The Pilot BPS-GP Extra Broad pen uses a 1.6 mm ball which makes it a very smooth writer. But I'd like to use the refills in other pens too and now I've found one they fit in.
For smooth writing and as an alternative to gel refills this is recommended, although it doesn't have more than (estimated) 1/3:rd longer write length than a typical G-2 and similar refill and isn't much cheaper, so it's not mainly for economic reasons I do this.
The tip is too wide to fit in those pens which take Parker style refills, so those are out.
In rollerball pens without a retraction mechanism the chance is good it might fit and I tried it in my Rotring Newton, which don't use much otherwise nowadays. The BPS-GP <XB> refill is a bit too long and the tip protrudes a bit more than looks really good, but otherwise the fit is good.
In the Core the fit is better.
That the tip protrudes circa 1 mm more than I'd like does not make it worse writing with and there's a enough metal inside the pen to make it stable and solid, so that's not a problem.
You have to cut down the refill a bit, but only about 2-3 mm into the ink part, so that's not a serious waste. Although the spring plate in the barrel doesn't engage narrow refills like this, exact length doesn't seem to be critital.
Relatively easy to fit.
What's needed is to wrap the refill in tape to approximately the same diameter as a gel refill and about as far from the tip to make it engage the spring. Since the tape may slip, it's even better use the tube from a G-2 refill and glue or tape the ballpoint refill into it
It needs to be shortened a bit too, but to the same length as a G-2 refill as there's a cutout in the retraction mechanism, so no ink needs to be wasted.
This may not be the best application for the refills, as they sometimes don't seem to like being carried tip down, as the ink may gather on the ball.
It'll fit, but as the retraction mechanism isn't the same as the G-2ex's, so you'll have to cut down the refill more, so it's not a good alternative in that regard.