Honestly, once the SHTF you are going to need at least a M14, and about 5000 rounds.
You are wise in going for 7.62x51 as everyone knows after about 100 yards the 5.56 is rubbish! Ive seen people being shot at 100yards and still continue to fight like nothings happened.
You should also seriously consider a pistol too,
Don't stuff around with 9mm though, no better than 22lr.
Get yourself a 1911 and about 2000 rounds for that.
And consider getting a decent scope for your m14, at least a 6-24x50 because once the SHTF its pretty much going to be sniper battles galore.
You can watch the news and just see that within a few years the worlds going to be in complete mayhem.