Cannons need windage. Cannons are not just big bore muskets. They are not hand held thus not hand controlled. They are shooting large heavy projectiles with large heavy charges. They are very different.
Patches-no patches in cannon bores. The larger circumference creates greater friction/resistance on trying seat a ball in the bore and it is is very easy to stick a ball in the bore. Then If you do get the ball seated you get extra velocity due to increased pressure. Increased pressure means increased recoil. That brings on issues of carriage control. The excessive recoil, way above the values a longgun, can also batter and break carriages. The extra velocity is just not needed in cannons. Patches are counter productive in a cannon.
I do use wads to hold the ball against the charge. Sabots were commonly use in Cannons with the ball strapped on front and a charge strapped on behind the sabot.
If you are shooting a mortar and you cannot see the ball fly, you are missing half the fun. Mortar charges should be light and you should be able the ball fly all the way down range to impact. Part of that is safety. If you can't see where your ball is landing how do you know it landed in a safe area. You may not always be able to find the ball, but if you can see the area where it is landing, you can know if it is landing in area that is safe.
My golf ball mortar uses 24 grains of Fg and will send a golf ball 30 yards are so. A steel or zinc ball will go slightly further with the same charge, about 40-50 yards. 90 grains gets me about 100 yards.
Here is a load chart for popcan mortars.
Did some bracketing to test preliminary to building a load table for the Lady Jane.
The powder chamber is 1 1/4" in diameter and maximum suggest safe charge is 300 grains of FG. 300 grains fills the chamber. Chambers larger than .7 cal should use Fg. Do not use FFG.
I just did four loads with two different projectiles-standard popcans full of cement and zinc round ball. Some surprising results
weight can ball
load 1, 25% 75gr 150' 220'
load 2, 50% 150gr 336' 424'
load 3, 75% 225gr 389' 663'
Max -100% 300gr 368' 835'
Average can weight 21.6 oz.
Average ball weight 36.2 oz.
Barrel elevation-45 degrees
That last can was spinning end over end, very fast from shot to impact and jumped 4'-5' in the air at impact and took off spinning down range.
My bowling ball mortar will send a 15 pound ball 440 yards with 6 ozs.and 652 yards with 8 ozs. I have also fired 16 ozs of black powder with a 15 lb ball from this mortar.
With all these loads listed above you can see the balls fly all the way to impact. Even the one pound load, you can see the ball fly all the to the impact zone, somewhere between 1-1/2 and 3 miles down range. we can see where they going, we just can't find them.
Good luck with the delrins. That 30 grain chamber should be good. But a caveat. You have not provided us with a picture of your mortar or any dimensions such as diameter of the powder chamber and diameter of the barrel over the powder chamber..
Without seeing that info it is hard to say what you have is safe.