Not a 4-lber piece, nor French, and made in Bengal, yet this quote of tests is made about the wooden "bottoms" in the canister for a 13-inch mortar experiment made by artillerists. I can not comment upon whether similar effects helped with the French, in practice or intent. - R
"The balls were put into tin canisters, having a strong wooden bottom of about 3½ inches thick.
It was owing to this bottom that the projectiles issued in the compact manner they did; for closing over the charge of powder, and fitting compactly the rounding of the bottom of the bore of the piece, the inflamed fluid of the powder had no means of insinuating itself between the balls, and causing them to diverge or spread, but was obliged to push forward in a lump the whole loading."
then, a little later on, comes the comment:
"These grape, viewed in the air on their way to the breach and column of assailants, had the appearapce of a flight of small birds so close, that the light could just be seen between each other."
Both on page 405 of Jones, John Thomas. Journals of Sieges Carried on by the Army Under the Duke of Wellington, in Spain, Between the Years 1811 and 1814: With Notes. 1827.