There are a couple things that is giving you the problem and more powder is not the answer, might even make it worse.
The concave base you mentioned. is it a cup base? dish base? or hollow base?
Back in the late 1800rds the transition from muzzle loaders to breach seated cartridge rifles were making a transition and what worked well in the muzzle loaders using the hollow based bullets to help seal the bore, rifled or the smoothies were carried over to the cartridge rifles.
Company's like Sharps that sold patched bullets had the twisted patch tail and this gave a problem either sticking in the deep hollow base like the mini and caused accuracy problems but the twisted tail also gave a problem with accuracy because the wad would get pushed over the twisted tail and cause a gas leak because the wad edges got pushed away from the bore cutting grooves on the bullet shank. Man I looked at a lot of recovered paper patched bullets with damaged bases with different ways to protect the bullet base to improve the accuracy using wad stacks and patched bases.
Below are some recovered bullets with the patches still stuck in the cup based bullets a friend sent me with his instructions on how he loaded them and wanted me to load as he does. He lives out of the snow belt
and saw dust just don't give the same results as fresh soft snow does
Those were shot at short range of around 50 yards and the patches rode with the bullets.
What I found was the patches had lube stuck between the base and the folded paper how he patched the cup based bullets, he did not use a twisted tail that would have filled the cup and hold the wad from getting reduced in diameter and letting the lube wad get pushed between the wad and filling the void between the paper and bullet. also you can see the bulets have wrinkles on the shank from being deep seated in the case. One of those bullets(second from the left has a gas cut cutting the skirt through).
Also the skirts were badly deformed by the wad stack making them thinner.
The thinner the skirts the more problem for having the deformed base.
I once tried to develop a lead bullet I shoot for the matches and I also swage a lot of my bullets using both of the Corbins tooling and I wanted a bullet that had a rebated flat and hollow base like the Lapua and Sierra Match bullets but a shorter elliptical ogive. The first problem I ran into was keyholes at 200 yards and even at 100, well time for the snow fall later in the fall
well what I found was the base was greatly deformed from the wads and gas cuts.
What I'm getting at with these cup based bullets you need a hard wad that won't get pushed unto the cup based bullet and let the gas do the damage. With that rebated bullet in the photo I solved that problem using 3/16" bread flour over the wad to protect the fragile base and that solved the accuracy problems and it shot very good in my .40-70 Sharps.
But making that bullet is more time involved to make. A good cast flat based bullet is hard to beat.....
Corn meal under the bullet will look like #1 of the bullets in the bullet tray. That bullet had no wad to see what the black powder does to the bullet base and you can see the granule dimples. Corn meal will do the same and also badly damage the cup based skirts.
Gas cuts and damaged skirts will cause a gas leak as the bullet clears the muzzle with uneven pressure on one side causing keyholes or oval holes through paper as well as bullets to long or short for the twist of your rifling.
Sorry for the long book.
Kurt