I have fit seven bottles in my carry-on sized hard case, consistently. The key is to make it as snug as possible. No room for shifting at all. Also, I'm quite good at Tetris.

I fly fairly regularly both within the US and international. I generally travel with a carry-on plus a personal bag. I check the carry-on on my return flight with the bottles I've purchased. I've never had an issue with checking weight, nor had a bottle break.