If headspace is off it can be DYI fixed with a few exceptions.
It depends on what is actually wrong as to how easy that may be.
Bolts vary a couple of thousandths and swapping bolts will usually fix a tolerance stacking issue.
Obviously past a certain point it takes someone who knows what they are doing with access to the necessary equipment.
In a lot of cases it is probably better to return the defective barrel assembly.
Of course people who don’t check are limiting their options.
Ranges have no liability for someone damaging their weapon or injuring themselves.

Actually - I spent a few minutes looking it up - and didn't find much.  I don't have any experience with the M249 SAW chamber, I just have always noticed how LOoOOOOOOooong the headspace is compared the M16 chamber spec.  If there is something specific around the design of their chamber that explains long headspace then please share it.  I've just always assumed they are loose as a goose for reliability above all concerns.

As a reloader, we get TONS of SAW-fired brass.  You can always tell the difference when resizing, there is a bit more resistance resizing SAW fired brass cases back to minimum (full length) spec, as opposed to M16 fired brass.



As to headspacing, I actually have the full match set.  I don't just check min or max, I can tell exactly where a chamber is in between:

http://www.midwayusa.com/product/549020/forster-complete-match-headspace-gage-set-223-remington