First of all finding, 2" with o rings will be next to impossible. 2. What expansion, your worried about nothing. 2' in the ground, worry aout a truck driving over it? All this is for not, if you like the lighter stuff fine, use it and get greater flow because of the larger interior diameter. It will take the PSI. The hammer you speak of results in a quick acting valve, use gates instead of ball valves. Expansion, unless you have exceptionally variable water temps, don't worry. If you are concerned, don't trench perfectly straight and add some jogs. Sch 40 is nice because it will resist physical abuse both while in transit to you, while you stick it in the ground and from some rocks. Water/air hammer can disinigrate even SCH 80 pipe. When you glue it (forget about the o ring idea) prime it and use grey glue, its is the heavy bodied stuff that won't dry as fast but is the best for any voids. It is the only glue that you can use on 8" and up pipe. The primer etches the top few hundreths of an inch of PVC making it soft and getting the best cohesion/fusion between the mating surfaces. Now, go trench, glue as much of the pipe above ground, let it dry, throw it in the trench, cover it up.
P.S. Put a tee in every now and then, its easier now then later, Rat...
