I've been having this issue with my balearic sling. Its not every throw, maybe every 1 in 5 or 10 throws but I'd like to stop this from happening.

When this happens it feels like my projectile is getting caught somewhere on the pouch or the release cord as I throw and the projectile goes hard angle to the left, way off of where I'm aiming. Not sure if this is an issue with the pouch to being too small or too big. Or possibly something else. I'm curious if others experience this issue with their split pouch slings.

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Here are some photos. I mostly use tennis balls as ammo but I have experience this with rocks as well. Another factor I think could be causing this is my grip. I use my index finger to hold the retention loop and use my thumb and index finger to pinch the release cord.

Thanks, very helpful insight. Here's a photo of the other side showing the exact taper points. I starting tapering around 2 inches from the end of the pouch and tried to keep an even taper down to the release knot. 10 strands tapered to 3 strands.

So instead of trying to have an even taper, should I taper more strands at the start after the belly and slow the taper as Im working to the end of the release cord?

Tennis balls can also be unforgiving in balearic slings IMO. You need the wrist angle correct or the ball will get dragged to the left by the sling since it's so light and balearic slings are relatively heavy. I always hated using tennis balls with any sling, but it forced me to adjust my release and now it works much better.

In terms of the wrist angle for clean release I orient my hand as if I'm doing an open hand slap. Previously to this my wrist would rotate during the throw into a palm facing down position, which does not lend itself to clean releases (although the right combination of sling and ammo minimises the problem). Not sure what other slingers are doing.

Given that you know what a "belly" is... (kudos, I always have to go look it up in other posts) this may be too simplistic of an answer and you probably already know this. But in case you don't know, or others don't know...

The main reason I have a fouling of a shot (that jerks to the left  for a right handed throw) is because of a twist in the cords when I load the projectile. If the cords cross when you hold the sling by your side, it will almost invariably cause a fouled shot, the release cord gets tangled with the retention cord and releases super late. From your description, this is what I would guess is happening. Make sure that your sling is fully untwisted every time you load a projectile. My apologies if this is already known to you.

Another probable cause of this issue is what AA mentioned, which is just having a heavy release cord. It causes more deflection of the projectile upon release, compared to a lighter release cord, all else being equal. Essentially the inertia of the cords causes it to "hang on" to the ball longer before the release cord can move out of the way, deflecting the ball.

Thanks for all the input guys, I will try to incorporate these tips. Unfortunately I don't have a yard to do some quick slinging so hopefully I can get out to my local baseball field in the coming days and film a video.

Thanks for those videos David, its helpful seeing it unfold in slow motion. I think my misfires are very similar to whats happening in your bad release video.