SpudFiles

If you were to sleeve your combustion chamber would it increase the strength of the over all gun, without taking into consideration fittings ect...

no it dosn't increase pressure because there is a gap in the pipe and the only pressure effective pipe is the outside.

what kind of pipe are you using because almost all except cellular core is fine for combustion

plus 4 inch sleaving would be hard as hell

The pipe that I am using is normal schedual 40 PVC and what if you were to solvent weld it to the other pipe, assuming it could be put in fast enough in before it hardens.

If you can find two lengths of pressure rated pipe, and one of them can actually slide into the other without any cutting, then ya, to solvent weld those two together would make a pipe that can hold higher pressure. Just be careful, though, because I don't imagine that the sleeved pipe would be able to hold double the pressure of the single pieces of pipe. What is this for?

"The pipe that I am using is normal schedual 40 PVC and what if you were to solvent weld it to the other pipe, assuming it could be put in fast enough in before it hardens."

The glue dosn't harden it melts the plastic together. you could easly get it in assuming that it wasn't super tight fitting

if your pipe is sch 40 and pressure rated there no need for the sleaving. combustion only creats ~50 PSi depending on the propelent.
4" PVC is rated to 220 PSi with a huge safty margin. you will be just fine