The 20P is my first "wildcat" adventure so I opted against needing to move the shoulder. Technically, there is fireforming involved to get the 30 deg shoulder, but I get what you're saying. While I've always liked the idea of a 30 or 40 deg shoulder, for a 223 based 20 cal, it just didn't offer any real advantage to me.Lefty-- you don't HAVE to fireform any of the cartridges that you mentioned, except size the necks to 20 cal. Load them and shoot them.
My first experience with an Ackley cartridge was with a .223 Rem. Some of the best groups with that rifle were when I was forming the cases. The same with my 20 Tac. All I did was run them through a sizing die to form the neck, load them and shoot them.
I also don't care for these proprietary cases, i.e. the .204 Ruger. You have to buy a special case to have the advantages of the performance. You can forget about trimming cases and split case necks. You can wring out the performance with the ubiquitous .223. The advantage of shooting these modified cases, as far as I'm concerned, is case life. The sharper shoulders and straight walled cases are the key.
As far as not being able to get a "sploder" for an AR, you will have to take that up with the barrel people. The only problem I can see, is because of the straight walled case, there might be a problem ejecting the cases. Some on this forum have said that because of the sharp shoulder they have had feeding problems.
As far as the Sploder goes, I agree. I'm just saying that I don't understand why someone who offers 20 Tac AR barrels wouldn't offer 20 Sploder barrels. I've heard that about the straight walls, but everyone I've talked with that owns a 223AI upper says they feed fine and they love theirs.