For pen turning, I use BLO as a way to add depth and contrast to the grain, and enhance any chatoyance present in the wood. I don't use it as a complete finish. I just wipe on a generous coat with a shop towel, allow to soak in for 10 minutes or so, then wipe off the excess and burnish at high speed for a minute or so to dry it and fast-track some curing. Then apply a CA finish as usual over the top of it.
I have used it as a finish for gun stocks. Application is largely the same - wipe on liberally, allow to sit ~10 minutes, wipe off thoroughly. But I end up doing numerous coats over the course of, usually, weeks, with lots of time in the warm garage to cure between coats.
TBH I haven't really put a BLO-only finish through any kind of testing. If it's raining outside, I probably don't really want to be out there shooting my Mosin Nagant anyway. I suspect it would stand up to a rainstorm if it was carefully dried off afterwards, but for a pen, I think BLO only would be pretty poor as a "protection" finish for the wood. Certainly it would do almost nothing to protect the wood from abrasions or dings.