IMO, you can't go wrong with any rock river arms rifle in a bull barrel. If all you want is a bench rifle for shooting small groups and long distance shooting, then it is awesome. Hell, even varmint hunting! LOL..... I know the A4 stock is not all that "tactical", but i'm a HUGE fan of this stock for rear sand back purposes. AWESOME entry level stock for target shooting with owl ear rear sand back. More tactical "adjustable" rear stocks are a real pain in the arse with it comes to rear sand bags. I think a 20" varmint A4 would be perfect IMO. About the only downfall I can think of when it comes to .308 rock rivers is they can't use pmags, but my local store had loads of rock river .308 mags in stock for $30 each...... on the other hand, 5.56 rock river rifles can use pmags. Have you checked gun broker for brand new rifles for about the same price? I'm just wondering?
I have the 5.56 EOP varmint rock river in an 18" ss bull barrel and it is the most accurate rifle in my collection and it only cost me about $1250 after taxes and fees. My rock river EOP with an 18" barrel is actually rather light weight with the scope and I wouldn't blink an eye using this rifle in the woods hunting above and beyond using it for shooting groups at the range. I've owned 4 total rock river rifles in my day, and currently own 2, and I would purchase another rock river rifle in a heart beat. Every rock river I've owned has been sub-moa shooters on average with the right optic. Also, in comparison to most dpms bull barrel rifles that are only in .223..... the rock river bull barrels come in the "wylde" chamber which is great for both 5.56 and .223. Their website shows an accuracy of 3/4moa and IMO it is really close to that with the right load development and ability of the shooter. I do not always get .5 groups, but I do get them rather often. Keep in mind the price of ammo! That is the main reason why I have my bull barrel rock river in 5.56 as it is used damn near all the time at the range for shooting groups up to 300yards.... feed this rifle some 60gr vmax and it is crazy accurate. If you're only going to be using it for target practice, and will be shooting at 300yards,,, maybe up to 500yards, then I would really consider the 5.56 in comparison to the .308... not only just for saving money on ammo, but it will be just as if not more accurate IMO at or under 300yards.
I know gunbroker has this 20" varmint A4 IF you want a 5.56 for $1249, which is actually not a bad price:
Rock River 20 in Varmint 223 ar15 ar 15 5.56 : Semi Auto Rifles at GunBroker.com