to its huge relative weight. But with tennis balls you'll see quite a but of curve at 15m and tons at 20m if you don't have a neutral release. Not necessarily a problem if you get used to it, but better to decide if you'd rather have a neutral throw than have to compensate.

[One thing I do as habit now is load the tennis ball into the pouch with the logo facing the back of the pouch(at release) so that you'll see the logo after your throw. This way you can get a good gauge of your spin control with every throw and adjust over time.]

That is very clever!!  Amazing idea!