Are you talking about a square cornered hole? If so, it can only be done by broaching or on a shaper.

I have done 2 breech block mortices now.
I roughed them out by drilling, then extended end mills, then finished by making a filing jig with HSS pins to keep the file straight and limit how deep it would cut.
There is a book out there (can't think of the name ATM)on making your own falling block action c/w plans for a kind of shaper adapter for the quill of a B/P mill. I made one but found it difficult to use, then made my filing jig. Rather tedious but it can be done.

You can also do it in a Bridgeport type mill. Bridgeport made a "slotting" head that went on the back of the turret. You see them on Ebay, every now and then. There are also dedicated Slotters. I used to have one, back in the mid 70's. It had a round, indexing, 12" table, about the right size for gun work.

But, you can also do one of, type stuff, using the quill on the BP. 

Grind a square tool, long enough to clear the bottom of the part and then, feed it in .002-.003 at a time. Do a rough then a finish cut.

The square tool could be made from a square HSS tool bit or a lathe parting tool. You'll have to find a good way to hold it in the quill. I would recommend a end mill holder so, it can bottom out (not hurting anything) and won't push up in the collect.

Frank DeHaas

Gerry,
I have a couple of different slotters in my shop that I've used over the years and a Bridgeport brand was another that I had but was happy when it went away...  I retro'd a K&T attachment to my #2 cincinatti and it works about as well a good Pratt and Whitney. 
A couple of things, is your Bridgeport a variable or a J-head?  The variable you have to put it in back gear, run it all the way down before you shut it off and a tool will still turn on you - some.  A j-head, leave it in high gear and throw your side lever forward and it will lock the spindle.   
I use either a 3/8" or 1/2" piece of cold rolled round, and silver solder a 1/4 or 5/16" HSS tool bit to the round and grind it to my desired shape, generally about a 60° or more included point and use that for cutting my mortice.  Be careful not to get things too hot.  Recently, switched from using HSS tool bits to using Tantung as it is a little more durable but it's more work and sometimes hard too find, even on eBay.
I use my BP spindle for squaring almost all my castings, but when its an action from a piece of stock, the slotters are a better choice.
As far as a clapper box or some such animal, I don't worry about it.  Run a neutral or negative rake on the face of the tool, plan on roughing things out and I've taken as much as .025" in that pass, but follow with a re-sharpened tool and scratching with a .005 or less finish pass.  The '74 Sharps is about the longest mortice to cut, plus the dual extractor cuts and I've had them turn out straight and square.
There's a photo of a tool using tantung and one of several Hepburns I did.  The tool shank diameter is dictated by what size mortice you're working on.  The hepburn doesn't give you a lot of room to manuever in.
Here's a most recent project, squaring the mortice on a Zettler, there's three of them now with this part done...  A little patience and finesse, which you have, they'll turn out fine.
Greg

Last time I did a square hole I used a keyway broach. You would have to make up a guide to make it work.

Looks like it turned out well!