[xsrdx]

Define "successful".

Longest period of (US) military service - the M16 has been "in uniform" longer than any previously issued rifle.

[yarddog]

I don't know the exact dates of service but I would guess the 1903 Springfield saw longer service. From around 1903 to somewhere around the late 60's. I have heard they were still being used for precision shooting in VietNam, at least, early on.