Canoe gun.

[Coinneach]

Consider Pecatonica River for an affordable kit.

 

[skwerleater]

Maybe consider a Pedersoli trade gun and cut it down to your desired length. There is one for sale on here now. My friend bought one last year and it’s a pretty nice piece.

 

[bpd303]

I have never "built" a gun either from scratch or kit, however I have assembled a few from scrounged, bartered, begged and various avenues for acquiring the parts I wanted.
Here is the Buggy rifle (canoe gun), I assembled years ago. It is a sweet shooting rifle. <AMR> Stock, back action lock, 15 1/2 in Green Mountain barrel, forged steel ram rod & sling swivels. I have a total of $200 in it.