These PTR PDW (MC51 clone) usually have two very common problems:
1) bolt hard to open when using the lever
2) did not completely cycle when firing (due to problem at locking piece and barrel very short).
G3 with short barrels (under 12") need special locking pieces and some other works to cycle reliably, because the G3 was never designed for short barrels and with the usual setup need all the energy can have to cycle.
With short barrels these guns start to have problems and this was discovered by the SAS when they started testing the MC51 ordered in the USA.
