My survival weapon of choice would be a Thompson G2 Contender, hands down.... Period...
Its an extremely versatile, and VERY accurate weapon. Simply change the barrel to change the caliber your hand or shoulder can tolerate. Specifications, you ask? 3.5 lb weight for 12 in barrel models, overall length 15.5 in. It has an adjustable trigger, and you can use centerfire, AND rimfire barrels on the same frame. Outstanding sights, and is drilled and tapped for scope sights.
Caliber selection ranges from the sublime, to the insane.. The " slightly better than a pellet gun" 22 short for the taking of rabbits and sparrows, a .45 acp/ .410 combo( the .410 being the most useless/ineffective shotgun shell ever invented in my opinion) , to downright Diabolical, Pantella sized, African "Big Five" stopping calibers are all available for your shooting/ shoulder pleasure... And all have been used globally , with proven results. ( do a web search )
I personally could care less about ammuntion weight.. This is my survival we're talking about...Meaning: I will gleefully carry 10 lbs of cartridges , plus another 10 on top of that... As far as weight goes, in a true wilderness survival situation ammunition ranks right up there with water as far as i'm concerned.
just my 2 $
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victory begins with a mindset...