are trying to short TLT such that your broker can't find any shares to borrow from longs. In situations like that, even if you could short it, short interest charges are likely high. Just like what happened to USO late last year. I'm going to 班門弄斧 here. If you are confident, you can sell ITM calls, either long dated or roll short dated each month. This way, you can take in some extra time value of the premium.