Birdshot from a rifled barrel generally tends to be pretty worthless beyond a few feet. I like to experiment with odd-ball loads and have loaded birdshot in .45 Colt, .45 ACP, .45/70 and .444 with pretty much the same result in all, the rifling just spins the shot out in a radial pattern, scattering so wild as to be useless other than for snakes at 10 feet.
Your mention of 3/4 ounces of shot and 20 grains of Unique scares me! You have to remember that when you fill the case with lead it is not at all like a bullet seated normally, you cannot just use the data for a bullet of similar weight. You cannot use 45/70 or 45/120 data at all but should use .410 shotgun data. All in all, it is not worth the bother.
I also tried loads 45/70 with three .457 roundballs and at 65 yards only one ball would even hit the 18x24" target. At 35 yards sometimes two balls would hit somewhere on the target but unlikely to be anywhere near the point of aim. Really, even if they shot accurately, I can't think of anything they could do that a single bullet wouldn't do better. Five .457 balls would total about 715 grains and once again, you cannot use the same powder charges you would use with a 715 grain bullet, even if you could find data for such a bullet. When you fill the case with lead, smokeless powder will not behave at all the same as when loaded with normal air space.
I understand your desire to experiment but please abandon this one, even a very experienced handloader would we skating on thin ice when working with such bizarre loads with no published data. I'd not like to see you on You Tube blowing your face off!