I like riding my 500cc single-cylinder crappily restored 1955 AJS just for the feel of it. It's far from perfect and by modern standards slow. It still handles better than me.

You make the point abt shooting a pistol for self defence. The accuracy necessary for that purpose is easily simulated by a BB gun. That's probably why reasonable quality BB guns exist on the world market. No point discussing pistol self defence here in the UK other than theoretically - but even then you can't simulate the noise and large recoil from a M1911 .45 auto and the resulting 'shock factor' of having fired it.

A smooth-bore BB gun is fun but in my experience they do not shoot better than me. In a hobby you get a degree of satisfaction based on what effort you've put in. I believe smooth-bore BBs will deliver little satisfaction once some effort has been put in. Others will disagree.

On the other hand, pure speed of shooting at suddenly appearing mechanically-presented large targets might even be fun with a repeating spudgun.

From slow motion video on youtube it can be seen that a blowback CO2 pistol dips down in the hand after firing. A fire-arm pistol most definitely kicks up, with or without a ported barrel. How does that prepare the novice shooter for double-tap or fast firing?