Somewhere in my crates of photos, I have some of me with my long sold HK51S. It was, (and still is in someone else's collection), an awesome weapon. I needed the money to fund an extended stay in Africa and the deal was too good to pass up, so off it went. I remember spending $500.00 on the HK91 and had the conversion done for around the same dough. God I miss the 80's...
Flemming converted mine to a 12.5" with the wide MP5 front end. I understand 8" models were available, but I can't imagine it really. The 51S was a handfull - especially in full auto, but it was quite a hoot to shoot. 300-yard shots in SA were no trouble at all - center mass all day. Things got much more loose in FA, but I do remember making hits from a benchrest out to 500 on a full sized silhouette, using the then new fangled Aimpoint, (I still have that around here somewhere). I had a collapsible and a fixed stock and found the fixed did not have the "cool factor" but was much more realistic for accurate fire, (at least for me).
Standing next to this gun would feel like the air was being sucked from your body and the muzzle flash completely overwhelmed the flash suppresor. Even in daylight, muzzle flash was stunning - at dusk or at night it was surreal. I had a friend who had his hand slide off the forend during a mag dump - how he did not shoot his hand was beyond me, but his hand went forward of the muzzle, missing his digits, but tearing up his hand with burns an concussion - he wasn't right for weeks...
I can't fathom shooting this gun from inside a vehicle....
I can't see it being a practical combat weapon, but hey - what do I know?