P.S. Is there any way to see the stats of an item in the barter screen BEFORE buying the item?
No, not really. You can "see" it, but that generally won't tell you much for weapons or armor. Save, buy it, put it on, check the stats. Then reload if you don't like it.
As a note, Fallout uses three types of armor stats.
1) AC. The higher this is, the harder it is to hit you. Basically it decreases the hit chance for attackers. This is why you'll get different hit percentages for different enemies in melee.
2) Damage threshold. This amount is subtracted from all damage of that particular type. If the threshold is higher than the damage dealt, you get the "X hit Y for no damage" message.
3) Damage resistance. This is a percentage reduction of everything not stopped by the threshold.
Great info, thanks. So let me make sure I understand all of this correctly. I'll illustrate with a hypothetical example:
Suppose an opponent has a 75% (unmodified) chance of hitting me. With leather armor, my AC is 25, and with metal it's 20. So depending on which armor I am wearing, his chance is reduced to either 50% or 55%.
If he does hit me (say, with normal damage), then there is some unmodified amount of damage that is presumably random based on his weapon type and (I guess) my distance from him and other factors. Let's say this amount is 10. The DT for metal armor is 4, vs. 2 for leather. So this 10 is reduced to either 6 or 8. In both cases these are positive numbers, so his hit was successful.
Then, metal's DR of 30 reduces the 6 by 30%, so he does 4.2 damage to me, which will I assume be rounded down to 4. On the other hand, leather's DR of 25 reduces the 8 by 25%, so I take 6 damage. In either case, the final number (4 or 6) is subtracted directly from my HP.
Did I get all that right?
 Oh ho, I found a page in the Wiki that looks like it spells out all the details: http://fallout.wikia.com/wiki/Fallout_combat
also relevant: http://fallout.wikia.com/wiki/Critical_Hit
Here's a detail I wouldn't have guessed:
"Note: The Fallout 1 engine does not implement the ammo modifiers, so there is no difference between JHP and AP ammo."
So I guess that means in practice the two types are just a nuisance because they can't be mixed together in the same gun at the same time. I still hoard both types because I fear running out.