SAW Ultimax 100 is a very well-rated weapon.
For your info, it was almost adopted by US Army and Navy.
But when it was presented to them for evaluation, it was still
under improvements (not sure if it was the Mk II to Mk III
stage).
So US chose M249 (FN Minimi) instead to replace their M60.
Why SAW do not work well with blanks are due to serveral
factors,
1. The 30-rds m16 mag was a "improvisation" and do not fit well
into the rifle. the loose attachment makes feeding very poor.
2. Feeding is poor because blanks are wedged into the chamber and
cut by the bolt, thus creating a "double feeding"-like IA.
3. Very often blank attachment is not screwed to maximum tightness,
this lead to gas escaping and thus not creating enough blowback for
the rifle's action cycle to complete.
4. Poor maintenance by operators, partly due to lack of exposure to
the weapon by their superiors themselves, leading to many people
unfamiliar with the weapon.
During live-firing, SAW fires reliably with most IA caused by
magazine attachment issues.
