Bill, there are a couple of reasons why they spit, and when they do it can be bad.
As Ric mentioned, the cylinder may not be aligned with the barrel, a good gunsmith with a range rod will be able to help you there. The cylinder stop does wear and when the cylinder rotates and comes to rest may be just that tad ut of alignment that will cause it to spit.
Another is the forcing cone angle, unfortunately all except the top end target pistols come with a relatively sharp forcing cone angle, built for jacketed projectiles. What is best for lead is a slow tapering forcing cone, my Manhurin can almost fit the entire length of a 148HBWC before striking rifling.
Shooting HBWC's will also cause it to spit, lots of gas wanting that skirt to expand, no matter how small the B/C gap is, nature of the beast.
Your projectile certainly isn't too long, having shot 160HBWC's through several different revolvers from the standard Smith twist down to my current revolver with a 1-8 twist and all shot fine.
I'm not sure which part of your revolver was dirty but would imagine you are speaking of the cylinder face or the back of the barrel/forcing cone. They will drag if the B/C gap is too small or incredibly dirty, you have a DW, is the barrel square to the cylinder face? Never shot one sorry, but if you can swap barrels then is there an issue?
Doe sit spit with ligher/shorter projectiles, have you got access to a couple of 100gn for example.
And YES, there is a lot of powder and gas that comes out of that gap, maybe not necessarily lead. Try the paper test, fire the revolver with the B/C gap next to a piece of white paper, big shredded holes say lead, powdery holes say gas or powder.
You never mentioned what powder you are using behind them, makes a small difference. Fast powder= lots of gas right now, slow powder=burns right down the barrel.
cheers
