bait pump

my son has started sea fishing (mostly beach) and has been buying his bait online so i thought i would be nice and buy a bait pump to save money in the long run, i have seen that they come in two sizes 28mm and 50mm, which would be best to buy and why?

So here's the deal: I bought a bait pump (28 mm) and I am frustrated with my inability to get lugworms with it. I got a handful the first time (with lots of attempts). Today I tried for at least 30 mins, probably some 50 casts or so and only found two tiny little guys. I have read various posts...