A "Belt Mountain Base Pin" is a slightly oversized base pin made by Belt Mountain. What is does is tighten up the cylinder play so that you have a tighter lock-up when firing the pistol, which MAY increase accuracy (it did with mine). Some folks have also had problems with Ruger (and maybe other) SA revolvers when the base pin started backing out under heavy recoil. BM base pins are available with a locking screw that eliminates that possibility. I bought mine with the locking pin, but I don't use it since the pin, with it's tighter tolerances, locks in plenty tight enough on it's own.
You can buy them for less than $30.00 (I paid about $22.00 for mine).
~Rat