Way off the mark

A very common problem with the Mossberg 715s is that they can't be sighted in.  Very often, the sights don't have enough adjustment room.  Most commonly, they shoot low and left, though mine shot low and right.  The problem is that the receiver isn't located properly in the plastic shell.  The sights are located on the plastic shell, and not attached to the gun it's self.  To fix this, the action must be relocated inside the shell.  I started with milling the receiver where the mounting screws go through.  I milled off .050" from the back of the receiver.  Then I shimmed up the front of the receiver about the same amount with some washers.  For reference, .050" is about 16 sheets of notebook paper thick.  The left/right alignment is adjusted by biasing the receiver in the shell as the bolts are tightened.  The red arrow shows where I removed material, the blue arrow shows where I added shims to get the barrel to point up further.

The second problem is that there is some play in the receiver mounts.  Any time you took the action out of the case, you couldn't be sure it would go back into the exact same spot.  That means you'd have to resight it every time you took it apart to clean.  There is also enough play that it could move around under recoil.  The solution here is the same as on the Mosin, we're going to bed the action with JB Weld.  In order to not have the JB Weld run everywhere, I made a dam with modeling clay.  I also scuffed up the plastic to give the JB Weld something to grab on to.

Then I mixed up the JB Weld, smeared it in, and bolted the receiver back into the shell.  The receiver had a heavy coat of silicone and car wax on it to keep it from sticking to the JB Weld too.

With the clay cleaned out and the receiver removed, you can see how the JB Weld perfectly conforms to the receiver.  It's much more obvious how well it works on this than the Mosin.  Now the receiver will mount in exactly the same place each time it's removed, and it won't move around as the gun is being fired.

With the receiver adjusted and bedded in the shell, the sights could now be aligned, and the gun can be taken apart without messing up the aim.