I've shot some unofficial Appleseed with my SU-22C (Appleseed friend giving me some second hand training) and the 27 round mags are a major disadvantage when shooting prone IMO. The SU-22 is too high, and rather than acting as a bi-pod, the rifle is teetering on top of the magazine in a very unstable configuration. I like the 15 round mags much better for shooting prone or off the bench.
I bought magazines directly from Black Dog, and no surprise, the 44MAG.com prices are MUCH better.
http://www.44mag.com/category/black_dog_machine
Palmetto State has good prices too.
http://palmettostatearmory.com/BDM.php
For Appleseed, you really need the 50 round drum.

Someone should make flip down legs to make it function like a bi-pod, so the SU-22 can be used like a mini SAW.
http://www.44mag.com/product/50_drum/black_dog_machine
My recommendation for a very good sling for Appleseed is a Turner synthetic sling. It looks and feels about like a leather sling, but it's more durable and I think a bit nicer. I don't have one because I don't shoot Appleseed yet, and I didn't like the weight. You probably won't like the cost, either, but it's a very good shooting sling. It's probably fair as a carry sling. Most of the cheap lightweight slings are OK for carry but not very good for shooting. This might be a good choice to install on your SU-22 liberty training rifle for Appleseed, and then move to your *REAL* liberty rifle the rest of the time.
http://www.turnersling.com
Appleseed is great! The shooting is great. The history is great. It's amazing how much people improve their shooting by focusing on the basics and learning to do it right. Accurate shooting from a prone position isn't comfortable. The sling is tight, and your support hand is at an uncomfortable angle, but I'm surprised what a stable platform can be formed with nothing more than a rifle, a sling, and the human body.
If there's time, you might want to see about a trigger job for the SU-22. IMO, the SU-22 is capable of very good accuracy, but the stiff trigger is the weak link. After shooting my friend's tricked out 10/22 with the aftermarket match trigger, when I went back to my SU-22 I thought someone had enabled the safety.
It'd also be a good idea to find out which ammo your SU-22 likes the best. Cheap bulk pack ammo will probably be 2 MOA or worse, which isn't competitive at Appleseed. I still haven't done the big SU-22 ammo comparison. I have the ammo, and I'm just looking for the time. Unofficially, I've had pretty good luck with Federal AutoMatch in the 325 round bulk packs. I bet some of the mid-priced ammo like CCI 100 round packs should be accurate. Test it before you go for reliability and accuracy off the bench, and I'd have a backup box of ammo you know feeds and fires well in your SU-22 just in case.
Have fun!