I often have folks e-mail to ask me which is the best all-around rifle for retreat defense. The following may sound a bit like a proverbial Chevy versus Ford rant, but here goes…
To begin, let me state that I firmly believe that .223 Remington/5.56mm NATO is insufficient for self defense. That cartridge was designed specifically for killing woodchucks–not men. It does well at wounding men, which is fine for military organizations. (An incapacitating wound removes three enemy soldiers from the battlefield–the wounded soldier plus two stretcher bearers.) But the last thing that I would ever want to do post-TEOTWAWKI  is wound a looter. I want them 100% RBC ed, and I want to insure a less than 0.001% probability that they are going to crawl off and snipe at me and my family for the next day or two.
For serious social engineering, .308 Winchester/7.62mm NATO will do the job. It is also a fine deer hunting cartridge. Which .308? In essence, I consider M1As , HK-91s , FALs, L1A1s , and modern AR-10s  all functionally equivalent. All four are quite suitable for retreat defense. However, pre-ban HKs are currently way over-priced, and M1A parts and spare magazines have become much too expensive! Meanwhile, most AR-10s use very expensive magazines. (Note: A couple of AR-10 manufacturer have cleverly introduced variants that use FAL magazines. That is the ideal type to buy if you decide to opt for AR-10s.)
Three years ago, I sold all five of the M1As from my family battery and replaced them with five L1A1s and a Para FAL that had been converted to take inch pattern (L1A1) magazines. FWIW, I was an dyed-in-the-wool M1A owner from 1978 to 2002. I switched to L1A1s because of the profusion of inexpensive L1A1 magazines and spare parts. I had 47 spare M1A magazines and nearly one complete spare parts kit. Propitiously, I sold off all those M1A magazines shortly before the 1994 ban expired, for $30 to $45 each. (Some of them were still in original U.S.G.I.  wrappers.) Now, for less money than I realized from the sale of my M1As and their accessories, I have 138 spare magazines and four complete L1A1 spare parts sets, plus scopes for all six of my L1A1s. To borrow the modern parlance, the decision was a “no brainer.” Most of my L1A1s are built on pre-ban receivers. IMHO, L1A1s and FALs are the clear choice in today’s market. See The FALFiles  for sources. IMO, Century Gun Works (CGW)  of Gardnerville, Nevada custom builds the very best FALs and L1As. If you have a FAL or L1A1 kit, then Rich Saunders at CGW  is the gent to build it for you. The quality of work at CGW is followed very closely behind by T. Mark Graham of Arizona Response Systems . Mark is also a great gunsmith. I had Mark convert a couple of pre-ban SAR-48s to inch pattern specifications for me. Of the large scale production FAL clones, I think that D.S. Arms  rifles are hard to beat. See my FAL FAQ  for additional details about FALs and L1A1s.
After all that talk about FALs and L1A1s, you may wonder why I showed HK-91s as a “group standard” in my novel Patriots . Ironically, I’ve actually never owned an HK-91. However, several of my friends have, and swear by them. I portrayed them as standard partly in an attempt to make the novel appear less U.S.-centric. (At the time I would have otherwise touted the M1A.)
To re-iterate: I consider the L1A1, FAL, HK-91, M1A, and the new production AR-10 variants all roughly comparable in terms of reliability and putting lead down range. Here are my quick and dirty comparisons of all four rifles:
The M1A has an edge in accuracy–at least the more expensive match grade models. (But the charging handle is on the wrong side except if you are a lefty.)The HK-91 has an edge in reliability. (But it has inferior ergonomics and it’s action doesn’t lock open after the last round in a magazine is fired.)The AR-10 has an edge in light weight. (But it shares the AR-15’s filthy gas system design.)The FAL has the best ergonomics, and is currently the most reasonably priced.
At the time that I wrote the first draft of the novel. (the winter of 1990/1991), M1As and HKs were both roughly $700 and FALs were $2,200. (The FAL clones hadn’t yet hit the U.S. market.) If I were writing the novel today, I’d definitely pick the L1A1 to portray as group standard.
I prefer L1A1s over FALs because of their sturdier Maranyl stock furniture, bigger selector switches and magazine releases, their folding charging handles, and most importantly their ability to accept BOTH inch and metric magazines. (Tactically, that is an advantage, as the Brits found when they invaded the Falklands.) YMMV , but I do think that “inch is best.” And if you live in a State that borders Canada, I consider inch guns absolutely the way to go. (Since inch pattern spares and accessories are likely to drift across the border WTSHTF.)
OBTW, if you don’t yet have a copy, I strongly recommend that you buy yourself the latest edition of “Boston’s Gun Bible.” Among other topics, Boston goes into great detail about weighing the merits of various battle rifles. My review of Boston’s Gun Bible is included in my Bookshelf  page.
One closing note: If practicable and affordable, arm all of the defenders of your retreat with the same model and caliber of rifle, for three reasons:
Commonality of spare magazines
Commonality of training (Any group/family member can pick up any rifle and know how to use it–although its “zero” will probably be slightly different)
Commonality of spare parts