All the indications in all the chain fires I’ve had were that the flame came from the front of the cylinder. A couple times in the past I forgot to put grease over the balls. This time, they sometimes rose out of the chambers, apparently from air compression, and needed another push from the ram. This could smear the Bore Butter along the fouled chamber surface and open channels for the flame past the balls.
Most of my guns have short cylinders and chambers, probably not enough air to push back, and better fitting for the balls. Armi San Marco is long out of business, driven out by better-quality stuff by other companies. I haven’t seen their recommendation for round ball size; each gun might be a law unto itself.
The cap nipples are in deep cutouts (especially deep in the Walker; needs a “knack” to cap with your fingers) and there was no end shake and no indication of impact on the cooked-off caps. Pinched No. 11s, in my opinion, are more acceptable than slightly undersized No. 10s, which often need a snap to seat them fully and another to fire them. The pinched caps don’t go through cappers very well, though.
The Walker chambers have plenty of room for wads. I might try them some time.
I’ll take my pin gauges to the chambers, polish them out and cast some .454”s. That coffee can had better watch out next time!