I'd suggest that your 90 gr. load is excessive leading to blown skirts. Reduce by 30 gr. and start again. Given a standard minie 60 gr powder will expand the skirt on that minie well enough for deer accuracy.

I don't know what you plan to do with your Zoli but 90 grns is a lot of powder for paper punching and a little to much for hunting with the .58 I think. I have been shooting several of the .58 muskets for years and for targets I use 50grns of 2F Black Powder and a .575 minnie short, 250grn. The important thing on minnie or any hollow base bullet is to fill the hollow base with grease to help expand the skirt to grip the lands. I shoot 70grns and the same bullet for deer within 100yds. I tried the heavier minies and didn't get good groups. Round balls in the Zouave muskets with 3 grooves will shoot with 50-75 grns in my rifles great. Any more and I get blown patches and eratic flight.
I'm sure if you worked on the loads long enough you could shoot more powder but why. With the short barrels, unless you are shooting a 3-Band, you are not burning all of the powder. I would try less powder and fill the bases with grease and see how that does.

Mike

Since I first posted the question, I've spent some time shooting, and playing with different loads. I've tried the .570" round ball with the .005" patch with 40-60 grs. of Swiss 2f, Cartridge GOEX, and FFG Triple 7, going up in 5 gr. increments. I found that the LEE 480 gr. .575" traditional profile Minie (Burton) ball fits the bore, and worked from 40-100 gr. of the same powders in 5 gr. increments. I've come to the following conclusions:
1. The trigger needs a lot of work, since it takes 2 men and a boy to pull.
2. The rear sight quality is an anomaly, and needs to be replaced. I looked at several other Zoli made Zouave's, and mine is the only one with that poor of a rear sight.
3. I need to practice, practice, practice my loading technique. A lot of the poor grouping I think comes from not seating the bullet with the same amount of pressure every time. I bought an unbreakable ram rod that'a longer and bigger around, so I can actually use it without hurting my hands, and my accuracy improved. I also don't have to put my hand over the bore to seat the ball like I do with the rifles rod, which made me very nervous.
It's still a lot of fun, and a bit different than loading BP cartridge rifles, since I have to play with the powder charge as well as the bullet type, diameter, lube, and crimp(seating pressure in the muzzle loader).
Thanks for the suggestions. I may try the Enfield style bullets next with the base plug.

Byron