Note 1
If your gun manufacturer is not listed here, remove the nipple from your weapon and take it to your local gunsmith to determine your nipple thread size, or you can take the nipple down to your local hardware store (Truevalue, Lowes, Home Depot) and they usually have on display sample nuts and bolts in both metric and standard sizes you can use to help determine your nipple thread size.
Note 2
The H&R Huntsman nipples fits only old-style H&R Huntsman 45 & 58 caliber rifles.