Installing and Removing Buttstocks on HK91-pattern rifles

This is the exact same procedure using the exact same parts with the HK 91, the JLD PTR-91, and the CETME. I understand that the MP5 and its clones (SW-5, BW-5, and CA-94, among others) use the same design but smaller parts.

Make SURE before you start this that the rifle is NOT locked back. If you've got the charging handle locked back, it's going to put plenty of pressure on your recoil spring, which in turn will make it tough to remove the stock pins. Moreover, when you do get them off, the buttstock will rocket off of your rifle with the vengeance of an angry spring. Do not make the spring angry. Obviously, also observe all safety precautions, unload the weapon first, etc.

First, figure out which side of the pins you would press on to remove them. This side is NOT the correct side - notice, the two pins in the buttstock are showing holes.
This is the correct side of the pins to push on. Notice that it doesn't really matter which side you push the pins in from.
If your rifle is absolutely brand new and tight, you may have to gently hammer on the pins a bit to push them through. On any rifle that's seen a bit of use, this will not be an issue. Push with your finger as shown until they are flat with the stock.
Yes, you do so with both of them.
Now flip the rifle over and pull the pins out. If the pins are too tough to get out, then use a punch or dowel from the other side and a hammer to gently tap them out.
Both pins are out. As you can see, both pins are an equal length so it doesn't matter which one went in which hole.
Now you can pull the buttstock straight off. If you're using the collapsible buttstock, you can do this with it collapsed or extended; it'll come off either way.
The grip frame assembly can be removed now if you wish, to facilitate cleaning. You'll notice that the stock I'm reattaching isn't the stock I removed - I decided to use the fixed buttstock during reassembly, to show folks that changing stocks on these rifles isn't rocket science.
Line up the recoil spring and slide the buttstock on straight. It shouldn't have a lot of resistance, and should come to rest readily against the grip frame. Note that the rear of the grip frame goes over the receiver, and then underneath the front of the buttstock. It should line up perfectly with the front stock pin.
Now, just line up the stock pins and push them down.
Once you've pushed both pins in as far as they should go, it should look about like this.

-- SeanNewton - 17 Oct 2008