According to Samuel Baker, a 14 bore double rifle shooting round balls, with only 4 1/2 drams, would put a ball completely through an Indian Elephant's head, side to side. Of course, they used hardened balls. Slugs were tried by many & him and given up upon as being inferior due to THEIR lack of penetration.

I'm talking muzzleloaders here. In a muzzleloader, hardened bullets could not be used. Hardened bullets had to be undersize to get down the bore and as such, they'd fall from the bore, or worse, move down and lodge as an obstruction if the muzzles were tilted down, as well as being inaccurate as they didn't 'take' the rifling. A hardened round ball penetrated more than enough on the heavy game.

After ctg. guns were developed and slugs could be hardened, they replaced the round balls in some guns, but not entirely, as many still trusted the hardened round balls over slugs, especially in the large bore doubles, 10's and 8's for example.

Nitro powders allowed the use of small bore sizes for all large and dangerous game, ie: .45 and .50 cal., which were mere 'stalking' or deer rifles prior to the development of smokeless powders. The nitro powders did little to improve upon the killing power of the very large bores, ie: 12 and over, as they didn't increase the speed of the balls much at all, merely shot somewhat cleaner.

Shot will probably pattern just fine out to 25 yards on rising birds, but the rifling is against you and will blow patterns fast. You'll have to test, of course. Without any sort of choke, heavy wads are prone to making donut patterns - one must adjust the load, ie: wads to find a load that shoots well. It takes at least 3 shots to 'prove' a load, pattern wise. A single shot on a patterning board can have you using a useless load for hunting and that merely makes one upset with nothing but misses due to donuts (nothing in the centre of the pattern where the wad blew through. It doen's talwasy blow through the centre, sometimes removing the entire left, right, top or bottom of the pattern. It's all experimentation - then, you get to try to find a load that shoots the same in each barrel - nothing but fun!

I've used shot in my Hawken .58 (24 bore) MLoading Rifle for grouse to 25 yards, but past that the pattern was VERY thin. It was deadly, if you could get it's 11 1/2 pounds swinging. My 9 1/2 pound 14 bore would be much better, but have never tried it with shot. I have the wads, but that's as far as that went.
Rifling can grab some lead, but a tight patch usually pulls that out.