[@Sophrus] The needle rifle variants are potent in combat, but have the out of combat negatives of both a longlas, and an anti-material rifle. Their design makes them work largely like lasguns, which means they need batteries and a high degree of maintenance. They also need specialized ammunition, that is far from standard issue and would probably be harder to get ahold of then any variant of bolter ammunition. They're also limited to use as anti biological infantry weapons only.

The suggestion is appreciated, but Vespin isn't like Oskar. He's not got the muscle needed to pack around enough variety of weapons choices to deal with multiple enemy types as needed. So, whatever his gun is, anything it can't engage is basically immune to him. While I don't mind him not being able to take out a tank, his gun needs to pack the kind of punch that can take out a Tau crisis suite, or an ork warboss, because as a high-ranking sniper those would be his assigned targets, enemy high-command.

Edit:
[@Klomster] If it's that major an issue, I'll probably end up going with a modified Type XIV, that will incorporate a number of systems from an Ultra Pattern Mk IX. Technically, both are light-based weapons, so putting some money into the hands of the right tech priest, he should be able to get the best of both worlds.