3 Gallon Batch In 6.5 Gallon Bucket?

Would there be any downside to fermenting (primary) a 3 gallon batch in a 6.5 gallon bucket? I know for secondary, this would be a bad idea, but are there any ill effects for such excessive volume?

No downside at all. I’ve fermented 2.5 and 3 gallon batches in a 6 gallon carboy before. During fermentation, the headspace will fill with CO2, which will protect against oxidation. After fermentation, would advise against racking to a “secondary” vessel with a large headspace for bulk aging. Oxidation would become a concern in that case.

+1. I just did this a few months ago when I split a batch of Ipa to try different yeasts. No ill effects at all. Proceed, good sir.

No problems at all putting 3 gal. in your 6.5 fermenter. I have a 7.5 wine bucket and I have put 3 gal. beers in that. I call them yeast starters.

+1 again. I do 3 gallon batches. All in 5 or 6 gallon carboys.

Same here - I do 3 gallon batches in 6-6.5 gallon buckets and carboys. I have some 3 gallon Better Bottles for secondary fermenters if I want to age for an extended time, but that’s generally just for meads and sours/Brett-aged beers.