How to clear a serious AR-15 jam?

I was shooting my AR-15 at the range the other day and after shooting about 25 rounds my bolt locked up. The rifle failed to cycle another round and the bolt is stuck back about 1/4 inch from being completely forward. The bolt is completely jammed, I can't move it at all. The shell is expended so I know that... show more I was shooting my AR-15 at the range the other day and after shooting about 25 rounds my bolt locked up. The rifle failed to cycle another round and the bolt is stuck back about 1/4 inch from being completely forward. The bolt is completely jammed, I can't move it at all. The shell is expended so I know that there isn't a live round in the chamber. I can not unjam the bolt. I've taken the lower receiver off and I've tried yanking back on the charging handle and even sticking a flathead screwdriver through the ejection port and pulling back on the front of the bolt at the same time, I can't get it to budge.
Does anyone have any ideas on how to get the bolt unjammed?

I always keep my rifle clean and oiled and use decent ammo. Does anyone have any ideas on how to prevent this problem in the future once I get it unjammed?
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