When a wad is placed directly on the powder it creates conditions that lead to a bulge in the chamber. A lot of this is dependent on the size of the charge used the stronger the charge the faster the bulge or ring will appear. Of course if you ignore the ring sooner or later the stressed metal will fail. 
If a wad is placed .050” to ‘100” off the power and the powder is allowed to slump, or even better if the powder is oriented to the mouth of the case and allowed to slump to the rear the bulging is virtually eliminated. 
With any smokeless powder you can load past the action / barrels ability to contain the blast. As this was a trapdoor that has always been classified as a weak action it is possible that wad or no it would have burst the action.

40 Rod

Never been to the Quigley but sounds like the firing line is a dangerous spot to be with reported blow up's.

Probably had to put some sort of rule in even though it's going to have little effect. No science involved.  Only rule that will limit blow up's is Black Powder only.   

Don't get me wrong I shoot mostly smokeless powder. 650 shooters some with questionable ability shooting weak actions loading smokeless off internet information. I don't want to be there. 

Shooting BPCS with Black only rule never even think about blow up's. Schuetzen matches I go to know almost everybody and don't worry much about them either.  I did shoot next to one fellow this year that made me very nervous.   

Boats

Joe it can but way less likely too.  650 people some are going to be dumb.

No matter to me I won't be there.

Boats

Thought I might go one year. Not now. 

         Joe.

Three basic types of powder are used in our sport although they are often listed as two (black powder and smokeless).  These basic types are more directly labeled as black powder (and substitute), single-base smokeless (nitrocellulose) and double base smokeless (which has an added buffered nitroglycerine component).

(You need to Login or Register to view media files and links)

This upcoming Q match will be my 14th and I don't feel any less safe there than I do next to the guy at my rifle range.  Joe, you'd better give up shooting schuetzen too; probably less safe than the Quigley. Who was the California guy that blew up his highwall at a schuetzen match about 8-10 years ago with a double charge?  It was in the ASSRA journal.

Over the years I've shot the Q about a dozen times and can say it is as safe there as at any other match. As I remember there is a box to check what powder you use and as I remember black is about 85%.  There is always the idiot.  Once a shooter came to the match without loading any cartridges and Saturday morning just before the match started he sent his wife around with instructions to beg, bower or steal some Reloader 7.  We know there are always some that think faster is better and if you are a current or former schuetzen shooter, you know that doesn't work very well with old guns, cast bullets and smokeless powder.  The match has always been considered something of a traditional, buffalo era, type match and many dress the occasion and use only black.  Scuttlebutt says that many in the Forsyth Rod & Gun Club shoot smokeless at the match.  What's the phun in shooting smokeless in a Sharps or other old and replica type gun when the objective is to duplicate buffalo shooting of the late 1800's?