The "afsluiter" is the bolt mechanism (also known as "grendel" of course), so the "afsluiterdraagstuk" would be the "bolt carrier".

Quite a few refs on Google for "MP5" and "bolt carrier":
http://www.google.com/search?hl=en&q=MP5 "bolt carrier"

"The primary version of the MP5 submachine gun family is the MP5A2, which is a selective-fire roller-delayed blowback operated weapon. The bolt mechanism consists of a bolt head and bolt carrier. After firing, the rollers integrated into the bolt head are cammed inward against the inclined planes of the barrel extension and locking piece by rearward pressure on the bolt head. As the rollers are forced inward they displace the locking piece and propel the bolt carrier to the rear. The bolt carrier's rearward velocity is four times that of the bolt head since the bolt remains locked for a short period of time after the initial recoil impulse. After the bolt carrier has traveled rearward 4 mm, the rollers on the bolt head (which has moved only 1 mm) are compressed in, unlocking the bolt, opening the breech, and the two parts continue their backward movement together."
http://www.avguns.com/mp5.html

This is an exploded drawing of an assault rifle rather than a submachine gun, but it might be useful to you:
http://world.guns.ru/assault/ar15_blow.jpg