Q:
rifle build?
Will it work in PvE? Kind of if you want to farm Orr. If you want to progress through FotM? No.
Your choice of spec will output minimal damage without bombs/grenades or power. Rifles are better suited to power builds anyway.
As it stands at the moment, Engineer main weapons are not even close to optimal for damage as most of their abilities are utility oriented, if you want to be useful in dungeons you need a damage kit.
If you are hell bent on using rifles the majority of the time, go for a power based rifle elixir build, still far from optimal but should perform ok.
As for WvW, well if you are just running around with the zerg grenades are basically a necessity if only to tag people.
If you are roaming or doing small scale pvp then you will need a stun breaker of some sort.