If you already have some #10's and you just want to shoot the pistol a few times,,I'd simply remove the nipple and trim the dia down a bit so the #10 will fit correctly. It doesn't take much, just keep a slight taper to the nipple as it originally has.
Ordering a single lot of #11's just for a few shots here can be kind of expensive. Maybe you have other uses for the 11's though.
One other thing I see is that the hammer doesn't appear to be striking the nipple squarely.
That's just an observation based on the angle of the nipple and that of the face of the hammer.
The hammer is counterbored to catch cap fragments and maybe the counterbore is more correctly matching the nipple face. But in the pic, it appears the hammer striking the nipple at the rear edge of it.
That angle (along with the poor fit of the cap) isn't going to get you much positive ignition.
Correcting the angle of the hammer to the nipple to better the contact means bending the hammer in this instance,,or carefully recutting the counterbore in the face of the hammer to better match the nipple.
You can't do much with the angle of the nipple itself as it's a tang breech,,not a drum breech.
The latter you could turn a few degrees to match up better.
You might not get a perfect match between the two surfaces, but the closer they meet flat the better to smash and ignite that cap.
When you have the nipple out to trim the stem down to #10, clean up any battering from dry firing all ready mentioned and make sure the flash hole is clear in it and the breech plug
