I'll throw in some things concerning our potential use of steel shot and of the shot protectors potentially used therewith.
Most of us are shooting unchoked barrels, so any shot protection does not have to deal with the deformations and associated forces that come from pushing the shot load through forcing cones and/or chokes. Steel/tungsten-rated shotcups intended for modern shotshells, on the other hand, do have to deal with chokes and forcing cones, without scrubbing through or leaking shot between the petals, and preferably providing some cushioning during the reorganization of the shot charge as it is squeezed down through the constrictions. So those of us shooting plain unchoked barrels do not need as much protection as those shooting through chokes, and some of the hunters using steel shot in unchoked muzzleloaders report good results with various forms of home-made shotcups. In the somewhat limited testing I did at one time trying to develop a steel-shot load for my 16ga double, I found any shot protector configuration that resulted in two layers of business-card-weight card stock gave completely adequate protection, based on examination of my bore and of recovered shotcups or the remains thereof. In the end, I gave up on the development because I felt the recoil was too stout for the stock of a fairly light flintlock double, although I may try again with smaller shot charges.
Some shooters have reported success with steel shot within limitations they consider acceptable. Most hunters that report consistent success using steel shot in muzzleloaders are shooting ducks, and sometimes doves and/or "upland" birds in areas where non-toxic shot is required. I've found too few people reporting success with larger birds to generalize about them. Those successful with ducks and larger upland birds are mostly using #1-#3 size shot, sometimes #4 for teal and other smaller birds or for shorter ranges, and occasionally B or BB through chokes for the largest birds at longer ranges. They are consistently using more powder than the equal-volume load for the same weight of lead shot, commonly around 1/4-1/3 greater but sometimes up to 1/2 more. As an example, for a 1.1/4oz steel-shot load, the same volume of powder as a 1.1/4oz lead-shot charge is about 3dr/82gr, so 1/4 more would be 3.3/4dr/103gr (which happens to be the common 1.1/4oz-3.3/4dr "heavy field load" cartridge for a choked 12ga), while 1/3 more would be 4dr/109gr, and 1/2 more would be a rather stout 4.5dr/123gr. The same general principles apply for doves but often with somewhat smaller shot charges, and with shot sizes of #4-#6, maybe #7 but I cannot recall for sure. With conventional loads in unchoked muzzleloaders, these heavy powder charges would tend to open or completely blow the patterns, but the plastic steel-capable shotcups, and some of the homemade ones, tend to tighten the patterns and compensate for the heavy charges. On the other hand, chokes give very different dynamics in patterning, and can often benefit from higher velocities than cylinder bores. FYI, a volume of powder equal to the steel shot can range from 3/7 to 2/3 more (depending on shot size and measure diameter) than the lead-shot equal-volume load, which would be heavy loads of 4.1/4-5dr/117-137gr in the case of 1.1/4oz of shot. As is often the case, those reporting success with lower velocity loads tend to use larger shot sizes to get adequate penetration.
Muzzleloading smoothbores tend to be limited-range tools in general and steel shot tends to further constrain the range. With traditional muzzleloaders and B.P or substitutes, patterning and recoil (both shooter tolerance and possible damage to the stock) tend to constrain the maximum velocity, and velocity and shot size constrain penetration. Even with larger shot charges from choked barrels, I don't recall anyone mentioning effective ranges greater than 35 or 40 yards, and many, especially with unchoked barrels and/or smaller shot charges, tend to cite maximum consistently-effective ranges around 25 or 30 yards.
Regards,
Joel
