Rates of Fire: Cyclic or Follow Up
While I would not disparage the use of rifles in defense against bipeds, when I think of them for self-defense, I generally think of defense against wild animals when I'm hunting or camping. In Wymoing, Idaho and Montana with the reemergence of wolves and grizzly bears, SD against these apex predators is probably the job of a rifle if its available. During hunting season, the report of a rifle in these areas now often equates to the ringing of the dinner bell for some of our furry friends.
I guess another use for rifles in defense would be in those communities that forbid pistol ownership. A good carbine might work out well there, but I think that I would opt for a shotgun instead. Of course I certainly would never opt voluntarily to live in such a place, many may not have a say in the matter.
Bedlamite,
Back in my LE days, the average officer (per FBI stats) fired 4 to 5 shots in just a couple of seconds during the hours of darkness at a distance of 4 to 7 feet. I'm not sure what it is since the turn of the century, but you're pretty close.
On the other hand, follow up shot speed is not fully dependent on a cartridge's length, although it can be part of the equation, particularly when determining the cyclic rate of fire of a weapons. Your example of cartridge length affecting repeat fire really refered more to cyclic rates of fire as I read it.
If my assumtion is correct, then several more important factors than overal length of a round come into play as to determining cyclic rates of fire (maximum mechanical firing rate not considering belt or magazine changes):
1. Slide or bolt weight
2. Amount of gas allowed to impinge the piston in a gas operated weapon (some are variable and some are self venting to even out pressure spreads in different ammo)
3. Pounds of force required to compress or expand the operating rod spring
5. Gas cups to enhance recoil on recoil operated weaons (MG 34 or MG 42)
6. Cleanliness or degree of fowling in the weapon
7. Lubricants used
Examples:
A. The German MG 42 of WWII had a very high cyclic rate (1200 RPM and higher), yet a fairly long cartridge (7.92 X 57mm)
B. The US Model 1917 and 1919 machine guns had much slower rates of fire (550 to 600 or so), yet the overall length of their US Cal 30 round (30/06) was only something like 5mm longer than the 7.92 X 57mm.
C. The US M3 Greese Gun sub-machine gun has a very low cyclic rate of fire (450-500 rpm), yet a very short cartridge (45 ACP)
D. The M1 Thompson, firing the same cartridge as the M3 has a fairly high rate of fire (700 rpm or so)
E. The current German GP machine gun is the G-3, basically a WWII MG 42, yet in its heavier bolt version, its cyclic rate is only about half of the WWII version's rate
F. The original UZI had a moderate rate of fire, but its smaller offspring (Mini and Micro Uzis) firing the same 9mm have horribly high rates of fire due to the lighter bolts.
For me, fast follow up shots or double taps, depend more on the amount of recoil, trigger mechanism, weight of the weapon, the grip design of the weapon and the degree of accuracy that I need on the second or third shots instead of the actual cyclic rate or length of the cartridge.
Other than in full auto weapons, most of us cannot take advantage of cyclic rates of fire in self-loading weapons and cartridge lenth does not really impact the speed of repeat shots.
So, after that rather lengthy diatribe, what would I choose for a home SD rifle? My choice would be what I've known and used at work for the last 20 years or so. While I can't bring my M4A1 home, I could easily mimic it with a similar AR-15 with a 16" barrel, flat top and an Eotech sight. Yep, that would be it.