Hello Winn R,
"Machining an oblong slot on the inside of a receiver"... hmmmm !!
If the receiver is a round tube then that is a real challenge. It would call for broaching or chain broaching or shaper work. The op-rod grooves in a BAR receiver are like that though the receiver is not a round tube it is enclosed.
If the receiver is open to the outside then a mill with a key seat cutter will often do the job. Also again a shaper can reach in there and scrape the groove.
If the receiver area to be grooved is directly opposite the open side then an extended endmill cutter could reach in and groove it. Look through some tooling catalogs for the cutter best suited for your needs.
If still stumped on this cut post some pictures and we can all voice our solutions.