Firstly, they did not fire 2600 roundshot and 400 canister rounds, but 2516 rounds (or more roughly, 2600 rounds), of which 362 or 368 or 400 rounds were canister.
Cartouches employées 2516 dont 362 à balles.
Consommation: 2516 cartouches, dont 368 à balles.
Also a rough estimation of 2600 rounds:
Les troupes d'artillerie ont ou dans cotte journée, hors de combat, tues ou blessés, 4 officiers, 52 hommes; 53 chevaux ont été tués en moins de trois heures, il a été tire 2,600 coups de canon ou obus, dont 400 à mitraille.
The total expenditure of gun ammunition by Senarmont's 30-gun Battery was 2516 rounds, of which 368 were case-shot.
So Senarmont's artillery used 2516 rounds in the Battle of Friedland, of which (dont) 362 or 368 were canister.
For 'dont'
http://www.larousse.fr/dictionnaires/francais-anglais/dont/26325
Secondly, the 2516 or 2600 rounds were not fired in 30 minutes. But, as the 'Journal d'opérations du 1e corps de la Grande-Armée' indicates, in less than three hours (moins de trois heures). How many rounds were fired during the 25 minutes (vingt-cinq minutes) when the canister fire was 'most terrible' (le plus terrible) (Marion, p. 141, Girod de l'Ain, p.360) ? I simply don't know.