Why not just do this?

Yes, a full auto version of the IWS. 

Seriously though, wouldnt an APFSDS with 20k ft lbs be more destructive than an API round?

I came across this truly superb article on the effectiveness of a 20mm AMR. 

Worth checking out. Theres penetration fragmentation data, long range ballistics, recoil impulse data, surveys of current off the shelf systems, and potential design features of a future AMR rifle:

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2009/infantrysmallarms/thursdaysessionxi8503.pdf

Theres 48 pages of great data in there, but the conclusion is:

Yes, that is an excellent presentation.

Regrettably, little appears to have progressed in the 12 years since I first read the article.

I’d like to see a 20mm AMR broadly adopted, not limited to SOF. Infantry and engineer platoons could benefit from the capability.

I would opine that the lack of progress is due to a 20m AMR being too much of a niche weapon, even for special forces.

For infantry platoons, a multi-purpose weapon makes far more sense, hence the USMC and Army choosing the M4 CG.

YouTube

Why the M4 Carl Gustaf is so deadly

Saab's new Carl Gustaf, the M4 or M3E1, is roughly 6.5 lbs lighter but is smarter and far more lethal than its predecessor.More Information: https://www.mari...

Lynx looks exciting but honestly many 50BMGs i shot were rather soft on recoil due to massively effective muzzle brakes so i am kinda skeptical that this is cutting the recoil much. And its also designed as short range anti material rifle with sub 1000m practical range.

https://www.youtube.com/watch?v=FDfLKpOkWVg

YouTube

GM6 Lynx Semi Auto 50 BMG Bullpup 4K UHD

12,7x99 (.50 BMG)GM6 Lynx is a revolutionary, unique new weapon combining light weight with portability, mobility and heavy fire power.FEATURESMost advance A...

I actually like the gm6 and the niche it puts itself in.

For something like a vehicle checkpoint where you may need to stop a vbied coming at you, you don't actually need more than 1k meter range as much as you need the ability to pump multiple raufoss or m8 API into something QUICKLY.

By being shorter and easier to fire quickly it does this better than other options. You can conceivably hoist this over a jersey barrier get behind it and pump rounds quickly then move if necessary.

Maybe there's something I fundamentally misunderstand here but I can appreciate the tradeoffs they're making, including the >1k meter precision sacrifice.

Put bluntly in many cases the guy nearest this gun at the time it needs to get used probably doesn't really know how to shoot past 1k effectively anyway.

I know I certainly don't have the skills necessary to do that truly well.

To a degree I think it's a weapon good for the force and enablers we have rather than the Ultimate tool for the ideal paper force that has nothing but guys trained to shoot past 1k effectively and $30,000 high end enablers like 3rd gen flir fuzed night vision and thermal optics etc that can identify and resolve a target sufficient to make it within ROE to be permissible to shoot.

I was thinking complementary to Charlie Gutsache, not instead. 
It’s a truism that there are few problems that can’t be solved with more HE, but sometimes more is too much more.

That being said, I think it’s a shame the xm109 payload rifle wasn’t further developed. 

Indeed that is the case , Lynx was always meant for use mostly in Assault rifle type distances, 1000m shots even with 50cal require accurate range finding and ballistics calculations so not something you do in a hurry. The most fascinating thing is that gun can be carried collapsed when its barely longer than an assault rifle. But so far i havent seen any adoption of the gun anywhere . would at least expect some ME customers to buy for the cool factor alone.