Shooting a slufoot HR4570 conversion the first time with FullStream and 280longshot this weekend.

I had Bore sighted the scope prior to coming out to shoot and it was pretty dang close on the first shot at 50 yards made several follow up shots to get it dialed in at 50. Moved out to a hundred yards expecting similiar results.

At 100 I was not even on paper. After a few shots to verify what was going on I assumed Id blown up my ZA-3 Minox scope. We removed the breech plug to check the bores alignment with the cross hairs (poor mans Bore siter).
What we found was the breech plug was extremely loose. Probably my fault for not snugging it down enough. But the bore and the crosshairs of the scope were dead on.

Thinking that the loose breech plug might be the problem but not knowing how that could have affected accuracy We cleaned the plug, snugged it in the barrel with a socket and then shot at 25 yards. 1/2 low of bullseye??? Im back in business so I think. I shoot one last shot at 100 yards and vertically I am good but about 6 inches low. That was my last shot for the day.

Questions:
1. Could the loose breech plug have caused the accuracy issues?
2. Was 62 grains of n120 to much and did that cause the issue at longer distances as it was fairly accurate out to 50. I might add that FullStream was shooting 58 grains of the n120 out of his 45/70 conversion and getting great groups at 200 yards. Can 4 grains make such a huge difference?

I waited for a cool barrell between all shots
62 grains, barnes 195, light blue harvester sabots
Chroned speed 2830
None of the recovered sabots appreared to be blown

Thoughts from the experts???
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Vegiterian- An old Indian word for Bad Hunter.