Since I only use Hornady 75 ELDM, this looks like the best place to post my results.
I received my PSA 20" upper (SS, 1:7, adjustable gas block) about three weeks ago. I got it for the sole purpose of having a reliable, 1K capable AR15 gas gun loaded to mag length for PRS matches. Just couldn't get it done with my 18" or 20" 223 /556 loads.
Since then I've been testing a variety of powders: CFE223, H4350, 760, 748, H4895, Pro 2000MR, R17, R15 and ARCOMP. I've been high and low in charge weights with just about all of them, but I've developed four loads (two of which I've already tested with success at 1K) that I'll post here.
Components:
New or 1X Starline brass
75 ELDM seated to mag length or a bit less (2.255 -.258)
ACS 6.8 10-round mags
CCI Magnum small PISTOL primers (thought they were rifle primers, but they worked so well I decided to stay with them).
Magneto V3 used for all crono work
Loads tested out to 1K
25.1 Alliant Varmint - 2775 fps
28.7 H414 - 2825 fps
The above two loads, once corrected for elevation (up .5 mil) and windage (left 1.5 - 1.7) produced three consecutive hits each on the 1K steel - 18" x 30" with flashing strobe.
New loads
25 ARCOMP - 2900 fps - my zero is at 50 yards - three rounds - .250 hole edge to edge
27 R15 - 2910 fps - again, three rounds at 50 yards, .300 touching, center to center
I've not had any FTF cycling issues (other than adjusting the gas block for a particular load if needed) with the PSA / ACS mag combo.
I used Federal factory 75 grain TMJ for barrel break in and (re)zeroing. Not a 1K load, but very accurate and consistent at 2900 fps. It appears that Fed brass used for the load is trimmed to 1.6 so after firing once, trimming is required.
The new Starline was all trimmed to 1.590 so even after two or three firings, the resized case is still under 1.6 so I haven't had to trim anything yet.
In my case, I found that with the 75 ELDM, 2775 fps is the bottom line for a reliable supersonic load at 1K. In my part of the world, Density altitude numbers used are typically in the 1000 - 2500 ft range.
With all the loads I tried, it seemed the faster I tried to go, the "groups" would open up. It also was the case that I could load well above posted data (Sierra and Hodgon) without any pressure signs or just the start of them. To give one example, I loaded 28.7 of R17 almost to the top of the neck, seated the bullet without any crunch or resistance, crono'd at 2975 fps, with some ejector swipe, but was all over the place on paper. I did use this load at 1K, but after dialing up as before, I only managed 1 of three on target.
I plan to put the ARCOMP and R15 load on paper at 500 and 600 shortly. I'll let you know what they do.