Just got back from my first session with the ACP conversion in my Omega.
First, I cannot say how it affected accuracy. I am still working on loads with the gun, and do not have enough experience with it to accuratly comment on this.
Last session (regular plug) I found it shot 200 gr Shockwaves MOA at 110 gr FFg 777, and 2" with 220 gr Dead Centers and 100 gr 777. I could not shoot much, as I spent half my time trying to unstick the range rod from the barrel as I struggled to clean out the obstinate crud ring.
This session, things changed. First, I got off about 35 shots in a couple hours because the crud ring was almost non-existant. There was breech fouling still, but it was easily worked out in a couple strokes. Second, it really affected what would shoot. The Shockwave load that had been so accurate blew wide open... 2-4". The 220 gr Dead centers were about .5" inch worse. However, the 240 gr Dead Centers with 100 gr 777 shot a couple of very impressive groups with at least 2 of 3 bullets touching. I ran out of these bullets, but will order more and work up to max accurate charge next session.
I found priming/depriming with this system much easier. The breech still got some dirt into it, so it isn't truly sealed. But I like it, and it was worth the money just to get rid of the $%#! crud ring!
Incidentally, the plastic insert from a box of 209 primers works great as a holder for the ACP shells (I used a Remington STS tray).