World War Z Standard Infantry Rifle (SIR)

Max Brooks' novel "World War Z" has a rifle he created just for this war. The rifle, affectionately called the SIR, is a mesh between the AK-47 and the AR-15 on a wood stock. Its key features are the durability of the weapon, the accuracy and the unfailing reliability. The rifles displayed are my interpretation of the weapon system. A fixed stock with a bayonet (not displayed), an AK operating system, modified with a three lug bolt, a beefed up and polished AR-15 semi auto trigger group, and an 18", 1:8 RH twist to stabilize the longer profile Cherry PIE round. There are some differences in the three. The sniper variant has a 24", 1:8 RH twist with modified port to account for longer internal ballistics. The carbine has a 14.5" barrel, 1:9 RH twist and a left folding stock. The lack of stock forearm length negates a folding bayonet. A standard removable U.S. bayonet should fit. I added a flash hider as a personal touch. As for sights, the SIR has a 20" sight radius using an A2 style peep sight. The SIR-C has a 15" A2 peep sight. As for the SIR-LR, no sight was added. I imagine the extra material spent on a rail system would be a waste. With this in mind, I would mount the scope rings directly to the upper receiver, reducing the scope height from center of bore.
As for the function, the piston sits in a shrouded, removable tube for cleaning. The upper receiver slides into and pins to the receiver block and lower receiver. A guide rod and spring reset the bolt on each cycle, and rests in a fixed block on the lower receiver. The whole operation is kept simple and smooth. The right hand reciprocating charging handle has one small problem. When the bolt assembly returns into battery, an opening between the upper receiver and stock would allow debris to enter the rifle. To combat this, a pivoting dust shield inside the housing raises with each pass of the handle. A spring returns the cover into place after each cycle.
The safety would be a simple button safety, behind the trigger group. The magazine catch would be a simple button style AR-15's have always had. This would allow for anyone with an AR-15 magazine to use what was on hand.