Bad manners? What if it’s a wine they don’t offer. It seems every restaurant I want to go to near me bans large format bottles even if they allow for multiple standard bottles with corkage.
When I worked in the industry I saw lots of mags and two 5L bottles. Both were for largish parties and they paid a fee for glassware and service. One 1985 GDL and the other was a Margaux.
If you are expecting the restaurant to do the wine service then it’s understandable that they might not want to have to maneuver large formats. If you’re doing it it doesn’t make much sense but then many restaurants have wine policies that make no sense to me.
Ask nicely beforehand, order food eagerly, share with back and front staff, and tip well.
Once, I brought a magnum of 1989 Bricco Manzoni to Union Square Cafe, only to discover that it was just added to the wine list. No problem. The restaurant was still welcoming, and charged only a standard bottle corkage. Everybody was happy.
We left a good remainder for other patrons to share.
I’ve never encountered a place with an anti-large format policy. I’ve had places charge me double for mags and others charge me single. The one time I brought a 3L I think it was a single charge (and we decanted the bottle into multiple decanters and used those to pour for the table).
I’ve also brought halves and never had anyone charge me half.
Took a 3L Montelena (IIRC) to Maestro’s in Chicago for a bachelor party dinner. No problems at all. Spoke with the somm and worked out fair pricing no real issue. I’ve also taken mags and been charged 2 x regular corkage. Went to Osteria Langhe in Chicago with a mag of Barolo and they charged just the 750 price. In all cases I either e-mailed or spoke directly with someone to confirm.