I just put a G3 heavy buffer in an original HK G3 stock.
I also have the HK-21 soft rubber pad on it.
The combination works well for me.
The kick is around 1/3 of what it was.
There is an odd push to shooting the rifle with the
2 stage buffer.
While it soaks up much of the recoil ,it also slows
what is left of the kick down.
What you get is more of a push than the kick of
the original.
The push is slower ,and makes it easier to keep your
sight alignment between shots .
At 100 yards ,I can keep my iron sights alligned and on
the 16 inch targets between shots.
Making follow ups on the same target or on a new target
faster.
There were quite a few HK-91 stocks imported with Heavy buffers
already in them.
Be aware that if you install one on a standard stock ,you will have
to cut up and remove the plastic piece inside the buttstock,as well
as bore out the steel area where the buffer enters.
A heavy buffer is 23 mm wide ,the standard is 20 mm.
This is the piece that you run a bolt through and into the
buffer.
I'm going to modify the inside of my stock with resin so I
can still use the back bolt.
It would be done if I wouldn't have had an accedent with some
chemicals and discolored my stock !