Found myself a bit bored the other day and saw a load of paintball practice balls ( a soft rubber ball) lying around.
Thought I would make up some rubber ball loads for the 12G.
Loaded them in Winchester hulls, CCI primers, Clays powder using recommended loads for 7/8 oz shot in a Winchester 1 oz wad.
Loads looked fine, good crimp etc and very light of course because the balls only weigh a fraction of the shot equivalent.
Fired them and most were perfect, chronographing around 740 fps and punching through one side of a cardboard box.

The problem is that a few of them were obvious squibs, ball barely making it out of the barrel and lots of unburned powder. No doubt due to inadequate powder burn because of reduced "start pressure".

Now how do I increase this "start pressure" to ensure consistent ignition. Perhaps cardboard wads or the like.

Any suggestions appreciated.

If you are getting good crimps you shouldnt have a problem. 10mm card discs will work. place them below the ball. Seems to me the ball is too light.

.68 Cal Balls?

You could also try buffer material as used with factory loaded buckshot. The idea is to take up any space around the ball and ensure a tight crimp.

Yep, .68 Cal balls. Fit perfectly in a 12 G wad but weigh next to nothing so ignition is inconsistent. Thought about using buffer but I don't think space is the problem as the crimp is perfect.
The extra card wads are my next step, just to add some more resistance/weight to the load. If that doesn't work I might try some felt over powder wads and have the ball in a cut down wad over that.

The more I think about it the more convinced I am that your ball is too light(sorry!)Thats the reason for the unburned powder.

Try faster burning powder. Bullseye or faster.

And I think you're on the right track using multiple over powder card wads with felt as needed.

Cut down the plastic wads enough to only give the .68" balls some protection and keep them aligned and centered leaving the bore.

And be careful! It's all fun and games until somebody gets an eye poked out! And that eye could be YOURS!

PS - Don't shoot any of your friends with these experimental loads! Protective equipment or not..... these SOBs could be deadly!

Fixed.

Same load, same primer.

Cut the wad support columns off leaving just effectively an overshot wad and the shot cup/wad.

Loaded the powder then the plastic "over shot" wad, a fibre wad, then the rubber ball in the "shot cup" topped off with a card overshot disc.

No more squibs. Punches through just a single layer of cardboard box at 15 yards.

Might be just the right message for the really unruly bulls down on the farm.

I used to load rock salt in a similar fashion when I was a kid. My grandfather had a press from way back whenever and he only used it to load rock salt into 20 Ga. shells for when the cows and bulls wanted to be less than cooperative.

I would first trim down the plastic shell side about half way. Then load it like a normal shot shell. hand load in the rock salt, then use a lid sealer out of the lid of my grandfather smoking tobacco can. Just a slight trim worked perfectly. Then crimp it down...Usually only had to load 1 shell. 1 whack with the salt and the animals tend to haul ass!