"What's the correct step of modeling? Low poly then high poly, or high poly then low poly?"
That's kind of a tricky question as a lot of modellers will take different steps when modelling. Here's the typical way:
The first step generally is to create a midpoly. Create your mesh with correct topology across the board keeping quads as much as you can. Make sure to check your loops using edge selection + loop to make sure that there are no triangles in the way of them.
Generally, after the midpoly you make two copies of the model. One to be a high poly and one to be the low poly. Add your supporting loops and use OpenSubdiv or regular subdiv. Of course, I highly recommend opensubdiv.
Take your mid poly object and collapse all unnecessary edges that you come across. If you find areas that need more loops for the sake of quality, add them. Remember that modern gpus are easily capable of handling high poly counts.
What not to do
Don't go from a low poly to a high poly since you will have to deal with excessive numbers of triangles that come from the optimization that you put into the model and will make subdiv take far longer than necessary.
Going from a high poly to a low poly isn't a terrible idea but you have to remove far more loops as you've added many supporting loops for the suvdivision. Going from a midpoly is the best of both worlds as it's easy to collapse the excess edges for the low poly but there's no supporting loops for you have to deal with.
Ah the XM8, set as a competitor for a NATO convertible weapons trial to potentially replace the M16/AR15 platform back in.... whenever. Worked fairly good but was too complex to produce or something similar to how the XM29 was perfect in everything except complexity in manufacturing and cost effectiveness. I'm sure I'll have to look that up again, might be mixing up facts with something else. Long story short it was never adopted, but several hundred units were built, consumers made up by private contractors and security agencies. From my understanding it is no longer in production.
Its definitely a rifle that has to grow on you, I'm personally not in favor of those sleek curves, I prefer rigid bodies to my rifles to make them look less like a toy, but I like the convertibility and models with integrated optics; Would prefer a straight mag to a curved one...
Heckler and Koch are probably the leading in odd-looking polymer-and-steel weapons