First and foremost, I don't load BP cartridges; I always try to find the equivalent smokeless load for all my antiques and so far it has panned out great. I guess in your case, somewhere between 2.0 to 2.5 gr of Unique should do it. I have a similar piece in 320 revolver cartridge and mine is recommended to be loaded at approx. 1.8 to 2.0 (as it's a smaller + lighter bullet/cartridge).
However, there' one very important thing to remember when loading BP in a cartridge: you cannot leave ANY airspace. I don't know how you loaded the 5 grains in your loads and if you used a filler, but if you haven't.. it might be a good idea to buy a bullet puller and correct the error before firing it even once. Air space in cartridge + black powder = extreme pressures. This is not the case with smokeless powders however, but BP and it's substitutes are very touchy to airspace... so be very careful.
Besides, 5 gr of BP seems a little too little, no? According to most people who load bp pistol cartridges, you're supposed to fill the case full of BP and sit the bullet on top in a way that it compresses the powder (hence no air space). I don't think 5 gr will be enough to do that... unless you fill the rest of the air gap with toilet paper (no joke, that's how it's done).
Also, I assume you already know that BP is measured in volume and smokeless is measured by weight even though the unit of measure for both is "grains", right? Do not confuse the two or you could have a bad surprise.
Now... having said all that, welcome to the antique side of collecting (the most addictive, I may add). It's rough in the beginning to figure all that stuff out, but once you master the basics, it's nothing but fun. Don't let anything discourage you from pursuing the hobby! If the budget allows, of course... (because we all know antiques can be pricey sometimes.. most times... always, lol!)
