How about a "standard" M4-type AR-pattern carbine CARRIED INSIDE A GUITAR CASE?
It will fit, won't it?
You can get used guitar cases from pawn shops dirt cheap.
With a little effort, you could make a series of wood or styrofoam-block supports in there the hold the gun steady in position and prevent it from rattling around, yet still allow you to pull it out, coock it, and have it ready to go.
You might have to keep a 10 or 20 round mag in it, to let it fit in the case, but a 30-round mag could be next to it, laying flat.
It will fit, won't it?
You can get used guitar cases from pawn shops dirt cheap.
With a little effort, you could make a series of wood or styrofoam-block supports in there the hold the gun steady in position and prevent it from rattling around, yet still allow you to pull it out, coock it, and have it ready to go.
You might have to keep a 10 or 20 round mag in it, to let it fit in the case, but a 30-round mag could be next to it, laying flat.