The bounced creature happily goes back to its owner's hand, and all counters on it (regardless of counter type) just vanish
Oko, Lorwyn Liege / Oko, Shadowmoor Scion I feel like I had to include Oko, Lorwyn Liege / Oko, Shadowmoor Scion because it has five abilities if you do not count the 1-mana transformation trigger
To actively mill your opponent, youll need to catch them with Didnt Say Please, stick an early Eye Collector (probably the most important enabler), or suffer playing Merfolk Secretkeeper (a combination of very two underwhelming cards, Tome Scour and Kraken Hatchling)
If you have ten bullets in your deck, even if youre playing 70 cards and viewing them all as bonus slots that dont take away from your other cards, youre still one-seventh bullets, meaning an opening hand will have an average of one bullet, which is like taking a mulligan every game if you assume bullets are narrow and not worth a card in the wrong place