Living in the north central part of the country we have a chain of stores named Mills Fleet Farm and they have introduced a firearm brand name "Huldra Arms".
Huldra Arms - Gas Piston Rifles
These are gas piston deisgn AR Style firearms. The rumor is that these are Adams Arms sold under the brand. Anyhow they offer 5.45 caliber complete firearms and uppers. I bought one of the 5.45X39 uppers and used a Spikes Tactical lower to build a rifle. It has a 16 inch barrel and carbine length gas system. So there are two suggestions for 20 inch barrels: Huldra and Adams Arms.
It turned out an OK shooter using Russian Wolf ammo but it absolutely requires a person install an extra strength hammer spring to ignite the hard Russian primers. I found out Hornady offers some domestically loaded ammo for this caliber but it appears they use Russian steel cases which means possibly the same hard primers. I have a couple of boxes of the Hornady ammo on order. I was hitting three hundred yard steel plates with no problem with the sixteen inch barrel so you may not need 20 inches however I do remember that CDNN offered an East German Border Guard bolt action rifle 20 years ago with a longer barrel and the guys I know who bought them liked them.
Hornady Manufacturing Company :: Search results
Then there is the magazine issue you are going to have with this caliber assuming you are going to build an AR rifle. They include a 30 round ASC 5.45 magazine with their rifles and you need them... I ordered ten followers from Rick at ASC and tried them in various other magazine bodies and none of them worked. I tried Lancer, Brownells, NHTMC and Magpul but none worked. I recently located some surlpus Canadian Orelite magazines and I plan to try one of those soon. I got the followers to fit in the aluminum mag bodies but then they bulged so as not to fit in the rifle magazine well. In the end I ordered two more additonal complete 30 round magazines from Rick. I like 20 round magazines because I use them when I go prone or shoot off the bench.
Anyhow this is a pretty nice little caliber and reasonably accurate even with the Russian ammo. Hope this helps you out a little and good luck!