I have one of each in AR platform (not suppressed), not a lot of difference. 20P, you need to neck down brass. 204R, these don't feed well through all AR mags. What they both are...a blast in in the PD fields! Kind of like a video game. Once you try one, you won't need to bring as many rounds for the bolt guns as you had been taking.
When using 39 SBKs, or others, you have to adjust the seating to the max your magazines allow. The poly Pmag magazines might allow for a little more length, but if you use these you will have to try to modify them internally, the shoulders don't match-up with the profile of the 223/5.56 cases they are designed for. I also use an AR in 223 Rem, as stated above, it a fine choice for pdawgs. I use the 50 gr Vmax, which I do believe is more destructive to the PDs than anything I have ever used...not sure why, but parts are flying when these meet.
The best magazines I have used in an AR-204R are the Wilson combat 7.62x40 WT (made from Lancer L5AWM mags).
They are made for Wilson’s long version of a 300 blackout (my apologies to Wilson for the simplistic comparison). They feed well and don’t crowd the case shoulders.
Edit:just saw They are on sale 50% off at moment. (Delete this if inappropriate) https://shopwilsoncombat.com/762x40-WT/products/419/