I'm just going to post absolutely everything I can think of.
How about a Nock Gun? Basically the same as Graknorke's suggestion except we leave out the revolving, hopefully saving on complexity. The real Nock Gun fired all seven loads at the same time, but we don't want that, so we would just need to separate the firing mechanisms. Or, we could make a gun that can fire superposed loads, like we discussed last thread.
Percussion caps worry me because I've always agreed that it's better to have something pretty good now, rather than something unusable now and perfect later.
Something I notice is that the Loyalists have just now decided to design a rifle outranging the Brown Bess, and so have the only one that does so by a wide margin (our Samara only does so by fifty-odd meters). If we design something like the Baker rifle, omitting the breechloading and other complexity-inducing factors the Loyalists have tried, we can probably have something that can really break them up from range.
I almost hesitate to say it, but what if we arm our cavalry with lances? We seem to be falling into the same trap that 1500-1600 era cavalry did with the caracole maneuver, attempting to outgun infantry when we cannot do so. Fire will never be cav's strong suit, so why shouldn't we use a weapon that should ensure our victory by shock? Also, none of the Dunwich rifles have bayonets so if we get in with shock cavalry, we could cut up their formations pretty badly. Also, it should be just about impossible to fail in designing a lance. Winged Hussars anyone?
Overall, I think we should try to make something that outranges the Victoria, but can still be fired quickly. Cavalry firearms seem to be marginal to me because infantry will always outgun cavalry. Thus, I think something similar to a Delvigne rifle is definitely worth a try, so how about the design below?
A musket five feet long that fires a round ball half an inch in diameter. Four rifling grooves are cut down the barrel to impart spin on the ball. The rifle features a chamber slightly smaller than the bore at the back of the barrel, which the powder falls into when poured from a paper cartridge into the rifle. The rest of the cartridge contains the ball and a wooden sabot, which allows the ball to deform only enough to engage the rifling when rammed. The powder charge and bore thickness are designed to allow the rifle to be effective at 400 yards.
Links:
https://en.wikipedia.org/wiki/Henri-Gustave_Delvignehttps://en.wikipedia.org/wiki/Nock_gunhttps://en.wikipedia.org/wiki/Caracole