I'm new to rimfire (mark 4 lite) and have tried a couple different types of ammo. I thought thunderbolts would be good, they were cheap and shot pretty accurate for plinking. After my first 250 rounds barrel was hard to clean but I managed. After my next trip out it was awful, barrel was heavily leaded and squib round stuck in the middle. it is currently at the gun shop, hopefully they can fix it. So apparently cheaper is not always better (imagine that) Whats the best ammo? Do I need to shoot copper jacketed bullets?