There is a contention goop over the end of the bullet (conical or ball) keeps the barrel clean. Shrug.
In that case a lubed wad would help as well.
I can clean my gun in 5 minutes so I don't see the need to mess with goop that is a mess.
As long as the bullet is large enough to shave lead, its going to seal. A wad under is insurance for the case of a sprue or flat spot and the chambers need to be uniform round or you can have a flame patch past the bullet.
The test I saw where a chain fire was deliberately induced (with just one adjacent cylinder) did not flat say, but for a 44 BP, it was below .451 (standard offering is .451 ball though .454 is recommended smallest for that caliber)
Frankly I do not put goop over the end of my bullets. I also do not go below .451 and when I use .451 I put a wad under it.
I played with over just to see blast pattern. Yes they do catch that, I found none went through it. You may even be able to shoot them fine, I was removing a single one at a time as I shot the cylinder.
Flame arrestors are a fine mesh screen so its not a myth. Some are metal. Large batteries have plastic versions.
The gun I am shooting true BP in is a Walker and with the barrel coming off, soap and water for the barrel, the cylinder and any other shot BP cylinders. 45 cal nylong brush and it takes a minute to clean. I take the cones off so as not to trap water in that area.
I put them in a dryer that I use to dry wet brass. Oven is not so good as it does not have the air blowing. It would work but you really need a paper towel and low as it goes heat.
For the ROA and the NMA's, I use the same 45 cal brush, push it through to get the worst of the crud out and then use Carbon Killer 2000 drizzled on it and through once, drizzle in the frame end and back out, a patch and about 5 cycles the barrel is clean.
But the true lube at the end folks have all sorts of combo of mixes they swear by. Beeswax makes it thicker and is a common adder for a lot of those brews.
I will take a gun down at some point for the innards but at a guess every 10-15 range trips. Or you can fill with grease but I am not into that, maybe if I was shooting a whole lot but not for 3-4 reloads at the range I do.