I'm soon to be a proud owner of a LEGP M4gery.

I would also like to me able to shoot longer distances with greater accuracy.

I was thinking about getting a complete upper with a 24" barrel but they are quite expensive.

I saw that I could get a bull barrel with sights and handguards for around $200.

Which leads me to the follwing questions.

1) How hard is it to change a barrel (time)?

2) How much better is a 24" bull type barrel then a 20"?

3) Do you switch uppers or just barrels?

Switch uppers. You won't save any money the other way. You will need the tools & the time will be a bitch.

I switch uppers. You can get uppers without carriers and bolts for almost as little as you're paying for the barrel and hand guard. For instance, I recently found a 16" HBar upper for $175. Just use the carrier and bolt you have now.

Legal disclamer: It is wise to check head spacing on the new upper with the old bolt. [:)]

I agree.  Switch uppers.  I recently removed the barrel on my AR so that I could switch barrel nuts.  The job wasn't really that difficult, but it is not one that I would want to do regularly.  You might even consider selling your LEGP upper and using that money to get the bull barrel upper of your choice.  I would personally keep the LEGP upper to plink with and be rough on, and get a bull barrel upper for distance shooting.  I don't like to shoot a bunch of rapid fire through my bull barrel uppers.  But that's just me.  I can't see spending a bunch of money on a really nice upper and then treat it like garbage.

Thanks guys, I didn't think of that.  I assume it is easy to switch bolts?

So, which is better a 20" bull barrel or a 24" in terms of accuracy and fun factor.  Since I already have a 16" heavy barrel I'm looking for this barrel to be capable of making shots I could not make with the regualr 16" barrel.

You probably won't see a great deal of accuracy difference between a 20 and a 24, but since you already have an upper for plinking and mobility and weight is not such a high criteria, I would go ahead and go with the 24.  It may give you a little increase in accuracy and velocity as well.  I'd also look into a fluted barrel as it will keep weight down and dissipate heat faster.

ques

I do switch, I also have a complete set of Headspace Gauges from Bushmaster (they are fairly expensive). I use the standard initial check of lowering bolt in receiver and with light pressure pushing to closed position. Anyway, but I believe that most parts purchased are Mil-Spec so will interchange with each other. The USMC Tech Manual says it is dangerous for obvious reasons.

I switch uppers, you have to after you run through 3 mags on Full Auto.  Switch uppers, shoot all day long.  There is always a cool barrel/upper laying around somewhere...