Ok, I plan on getting my CAR-15 this comming thursday. One of the first things I want to do is modify the safety switch so that its not a PITA when shooting(Im left handed). Second: what is a carbine rifle exactly? I keep seeing the term and dont have a clue to its definition. Thirdly: for accurate shooting does a two stage trigger have a big advantage over the standard trigger? Fourthly: Can anyone give me some links to makers of bull barrels? Thanks guys

1. there is a selector that you can install in place of the factory selector to give you right or left select (ambendex..(? damn spelling!)

2. Main Entry: car·bine
Pronunciation: 'kär-"bEn, -"bIn
Function: noun
Etymology: French carabine, from Middle French carabin carabineer
Date: 1605
1 : a short-barreled lightweight firearm orig. used by cavalry
2 : a light short-barreled repeating rifle that is used as a supplementary military arm or for hunting in dense brush

3. two stage will help when benchrest or target shooting,  I dont see a need for this high cost option for general use / protection.

Main Entry: car·bine
Pronunciation: 'kär-"bEn, -"bIn
Function: noun
Etymology: French carabine, from Middle French carabin carabineer
Date: 1605
1 : a short-barreled lightweight firearm orig. used by cavalry
2 : a light short-barreled repeating rifle that is used as a supplementary military arm or for hunting in dense brush

Edited to add: Guess you beat me to it

In ARish, a rifle has a 20" barrel; anything shorter (16", 14.5", etc.) is a carbine.

As far as the trigger, I'd see how bad your stock trigger is first. Some are not bad, others are horrible. It also depends on what you want the rifle for. For a competition or varmint rifle, definately get a double stage. For a serious weapon, you might want to keep the military trigger and maybe just polish it a little.

I am not sure who sells bull barrels.  HBARs are already plenty heavy, and they are usually extremely accurate.