I was holding a preban SP1 today like the ones of old. I noticed that first off it was considerably lighter than my AR. The owner said it had a light barrel in it? Well that threw me off (newbie sort of). What is the difference, reliabilty speaking from a heavy barrel, light or bull barrel for that matter?

No reliability difference between light and heavy barrels. Light is nice and keeps with the original concept the AR was founded on. Personally, I wouldn't have an HBAR longer than 16" and prefer the M4 or A1 profile carbines. HBAR barrels are cheaper to produce due to less machine work required, that's why they are so common. To me, their best use is for varmint shooting or high power type matches not carrying or everday shooting.

Quoted: What is the difference, reliabilty speaking from a heavy barrel, light or bull barrel for that matter?

As far as reliability goes I don't think there is a difference. During prolonged periods of shooting a heavy barrel takes more time to heat up but the difference in that regard between an HBar and a standard weight barrel would be negligible. A bull barrel provides a more stable shooting platform. To say that an SP1 has a light barrel is somewhat misleading. It implies that most SP1's came with heavier barrels. The fact is SP1's came with the standard barrel for the M-16/AR-15 rifles that were being made at the time. If any SP1's came with heavy barrels I've ever heard of it before. If I recall correctly the difference in weight between an M-16A1 and an M-16A2 is about 1/2 pound. I wouldn't consider one more reliable than the other.

I have shot the M16A1, M16A2, and several of the current Hbar rifles from Colt and Bushmaster.  Personally, I could'nt tell a whole lot of difference, if any,  in accuracy between them and no difference in reliability.
I miss those little M16A1 rifles dearly.

well...., allegedly !!

the early light barrel had a 1-14 twist, it did not stabilize the bullet, so the twist went to 1-12 still not fast enuff, then comes 1-9 fast enuff but over heats a "pencil" barrel too much, so leave on more steel.....

my explination is a very simplified complex mater that has taken thousands of pages wrytten by hundreds of "X-pertz".........