Thanks Staale and David for replying with info I did not know about the pom-poms.
I cut and pasted what was on show from an item in Microsoft News, it is full of ads but I thought safe otherwise. It's obviously click-bait but thoughtlessly I forgot to add that important bit of info in my title, and hence, saving many from needlessly and tiringly 'clicking' on. I know, it's rough!
The captions in these click-bait shows are usually wrong for a whole range of reasons and so should be taken with a grain of salt at the very least, but the images themselves are always interesting I think, and worth a look. The captions weren't the point but image itself can always be taken a bit further if the viewer wants.
It's a cheap sauce, but it's still a sauce.
40mm, as David said. The same caliber as the Bofors but with a lower muzzle velocity so hitting was harder and range was shorter. A quad Bofors was roughly as big and as heavy as an octuple pom-pom but had a better chance of actually shooting something down. It may have required more crew to keep it fed in combat, it had five-shot magazines where the pom-pom had those huge ammo-belt canisters. Once you had emptied the pom-pom canisters it would take quite a while to reload of course.
Nobody will hit anything with a ring sight. Later in the war the pom-poms got remote power control off a separate directory which made things better. The Bofors is in exactly the same situation of course, it needs a director to be effective.
As for effective... ship AA was mostly expected to distract the enemy pilot and to ruin the his bomb aim. Hitting him is a welcome but unexpected bonus. I suppose the pompom was as good as anything in this respect, there certainly is an impressive amount of shells going out from an octuple installation! And if a 40mm shell should happen to hit your plane you will certainly notice it.
19 - 2pdr pom-pom looks impressive and a good view of the shell link. What caliber was the 2pdr, looks at least 30mm?
25 - multiple pom-pom firing and by the hands-over-the-ears of one of the crew, incredibly noisy. Standard equipment on Commonwealth ships early but were they effective (deadly)?