There are four reasons to duplex shot sizes, and all have proponents and opponents.
1). Multi species loads. The idea is big pellets for geese, small for ducks. A variation on this is multi the range load. Problem here is, can you stuff enough of the big pellets in to make a worthwhile goose/long range load? Meaning, 7/8 oz of BB's isn't likely to produce a killing pattern on birds that require that size. Also, inclusion of the big pellets will lower the overall pellet count in the shell reducing its effectiveness for smaller targets.
2). Pattern improvement. There are some who claim that putting larger pellets on top of smaller will help tighten up the pattern. Conversely, larger on bottom will help spread the pattern. In both cases, the smaller shot is the focus of the load. I've never tried this, so I can't comment on its effectiveness.
3). Load fitment. I've never heard of any doing this with two sizes of the same material, but it would work. Say your load requires too many spacers for your taste. Why not replace some of the smaller shot with a much larger size. 1/8 oz of #T takes up a lot more room than #4's for example. This is done alot with TSS/steel shells.
4). Cost savings. Why shot a while ounce of TSS, when all you need is 1/4 and it costs $50/lb?
So if you want to try the 1x4 Bi, knock yourself out. But be prepared for flak from naysayers.
From KPY Shotshell Ballistics
1350 fps Bismuth #4 shot gets 1.50" of gel penetration at 51.2 yards away and each pellet has 2.36 lbs of energy, with about 77 pellets in 1/2 oz. Speed at 51.2 yards is 613 fps. Time to 51.2 yards is .182 seconds.
1350 fps Bismuth #1 shot gets 2.34" of gel penetration at 51.2 yards away and each pellet has 5.62 lbs of energy, with about 62 pellets in 3/4 oz. Speed at 51.2 yards is 694 fps. Time to 51.2 yards is .169 seconds.
At longer ranges the Bismuth #1 shot will get there quite a bit quicker than the Bismuth #4 shot will.
There are about 193 pellets in 1-1/4 oz of Bismuth #4 shot
There are about 103 pellets in 1-1/4 oz of Bismuth #1 shot
Using his numbers, we've got a load of 3/4 oz #1's and 1/2 oz #4's with a total of 139 pellets. Not a terrible load by any means for ducks. Though getting those 62 #1's to make a usable goose pattern at 50 yards may be tough. But I'm sure it could be done. And who knows, it might actually work perfectly for you--dont know till you try!
The thing is, you cant make the one shell to rule them all. Small birds, big birds, close in, long range, heavy payload, low recoil....
I have reloaded some duplex loads but the only ones I have confidence in are goose loads with Federal High Density BB shot and TSS #7 shot, and the other one is HW13 #2 shot with TSS #7 shot.
The HW13 #2 shot and TSS #7 shot duplex loads are low recoil 1-1/8 oz 2-3/4" loads. According toe KPY TSS #7 shot and 12g/cc #2 shot are the same when it comes to penetration.
I have other duplex loads that I am sure will work on geese but I just have more confidence in loads that have all the same size and type of shot in them.
1300 fps 12g/cc #2 shot gets 2.50" of gel penetration at 67.6 yards with about 71 pellets in 7/8 oz of shot.
1300 fps TSS #7 shot gets 2.50" of gel penetration at 67.6 yards with about 46 pellets in 1/4 oz of shot.
I would rather shoot at ducks with goose loads than end up shooting at geese with duck loads.
Tried it last year. I went back to 1oz #4 for ducks and 1oz #1 for geese. Rem green hulls for ducks and rem gold hulls for geese. Mec 1 3/16 lead bar. I don't worry about the over/under weight between #4and#1
Why not? Both were excellent patterns. Plenty of pen at yardage and both were the same fps. I don't mind changing shells. I like lower recoil. I also shoot trap at 1oz at the same fps. I didn't feel the duplex was any advantage.
Boss recommends their 4’s to guys unsure about Their 3/5 combo, almost same pellet count. 4’s will kill geese at 45yards.
The 3/5 combo I made up had about 179pellets per load of 1.125oz in the 12gauge. Longshot powder, and S3 wad. 1350-1425fps. The 4/6 is a duck load/close in goose load. The 1/4’s are a bit more oomph than standard 4’s for late season, thick geese.