[All4BigGuns]

While normally I feel that the system of a game is intrinsic to that game, of all the systems out there, I feel that this one could do well for the Fallout setting.

Anyone feel like discussing what would need to be done to use SR5's system for this?

[MijRai]

I've played with making Fallout-esque things in different systems/settings before, it's pretty fun to do.  Going point by point from the Shadowrun system, though... 

1.  You'd need to flesh out mutation and a new version of Radiation rules, that's for sure. 
2.  Ditching magic is simple enough.
3.  Having limited Matrix stuff for when you can access old computers (make it possible to reconfigure those Sentry Bots and laser death fences) seems like the right idea.
4.  Figuring out the effects of various drugs wouldn't be too hard, nor would statting out the various weapons/armor/gear you come across, but;
5.  Figuring out a good mechanic for quality and degradation over time/use would be more important to work up. 
6.  Prices for various things will need to come up, as well as throwing out the majority of the cyber and bioware (there's limited stuff, but it is a post-apocalyptic setting; getting it/a source would be a campaign of its own in my opinion). 
7.  Making a character creation system without the Races in it might be necessary, unless you can come up with enough options besides humans, ghouls and androids (maaaybe Super Mutants as your 'trolls'). 
8.  Writing up all the dangerous critters. 
9.  Perhaps shrinking down the Attributes to match the SPECIAL of Fallout; Strength (Strength), Perception (Perception), Endurance (Body/Willpower), Charisma (Charisma), Intelligence (Logic/Intuition), Agility (Agility/Reaction), Luck (Edge).  That's quite optional.
10.  Definitely implement some proper weights and a carry capacity for people, because the survival roots are important; Ammo needs a weight too to make those Wanderers pack carefully.
11.  Flesh out the Street Cred and Infamy system so you can make a proper Reputation chart/system; who's friends with who is important. 
12.  Perks are extremely important; treat them a bit like Martial Arts/Positive Qualities. 
13.  There's also Karma to consider; adjusting Perk availability to some extent would be a good way of representing that.  The Cannibal Perk shouldn't really be out there for the paragons of kindness and dignity, for example. 

[All4BigGuns]

There are a couple points you made that I don't agree with. I will only go into one though. Having to track weight and carrying capacity adds nothing to any game. It is just useless bookwork that detracts from the enjoyment.

[MijRai]

That varies from game to game for me (tracking weight in Shadowrun itself is something I don't tend to do, though I prefer it in Pathfinder to some extent), but it'd be a necessity for a Fallout game.  You always have limits as to how much you can carry, and in a setting like Fallout, food and water need to be on that list.  It isn't like Shadowrun where you can go to a corner store and buy some food or pick up ammo.  Fitting everything and the kitchen sink on your back is what people will do if you don't put down guidelines on weight and carrying capacity.  Even if you don't want to track specific numbers, you'll need something.

If actual weight isn't what you're after, maybe do something like a slot/encumbrance chart/table; If you carry this many large weapons, then you take this much penalty, because everything in your walking arsenal weighs you down and gets in the way.  If you carry an obnoxious amount of food/water, then you'll have problems there, etc. etc.  It doesn't have to be numbers (though I'd do it that way myself), but something needs to be done about it. 

What other quibbles do you have?  I mean, 9 is definitely an Optional thing, which I stated. 

[All4BigGuns]

Well, honestly the current prices for a lot of the mechanically useful things is more appropriate for a post-apocalyptic setting than the base setting, IMO.

[MijRai]

Mostly I'd say increase the price of consumables and sensitive tech, not your basic kit.  Put an actual difference in ammunition prices for your various weapon categories (light pistol vs. assault rifle, for example; 9mm is going to be less valuable (and more common) than 7.62x51), make those devices people can't just produce any more have more value.  I agree that some things in Shadowrun are totally overpriced, and some of those prices do match up with what I'd expect in your average Fallout setting. 

Working up a system for reloading your own ammunition as a cheaper way to keep your stuff up would be a fun way to bring out shine (or scuff) of the setting while making things easier on the wallet for players who are interested.  Just make the components cheaper or procurable (melting down childrens' toys for lead, for example), require a Reloading Kit (Or just an Armorer kit) and let them at it when they're at home!  Require rolls if they want to doctor their batches up to make specialty ammunition they didn't acquire, like Explosive or hollow-point. 

[All4BigGuns]

Reloading ammo is simple. Just make it an Extended Armorer Test with a threshold of like 4. Just make it so that this use requires tools and the 'no tool penalty' is being unable to make the attempt.

[MijRai]

That would work too!