Ok, you mention you know about the bipod effect, but did you try removing it. The Handis can be suceptible to forearm pressure or more accurately, varying forearm pressure. First off, if it's putting them into one hole at 25, it's not the fifle thats putting them into 4-5 inches at 50. Something changed. The scope screwed up, the forearm pressure changed, something.
Check the scope mounts and bases, sounds loose. Something shifted. I had a Bushnell scope on my Win 94 Trapper 44 mag and it was shooting clover leafs at 50yds. next trip out was 4" groups at 50yds?
??
Broken crosshair that I couldn't figure out till last night when I took the scope apart.
Trigger consistance on the handi's is another issue. Make sure you are pulling completely to the stop, because the transfer bar can make for incosnsistant primer strikes.
Finally, DO try other ammo.