Over powder wads
 

In the short ten has anyone tried over powder wads to take up space and then an SP-10? This allows all the shot to stay in the shot cup. I tried a 16 gauge half inch wad but I do not think it got as good an ignition. Too small I would guess. Anyone tried this with 10 gauge over powder wads? Would that and an SP-10 increase pressure? Must have been or there is a reason is is not? Thanks for help you can provide....

Bob, use a 16 gauge cork or felt wad in the SP10 wad placed under the shot to raise the shot column up to proper height for crimping for the load you are using.

right - the 16ga wad goes in the shot cup

What is it you are trying to achieve by having all the shot in the shot cup, a tighter pattern? How did your gun(s) pattern with a filler wad under the shot? Most of the short tens were built and choked in the days of no shot cups and most pattern plenty tight with a filler wad under the shot.

It is tighter patterns I am after. I have been using a 16 gauge wad in the shot cup but that put a lot of shot outside of the cup with a 1 1/4 load. My EM Riley has more open chokes than my Remington and I was trying tighten things up a bit...and as an experiment just to see the effects.

I have done this using black powder 12 ga. loads. But only use the cup portion of the wad. Filler is hard card and fiber with the shot cup placed on the fiber filler. It does work but I would not use the cushion part of the wad as I'd be very leery of the filler jamming into the shirt of the plastic cushion and causing pressure problems. I cut the cup off with a sharp utility knife of a razor saw. Just use the cup.

As you've discovered, you will get faulty ignition using a 16ga wad over the powder, because you don't have a good gas seal, and a uniform charge.

Why don't you just cut your shells a bit shorter ?

The HEVI-SHOT company uses flax seed as a filler, so everything stays in the shot cup and the seed is placed on top. This would allow you to use the SP10 wad and keep the shot inside and adjust the amount of filler you need for each shell. Flax seed is cheap, less than $5 for two pounds.

Plus you can eat it while reloading and have a healthy heart!

Putting a filler wad on top of the shot has the opposite effect the OP is trying to achieve as it creates a spreader load. I don't know if Flax seed would have the same effect but I would put it on the pattern paper.

The best ways to tighten a pattern are; larger shot, slower velocity, harder shot (maybe try copper coated), and granulated buffer.

Perhaps you could try a mylar wrap and see what that does.