I just want to clear something up. I have now read on a couple of threads of people saying that the S400/MPR has a competition trigger or suggest that you could ad a competition trigger (by what I assume is the MPR trigger post and shoe).
The S400/MPR does not have a competition or match grade trigger. It does have a true 2 stage trigger. The older S400's had two sears in the trigger mechanism and the new S400/MPR has got 3 sears. The actual trigger (only the part your finger touches) of the S400 is replaced in the MPR by a post and shoe similiar to the EV2. The 3 sear trigger in my opinion feels better than the old 2 stage trigger.
No matter how much you tune or how fine you try to adjust your trigger it will never be a match grade unit. This is not any negativity towards the S400, which in my opinion has a very acceptable trigger. It would have been nice if AA made a match grade unit for the S400/MPR and offered it as an upgrade, but to my knowledge they don't.
Please feel free to comment if you want, this is just as far as my little knowledge goes and if I am wrong I will apologize for the wrong statements I have made.