Pipe guns are only useful if you get ammo drops that match the pipe guns you make.
No offense, but you have that backwards. You don't make the guns and hope they match the ammo. You pick up ammo that doesn't match your guns, then you make a pipe gun to use the odd ammo.
One pipe gun strategy, in my relatively untested opinion, is that you would use them primarily if you have some other firearm, like maybe a pistol. So you would use your pipe gun(s) for the first shot(s) then switch to your primary firearm.
For example, you have a 9mm pistol with 30 rounds and you pick up 30 rounds of .45 caliber ammo. You then manufacture a couple pipe guns that use .45 ammo. Then you initially shoot one zombie with your first pipe gun, then drop it and either fire at the same zombie or a different one with your second pipe gun, then switch to your 9mm pistol.
Alternatively, remember I discussed earlier using fire to kill zombies? You set 4 or 5 fires to cover all spaced between you and the zombies, then fire your pipe gun to get the zombies to charge into the flames, and maybe kill a couple of them. Then switch to melee as the surrounding zombies burn up. The pipe gun itself would probably be a half-way decent melee weapon, actually. Better than a pistol, at any rate.