During ejection of live rounds is not a magpul issue. I have that issue on mind Remington 7mm on a regular factory stock. I don't see it as an issue. Never happens when ejecting empty rounds.
In the Tech info on the Magpul site it says that it will not work with the Rem 700 DM models that has the built in clip and they are right. The factory trigger guard and clip will not fit into the stock but, if you install a Magpul trigger guard and clip it will work. I talked to their Tech Support more than once and they never told me this but, I told them that I gave it a try and it seems to work fine. Can't find a longer front screw though for the trigger guard. Any of y'all know who has them. I called Brownell cause one of the videos has the Magpul and Brownell Reps putting an assembly together and the comment was made to get extended screws from Brownell but, when I called they had no answer.
Poliflexibles SAC short action means smaller cartridges. Typically anything up to a .308 in length. Once you get longer than that it falls under the long action category. Things like .270 Winchester, 7mm, .30-06, .300 win mag, etc. It can vary a little between gun manufactures, but typically that’s the rule of thumb.