So 26 grams = A little over 401. Grains & being the Avg,.52 cal Spencer bullet is 435. ish Grains .
This would make it a Good sized Bullet; (The average .45 ACP is 230 Grains).
The bullet has the look of a fired Spencer but the bottom is to rounded & the Rifling is 'Pronounced'.
I'm thinking SG-Slug of some Kind , Tho I don't know the weight in Gr, of the Avg, 12 Ga Slug....I would assume it's more than 401. Grains .