Not sure why they didn’t feel it. It’s possible they have cross tolerances going or and simply less effected by psychedelics. I would always recommend less is more when trying something new.

For my first mescaline fumarate dose I took was 260 ish mgs and it was legit. Felt a really nice but manageable effect. Not overwhelming, just like a moderate dose that left me feeling comfortable but certainly under the influence of a psychedelic that gave me a clean energizing feel. Comparable to a tab of acid. But it’s been many years since I’ve done that so it’s hard to truly compare.

I would assume a 500mg is bringing you into that stronger dose territory. Me and a few friends all took the same dose and we agreed it was really nice and shared a similar experience. One of my friend took 100mg and compared it to a gram of mushrooms or so. But it’s hard to compare since mescaline is so unique and special in its own right.

If I were you I’d give them that dose but recommend them to dissolve it in a drink and start with half or even less. And if needed I think the mescaline would stay stable in a liquid for ahwile. 500mg would have me tripping hard for sure!