Other parts come from a company called Nine Ball and these parts are available for all AEP and AESMG's as many of the parts will fit the Glock 18c M93R and the MP7A1.
First off is the high precision barrel with an inner diameter of 6.03mm

Also available is a full metal spring guide to replace the stock plastic one, I found that by adding the washer from the original spring guide will increase the FPS by by a small fraction.
There is also the most important part, the spring. This has no rating of power marked on it but you can expect on average some good results from this spring allowing you to actually use the MP7A1 in a woodland enviroment.

Lastly you have the Piston head and bearings these are of excellent quality and just by replacing the bearings you will notice a big difference in the sound and feel of the weapon.