There are several types of A3 stocks, the 2 that I have run into are the HK commercial and HK military versions. They can have convex or concave style butt plates. In the pic above, the military style has a parked finish and generally the end cap is not staked in place. Place a screw driver in the slot and tap counter clockwise. The end cap should unscrew somewhat easily.

Should you have the commercial version, then the end caps are generally staked in place. The snap ring should be removed and the outer collar will slide off the back end. Refer to this website for a diagram of the A3 stock.

http://www.mg-42.net/G3_Rbuttstock.htm

The metal is hard a cobalt or carbide drill bit is necessary to take out the staked areas. If one is not available, a carbide 1/8th in Dremil bit will work. Once removed a set of vice grips is needed to latch onto the end cap. Make sure you only grab the end cap and not the housing beneath it, the end cap is only about 1/4" thick.

Now you have to shorten, the tube in the picture below. It should be .865" as measured below. Usually its about 1/16" above where the cotter pin retains the spring inside the tube. The internal parts do not need to be removed. Once done insert the piston with out the spring, then install the end cap finger tight. Turn the aseembly with the recoil spring pointing upwards. The piston will slide back and the tip of the piston should be slightly below the backing plate like in the picture below.