using Fg instead of cannon -grade?

Can I safely substitute Fg for cannon-grade powder in a 44mm golf ball cannon? How much should I reduce the charge? What would be a decent MINIMUM charge for testing? (the cannon is rated as "400grain cannon grade max load" when shooting projectiles)

The key word here is SAFE.

In my 2" bore cannon I use 300gr of cannon grade powder, I have used 1F before with the same amount.
My gun is rated for 1200grs of cannon grade. so no problems with smaller powder. Not knowing your gun I would think 200grs of 1F would be fine, If you are shooting actual golf balls the projectile weight is low, I shoot an 8oz lead ball from mine.
I also adjust the charge weight depending on the distance i am shooting. for targets at 100 yards i use 150grs of powder and still only need 1/2 a degree of elevation. at targets up to 600 yards i use 300gr charge with 1 1/2 Degree of elevation.
I have had several oppertunitys to shoot at extreme range, I used 600 grs of powder and 5 degrees elevation, we shot at a Van sized target at 1 mile distance, my gun made 4 hits out of 8 shots, the 3 pounder that was also shooting made 5 hits out of 8 shots, not bad fore smoothbore cannons. I hope this info is helpful.

Take a look at Switliks book. He has some pressure testing info that shows a 40% increase in pressure when using an identical load of Fg over Cannon grade.

Flintlock,

Your loads are very interesting. You are achieving great accuracy at extreme distance and you load is barely 50% of maximum safe loads. Just goes to show that these guns don't need to be loaded up to make them work...It's just a waste of powder.

Good shooting!!

Granular size goes up pretty evenly on black powders now available. Have found the same rule-of-thumb for other size substitution goes well here too. Cut back powder about 25%, by volume, when switching back from cannon to Fg. Good luck.