the gun in your picture would be ileagle unless you built as a SBR and paid for a tax stamp. you need a 16" barrel. you could do a pistol that way but you would need to loose the forward grip. you will need to get the trigger guard closer to the mag well. that might be tough as the trunion is were your trigger is.
I like the concept and had been thinking of a pistol like that so it would be more balanced. the linkagke and trigger could possably worked out. Im sure there is a way to get it built. If you are doing a blow back a custom trunion and bolt could be used and the barrel and trunion could be move about a inch further back into the reciver. This would make it even more compact. You shold make the stock a side folder while your at it. Go for it