Quoted:
The 6.5mm SSTs expand consistently down to 1800fps when we did calibrated gel testing on them. We did 10-shot samples, and consulted with Hornady on their results as well. They do testing on gel out to 400yds to maintain real-world RPM, which has an effect on behavior in the tissue.
http://shootersnotes.com/wp-content/uploads/2014/01/Vol-2-Excerpt-300x326.jpg
The 123gr A-MAX expands down to ~1600fps, but isn't meant to be a controlled expansion bullet.
At Grendel speeds, it works very well as a hunting bullet on broadside shots on deer. Its performance on hogs has been good, but not as good as the 123gr SST.
Same for the ELD-M, which is basically an A-MAX with the new tip just to streamline production. Grendel never needed the new tips, but 123gr ELD-M got them anyway.
Some newer projectiles with lower expansion threshold are:
129gr Nosler Accubond Long Range-1300fps
110gr Lehigh Defense Controlled Chaos-1500fps
I think the Barnes 127gr LRX is supposed to expand down to 1600fps. It's quite long, and more ideal when fired from a 6.5-08 class of cartridge, but has been used successfully in 6.5 Grendel to hunt Oryx from a JP complete rifle.
Several hunters with 6.5 Grendel SBRs or pistols are using the 129gr ABLR, with DRT or quick kill results as that bullet is extremely soft, but stays bonded. It works as advertised, and necropsy has shown immediate and rapid expansion on hogs, with full opening within an inch of penetration, then massive cavitation of the vitals, usually with exits on broadside shots.
I have also been using modified Hornady GMXs cut down to 94gr in 6.5 Grendel. From an 18", it does 2750-2800fps, expands really well and retains 99% weight.
http://i1085.photobucket.com/albums/j422/LRRPF52/65%20Grendel%20Reloading/20170322_092509_zpsls2xkk1g.jpg