1/4-28 is one of those overlapping threads. Really the only significant difference between a BSF 1/4-28 and a UNF 1/4-28 is a 55 degree vs. a 60 thread angle. At the pitch diameter that difference is less than .001, well within the tolerance zones of either thread.
If the UNF male thread was made to it's maximum tolerance diameters, it might be a bit firm going in. I have found however that they are usually made somewhere in the middle to lower end of the tolerance range, so it shouldn't be too much of a problem.
