A friend of mine has a reloading shotgun machine in his basement and often makes specialty loads upon request. He is able to make shotgun shells that have mixed shot sizes of various weights.
Are there any real hunting applications for hunters to use shotgun shells containing mixed shot sizes?
You are referring to duplex loads. I don't see a whole lot of duplex loads anymore unless its a mix with TSS (tungsten super shot – the densest shot available). Remington had a line of duplex loads that featured a mixture of 4 and 6's. The reason stated was the #6 shot would provide the pattern density and the 4 would provide more penetration.
"Buck and ball" loads are commonly available. They're often marketed for self defense, but where legal for hunting they have the same benefits on game, namely: The "ball" (or slug) has more reach for aimed shots on further targets, but if you happen to get a closer-in target you have the "insurance" and added stopping-power of the buckshot.
Mixed has had limited use. A wide range just does not make sense as you would just tear up the smaller game. Even with similar sizes would get longer shot strings (smaller slow down faster) so a moving target at a distance you are guaranteed to miss with part. If you do some pattern testing you are likely to get a mixed pattern bigger than the smaller shot alone.
