Bruce, there is no doubting the 40 mm bbl is stout enough for your charge. But, a heavy charge is not necessary. I have to ask you, have you tried 500 grains?
The safety guidance for load development is to start ridiculously low and work up.
Never use someone else's load to start. That is a basic safety rule for all reloading.
In building loads you will be surprised how much distance/difference a small increment increase will give you and how small a charge increase will produce the result you want.
The 40 mm caliber, 1.575 inch calls for 500 grains of Cannon or Fg-about 1.14 ounces on the chart. That is for a lead round ball that will weigh about 5880 grains. You aren't using a round ball rather a heavier and longer slug. The way the round ball reacts in front of the charge of black powder is not going to be the same as the longer heavier rifled slug. The slug has greater resistance and is going to generate higher pressure.
Let me show by example how that looks. Click on the pictures to see video's.
Here is a 1 inch round ball that weighs about 1500 grs. shot with a 180 gr Fg charge.
Compare this with 1 inch, 9 ounce (3938 grian) cylindrical slug with the same 180 grain Fg charge.
Same charge weight, heavier projectile, greater mass, increase in pressure, more recoil.
Field carriages usually don't break wheels or spokes from firing. What breaks are trails or cheeks and associated hardware. When the cannon fires the barrel lifts up against the trail and capsquares. You can see that in the video above. Wheels usually break in transportation-going down the road.
Yes the chart can be hard to figure. The original published in the The more complete Cannoneer
has an error on it that George removed several years ago.
Each small square going up the side equals .05 inch of bore or caliber.
Each small square going across the bottom represents 10 grains.
30 mm is 1.181 inch. The load on the chart shows a load between 240 and 260 grains starting load. Earlier I said 200, but that is not correct the chart says 240-260 grains. This is maximum recommended charge.
Were this my gun-30mm gun. I would set up a target made from one side of a refrigerator box at 25 yards. I would make a 6"-8" diameter black aiming dot right in the middle, top-to-bottom, left-to-right. Aim at the dot as best you can and use a starting load of 150 grains of Fg. Fire a couple of rounds and see it the keyholing goes away. Mark each shot hole after firing so you don't become confused with previous shots. Do not change your sights or your aim point during the entire test. Increase your charge 5 grains and shoot a couple more rounds. Repeat by increasing you charge 5 more grains, same sight sight setting and same sight picture. Continue until you either reach maximum or you get the accuracy you are looking for.
My guess is that you will find optimal before you get to maximum.
To help with accuracy be sure to raise your lead melt temperature and get your mould hot to eliminate the wrinkles. While the lead is melting, set the mould on the edge of the pot.