A .250" nipple would be one quarter inch. They should be the same.
You mentioned trying the nipple in a 1/4 x 28 nut. Most of the quarter-inch hex nuts you get at the hardware store are threaded 1/4 x 20, so a .250 x 28 would not go more than maybe one turn, if that. Not sure where you might find a 1/4 x 28 nut, although they may exist. Most musket nipples, as far as I know, are 5/16" or 8mm in diameter. Thread pitch varies.
It's a good idea to invest in a micrometer or precision caliper, as well as a little screw-pitch gauge, or at least a Screw Checker from Brownell's. These Screw Checkers don't cost much, and are nothing more than a pocket-sized hardened steel plate full of threaded holes, with sizes and pitch clearly marked. You try your screw or nipple in the various holes until you find the one it fits.
If you can tell us the make and model of the gun that is involved, the Muzzleloading Brain Trust can probably tell you what size nipple you need. Foreign-made guns are fitted with a bewildering variety of metric and standard nipple threads. One-quarter by 28 is probably most common for American-made long guns, although there are a few exceptions, such as the T/C Seneca.
Good luck! Let us know how it all turns out!
Notchy Bob
