Just curious because want to make a competitive team to battle with friends and I think Weavile + Iron Ball Fling seems good.
The power of Fling is determined by the user's held item. The move will fail if the user has no held item, or if the item is not one listed below.
To determine the base power of each item, check the list in the source
Hope I helped!
Fling isn't a very competitively good move. Especially considering it's one-time use, Weavile is designed to be fast, and Iron Ball would slow it down and make it susceptible to instant death, I wouldn't recommend it.
The power of Fling is calculated as: a table that is difficult to copy-paste, so click the link if you want to see it.
Also you usually can't use fling more than once per battle. Knock off, night slash, or throat chop are almost always better.