A previous build that I still have but is dismantled now was made for 8mm steel ball bearings but I wasn't sure how accurate it was going to be past 25-30 metres being a smooth bore system.
While they intuitively sound good, steel balls are quite poor projectiles.
First of all, a sphere has quite a high drag coefficient compared to a conventional "bullet" shape:
This means that it's going to lose velocity at a higher rate, and drop is going to be significant over range meaning that correct range estimation is absolutely crucial to accuracy at distance. The fact that steel also has a relatively low density compared to conventional bullet materials such as lead means that for a given diameter it will lack weight and therefore less momentum to counter air resistance - think of it as the comparison between throwing a golf ball and a ping pong ball, both are about the same size and shape but even if thrown at a lower velocity, the golf ball is going to go much further because it slows down at lower rate.
In your experience, how do round projectiles out of smooth bore perform in trajectory over 100 or so metres?
Not terribly well, especially if there is a significant discrepancy between the the diameter of the barrel and the projectile.
If you can close that gap, and accuracy means reliably hitting a 3 x 3 foot target than a 1 inch circle, then it might be acceptable.
This does not mean that you necessarily need a rifled barrel to hit a target at 100 meters - one can for example have good results with
shotgun slugs in a smoothbore but if you're relying on drag for stability, as velocity starts to drop then group size increases.
You've made a very respectable launcher with higher performance than most projects on this forum, perhaps if accuracy is your goal then you should consider
making your own rifled barrel (assuming purchasing one is not an option) and casting your own lead bullets?