I'm pretty sure all of the Huldras use the Adams Arms piston system.
Quote from the website:
The Huldra Mark IV 5.45 Carbine’s patented piston system technology will run cleaner, cooler, and longer than any other AR. The piston system parts and the barrel have been treated with our salt nitride meloniting process making them harder than hard chrome.
It's the short (carbine length) piston system instead of the mid-length version. I'm not sure if there are any merits or demerits to either one as I can't seem to find many guys online that have reviewed them and there aren't many locally.
Our local Fleet-Farm hasn't had any 5.45x39 ones on the shelf whenever I've been checking them out. Just 5.56
I'm eye-balling the 5.56 carbine version but I'm still on the fence whether or not I want to buy or build. I do like the way the Adams piston system is put together.
One guy locally here has the 5.56 carbine. I haven't shot it yet but he's been saying that it's been a reliable rifle. Not much of a review or recommend, but that's the closest I've come to one so far besides fondling them at the store.
Do you guys have Mills Fleet Farm stores down there in FL or are you buying online? My understanding is that Huldra is a Fleet Farm store-brand.