I've 'broken in' a couple of barrels from DPMS over the years with excellent results... Was it totally necessary, I can't say, but the end result was that they shot really good...I still have one that is scary accurate..
UtahShooter, in your case, I would make sure the barrel is as clean as you can get it...DTech has a procedure for cleaning his uppers that works well during a breakin.... Find some Foaming Bore Cleaner (Outers or Gunslick) and soak the barrel a couple of times in a row to insure that any copper and powder residue is gone from the barrel...Then find some of the cheapest military (Not Wolf) ammo that you can find and start the prescribed break in procedure...
First of all, it won't hurt the barrel, it will just take some of your time...and try to get a minimum of 100 rounds through the barrel, before worrying about real accuracy...If the barrel is capable of accuracy, it will come, if not, you will never wring it out, without additional expense and effort..
The procedure I use in a little different than what most would promote...One round and clean for the first ten rounds. Five rounds and clean for the next twenty rounds, and then every ten rounds, clean...Until you have reached one hundred..
When you are 'breaking in' a barrel, you are doing two things, knocking down (hopefully) any left over machine marks from the cutting of the lands and grooves, and secondly, burnishing the steel surface of the barrel...
Unfortunately, with the shooting the previous owner has done, and not knowing what was specifically done on his part, this may be an exercise in futility...The best you can hope for is to remove his dirt and debris and start over..I'm hesitant to suggest using JB Paste unless that is a last resort, as you can potentially cause more damage than you are helping..
You will hear a lot of guys, myself included, that indicate a clean barrel loses some of it's accuracy until X number of rounds have been shot and after a second X number of rounds have been shot, the accuracy starts to degrade again. You may have to watch for that to occur and live/shoot accordingly..
After you complete the barrel break in procedure, and if you are stuck with factory ammo, obtain a box of several of the various ones in your immediate area and then run some tests to see which gives you the best consistent accuracy....
Before I started reloading, I was pleasantly surprised to find that for one rifle, the Federal 50gr HP gave me some excellent groups and it was the cheapest on the market... on a couple of others, Black Hills ammo earned the spot...For a couple, it was high dollar and that's one of the reasons I started reloading for my rifles..