To the OP, a pump shotgun is an excellent choice, and the Mossberg is one of the most highly regarded.
I definitely recommend training. In the meantime, you might consider taking a look at this:
https://www.personaldefensenetwork.com/product/fundamentals-of-defensive-long-guns-dvd/
It's not a substitute for training, but it's a good introduction to the concepts, and there is a section on running a pump shotgun.
The important thing is
practice. Make it an ingrained habit to pump the gun after every shot, and shoot it enough that competently using the action release, working the action, etc. become deeply ingrained. And follow the Four Rules of Gun Safety religiously.
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Now, a quick rabbit trail regarding small-caliber rifles and HD...
in my own experience, using rifles for home defense in residential areas is highly irresponsible.
Depends on the rifle and the load. 55gr civilian .223 JHP or non-bonded SP penetrates less in drywall, and has less wounding ability after penetrating even one full wall, than either 9mm JHP or 00 buckshot. If the rifle is loaded with FMJ or some bonded core hunting bullets, or is a larger caliber than .223/5.56mm, it may out penetrate a pistol or 00 buckshot, but the lighter, more fragile .223/5.56mm loads don't.
http://www.how-i-did-it.org/drywall/results.html
Also see Roberts G.K., "Law Enforcement General Purpose Shoulder Fired Weapons: the Wounding Effects of 5.56mm/.223 Carbines Compared with 12 ga. Shotguns and Pistol Caliber Weapons Using 10% Ordnance Gelatin as a Tissue Simulant,
Police Marksman, Jul/Aug 1998, pp. 38-45.
"When used with effective ammunition, the 5.56mm/.223 carbine simultaneously offers both greater effective range and less potential downrange hazard to bystanders than a 12 ga. shotgun, handgun, pistol caliber carbine, or SMG, as well as far greater potential to incapacitate a violent criminal than any handgun, pistol caliber carbine, or SMG."
Roberts actually dismissed the lightish 55gr JHP/SP loads as not offering *enough* penetration for law enforcement use, though I personally think a little less penetration is ideal for civilian HD use. The heavier JHP loads that the Roberts et al. study rated most highly still penetrate less than a lot of 9mm JHP.
Bare gelatin:
Win 69 gr JHP-BT Match S223M
Vel (avg) 2758 fps
Ave penetration 11.9 inches
Ave frag 74.6%
Black Hills 75 gr JHP Match
Vel (avg) 2580 fps
Ave penetration 13.3 inches
Ave frag 44.1%
9mm Fed 147 gr JHP 9MS [for comparison]
Vel (avg) 1043 fps
Ave penetration 13.2 inches
Ave frag n/a
Gelatin after passing through interior wall:
Win 69 gr JHP-BT Match S223M
Ave penetration 13.3 inches
Ave frag 79.5%
Black Hills 75 gr JHP Match
Ave penetration 13.0 inches
Ave frag 38.4%
9mm Fed 147 gr JHP 9MS [for comparison]
Ave penetration 22.8 inches (!)
Ave frag n/a
The sound factor was also mentioned earlier. I will admit, in the heat of the moment, you may not notice the sound nearly as much with the adrenaline pumping. But if you've never fired any type of firearm inside a closed structure without hearing protection...the effects are very long lasting, and the increased adrenaline will definitely affect any followup shots without proper practice. The more you train, the more your mechanical instincts will kick in and take your brain out of the equation in those situations.
There are three factors that determine firearm noise/blast: (1) gas volume, (2) gas pressure at the moment the gas is vented to the atmosphere, and (3) whether or not a muzzle brake or comp is present; bore diameter also seems to play a role in the peak pressure, possibly because the pressure is dumped faster at the moment of muzzle uncorking. Gas volume is determined by the amount of powder in the cartridge; pressure at uncorking is heavily affected by barrel length and whether or not there is a barrel/cylinder gap.
The result, though is that a 16"+ .223 *without* a muzzle brake is about the same dB as a 4" 9mm or an 18.5" shotgun, and much less loud than a .357 revolver shooting full-power loads. Add a brake to the rifle (or worse, 14.5" with a pinned brake) and the rifle will be far louder, though.
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