The 6920 is damn near perfect for what it was designed for. Shooting people from 0-400yds and never breaking.
If you want accuracy don't chop up the 6920. Buy a rifle built for the bullseye not for the battlefield.
As hlee says, don't make it into something that it was never intended to do. It is a fine carbine weapon, and with the right optics, you will have no problem banging steel 600 yards away.
Keep it for what it is, or sell it. Buy a precision AR.
See a pattern?
You got a great rifle. Put an ACOG on it if you want. Put a Giessele trigger in it if you like, I'd recommend the SSA/E. If you want a rail, get the KAC M4 two piece rail and vertical grip, look on Ebay. You can get a B5 SOPMOD stock (just the stock) and have a much nicer buttstock for $99 or less. Don't change the barrel. Then you'll have a rifle essentially just like what's issued but with a better trigger.
Work on mastering that while saving for something else. It should be around a 2-3MOA rifle (possibly better) depending on ammo, so capable of hitting the E type steel at 600 reliably. We used to use these as the very first SDM rifles FWIW. Colt M4's, NIB, basically built like above but with a bipod.
If you get the Geissele trigger and SOPMOD stock, you could get a nicer upper that's more accurate and use the same lower you have to save money. You can also do that, and then build a dedicated lower later with a fixed stock, then you'll have two rifles, one a nice M4, the other a nice tack driver. OR save for a new rifle altogether. Maybe a 7.62, maybe the new 20" KAC M110/SR25 with the keymod or mlok tubes; guaranteed to be under 1MOA and comes with a target to prove it using FGGM 168's. Better ammo and time and it may shoot even better.
But whatever you do, don't destroy that rifle. Sell it or trade it on here and get something else if you feel you need to, but don't damage what most consider to be a fine rifle, you'll regret it.