Welcome! If you're new to the case, there is a TON out there to read...not a ton from his most recent trial, unfortunately...there is a lot of great information linked in the posts in this thread that are well worth the read.
You're right about RJS having a shotgun. RJS's father actually owned two shotguns. One, the one that the defense contends was the murder weapon, he had taken without permission to go shooting with his friends a week before the murder, and it was hidden under his friend's bed the day of the murder, so he did not have access to it. BOTH of those weapons were turned over to police and determined not to be the murder weapon. The buckshot wasn't the same as the ammunition used to kill Belinda either. Tons of info on all of the shotguns they looked at is here starting at page 50:
State's Objections in Kelly Seigler Case
If you keep reading, they actually talk about the kids speeding out of the neighborhood as well, beginning in the middle of page 55.
If you go on to page 64, the noise that the neighbor boys described hearing around 4:30 is discussed-another neighbor also heard it, but described it as a backfire from a car, and he actually saw the car as he was standing outside. This was at the intersection the boy's house sits on the corner of.
Lots of things about RJS seem sketchy at first glance, I agree with you. However, when you have all the facts, like both juries did, it's easy to see that it's all smoke and mirrors.