Adapter...you mean to replace the ADM with a better ARCA clamp or are you looking for ARCA rail for your fore end?
I'm assuming you just want a better clamp (personally, I'm not that big a fan of the ADM clamp).
In that case, either Area 419 or RRS
THIS CLAMP WORKS ON ALL STANDARD ARCA/1.5″ DOVETAIL RAILS Area 419 is always looking for ways to improve existing products when we are not developing new technologies. With the rapid growth in popularity of ARCA rails in precision rifle, we saw a need to improve this great dovetail technology to...
www.area419.com
or this
soar.reallyrightstuff.com
I have both. The RRS is out of stock right now (what else is new) and they stopped taking back orders, unfortunately. The reason I like it so much...aside from RRS quality (and Area 419 quality is top notch also) is that you can mount to either ARCA or Pic rail with it.
If you have to have QD lever on the RRS (which I do not....its fast and easy to put these on/off IMO), then you want this one but its mo' money (like twice the money). This is exactly like the clamp on the Anvil-30. Note that it has the pin (R-Lock) in the rail to match with their notched ARCA rail but can easily go on any plain ARCA rail as the pin is spring loaded but having to mess with it would certainly detract from the quick part of quick disconnect, right? If that important to you, you could call RRS SOAR and see how to disassemble to remove the R-Lock pin....but personally I was fine with the screw knob version.
soar.reallyrightstuff.com
They not longer offer the ARMS-LR reference above in favor of the SC-ARC but its identical except it doesn't have the R-Lock pin and can be had for less money and they can be had from retailers who have stock. Also doesn't have the locking pin for the QD lever that the SC-ARC has.
The ARMS-LR SOAR Lever-Release Clamp is the perfect fit for shooting enthusiasts. With RSS's Patented Dual Clamp design, the ARMS-LR easily attaches to RRS-compatible dovetails and sections of Picatinny. Designed so users can attach directly to any ARMS-17S compatible bipods or any other...
diprecision.com
Both of these clamps have the ARMS 17S mounting hole pattern which is what your Atlas (and many others) have.
Hope this is helpful.