I wil challenge the players behavior as unjustified. And I do so on the basis of a very simple (implicit) rule that is very much put on a podest in D&D, especially in 5e:

##If You [the GM] don't like it, Change It!

If the GM feels that his ork tribe is equipped with Longbows and has a +6 for that, this is *his* decision and it is not in the right of the players to challenge this decision. 5e makes it a **big** part that the GM has every right to change the world as he seems fit, to invent monsters and change them as he wants.

In this vein the GM did decide to change the Orks to be something he wants: Killersniperorks. True, their Challenge rating was obviously too high for your party, but it was his right to do so. But the Players have no basis for their argument if it only starts with "*In the monster manual...*" because D&D 5e is a game that practically yells at the GM "**Invent your own stuff, we just give you a toybox**".<sup>this lazy *invent everything yourself* attitude is what I hate in some games</sup> And this very thing about 5e takes away the players very ability to challenge GM decisions *on the basis of the monster-manual*: The GM is the final and only arbitrator, especially when it comes to monster statistics.

And he has to be able to do this because there is one big thing that designing monsters in D&D5e demands of the GM: **Playtest it**

##TL;DR

As mean as it sounds: don't challenge the decision in itself, ask the GM to explain how challenging he thought this should be. He might be still playtesting this specific variant of orc and adjust it for further use!