Anyone had this problem...?

[mosigdude]

I just finished my first 300 Fireball build on an AR platform and started on load development. I topped the flat-top upper with rings of the appropriate height and set the scope. I tested subsonic loads with no difficulty or problems. When I switched to testing my supersonic work-ups the problems occurred...
Shots #1,2,3... no problems, decent accuracy, consistent velocity and no signs of excesive pressure. Recoil was a bit stiffer than I am used to out of .223, but I'm also pushing a slug about 3x the weight so I didn't think much about it. Either way, it was nothing that alarmed me and was not what I would consider excessive. Shot 4... I noticed that the chrono errored and I didn't see the shot on the target. Upon further inspection, I found that the chrono was shot (thus the reason for the error) and the bullet appeared to have struck the chrono at a very steep downward angle. I had been shooting off bags with plenty of clearance on the chrono, as I have done for years and years with other load work-ups with zero problems. I took the rifle apart, checked for damage and found nothing alarming. I reassembeled the rifle, moved the chrono and fired three additional shots at the same target. All three shots made a very nice group at the base of my target backer (approximately 24" lower than the original first 3 shots and point of aim).
Finally, unable to determine the cause, I removed the scope and found that the "guts" of the scope were loose and rattling around inside. This scope was not overly expensive and I had just planned to use it as a temp scope for load development so I was not, at first, concenrned. I replaced the scope with another nearly identical, continued load testing a few days later and had the same result again after 14 shots.
I know the actual recoil of the 300 Whisper is minimal and even inexpensive optics would have no difficulty with this. My question is this, does the AR platform generate a different recoil signature that is damaging to optics by the buffer striking the rear of the buffer tube or are these scopes just junk. I purposely didn't mention the brand of scope because I don't want to get into a ford/chevy brand war, but I am concerned that when I put my better scope on that this will occur again. I will say I've used this brand of optic in the past on several different rifles, from rimfire to 300 Win Mag with very good results.
Any input is appreciated.