Let's take a look at the cartridges that your current rifles are in:
M1A: 308
AR15: .223
AK47: 7.62x39
HK USC: 45 ACP
L1A1/FAL: .308
Seems to me that you are lacking in a Magnum cartridge that is flat shooting and has longer range than .308. Also you don't seem to have anything that can really double as both a hunting rifle and a sniping rifle. Though your M1A could do it in a pinch..you can get more accurate results with a really good Bolt action Rifle.
May I suggest something in 7mm Rem Magnum or 300 Win Mag or 300 WSM ? Your BEST results would be with a custom built rifle.
Stolle and Hall make great Actions for a Rifle.
Or you can go the Factory route and get yourself a Remington, Savage, Tikka, Sako, Winchester or Ruger Bolt Action.
Good Custom and Semi-Custom Rifle Builders: Robar, McMillan, Chandler, Jarrett.
Good Barrel Makers: Lija, Krieger, Shilen.