U MIGHT survive needing 20 rds ONCE(by luck) or 10 rds twice (by luck) but if skill and proper tactics is going to saveyou, you will be in VERY few altercations that you dont win with 3-4 shots, tops(or break-contact, slip away, etc. )
With only 4 rounds expended per fight, then you can save yourself a bunch of cash and get a super reliable Enfield No4 MkI for about $150. Lot's of penetration and killing power plus it holds 10 rounds. It also has vastly more range than a 11" CAR-15.
A 12 year old in the 'thick cover' with a .410 will only need one round for gunkid. While the 11" POSCAR will be worthless, the 12 year old will be luck with a bunch of twinkies gunkid will have in his '30 lb' pack.
This "four rounds per confrontation" is an AVERAGE of Law Enforcement versus scum, and civilian versus scum, not an average in the types of scenarios described in SHTF. Look at the Miami Shootout and those two idiot bank robbers' in LA. A couple more than four rounds wouldn't you say? Hundreds, maybe thousands of rounds were exchanged before LAPD granted those moron's their death wish.