Thanks to @frankgoss: 
HK770 .308 44 years new
Issue: Appears to jam when cocking lever is unfolded and pulled towards butt! Remove magazine and bolt gap 8mm?
Solution: Disassembly and grease bolt sides by rail groves and one drop gun oil on rollers
What happens when you start unfolding cocking lever, as you open it, it starts to unlock the locking lever lug on the head of bolt and makes a"click " sound when a support lever goes back above the cocking lever. That is normal.Then cocking gets harder.Keep applying palm up pressure to cocking lever and pull hard towards the butt and it should start to rack. Lock on catch and always let it slam home.
Why so hard to cock lever on catch?
Many new springs and levers. Locking lever small spring 44#, next is support lever and spring, next is firing pin spring inside locking or bolt connecting piece, next is hammer and hammer spring, while all these levers and springs are compressing in different directions , mostly down, the stout recoil spring compresses and wants to go to battery.
The mistake I was making was it appeared the 770 was jamming after that "click". What appears to happen is the locking lever lug did unlock, but only moved the locking piece back 5-8mm towards the butt. I disassembled , used padded flat head screw drive and twisted carefully the head from barrel extension. It moved, because the lug did not completely move up on the head all the way. It was not jammed, but those other levers and springs all needed force to move them.
For now when I here that " click" keep palm pushing up and pulling towards butt.
Just too new and has to be oiled and greased. "click" sound is support lever returning above cocking lever when locking lever lug is moved off head and appears jammed. Take magazine out and see bolt gap now 5-7mm .Two step see ROCK ISLAND FORGOTTEN WEAPONS HK SL6&7. Just keep applying palms up and pulling back and it should brake free and rack.
HK770 .308 44 years new
Issue: Appears to jam when cocking lever is unfolded and pulled towards butt! Remove magazine and bolt gap 8mm?
Solution: Disassembly and grease bolt sides by rail groves and one drop gun oil on rollers
What happens when you start unfolding cocking lever, as you open it, it starts to unlock the locking lever lug on the head of bolt and makes a"click " sound when a support lever goes back above the cocking lever. That is normal.Then cocking gets harder.Keep applying palm up pressure to cocking lever and pull hard towards the butt and it should start to rack. Lock on catch and always let it slam home.
Why so hard to cock lever on catch?
Many new springs and levers. Locking lever small spring 44#, next is support lever and spring, next is firing pin spring inside locking or bolt connecting piece, next is hammer and hammer spring, while all these levers and springs are compressing in different directions , mostly down, the stout recoil spring compresses and wants to go to battery.
The mistake I was making was it appeared the 770 was jamming after that "click". What appears to happen is the locking lever lug did unlock, but only moved the locking piece back 5-8mm towards the butt. I disassembled , used padded flat head screw drive and twisted carefully the head from barrel extension. It moved, because the lug did not completely move up on the head all the way. It was not jammed, but those other levers and springs all needed force to move them.
For now when I here that " click" keep palm pushing up and pulling towards butt.
Just too new and has to be oiled and greased. "click" sound is support lever returning above cocking lever when locking lever lug is moved off head and appears jammed. Take magazine out and see bolt gap now 5-7mm .Two step see ROCK ISLAND FORGOTTEN WEAPONS HK SL6&7. Just keep applying palms up and pulling back and it should brake free and rack.