Yep, you need to set the boring bar cutting edge so the bar cutting edge is in line with the postive movement of the head adjustment that is going to be advanced out wards, and with a slight positive rake to the line.

The lines top to bottom line is the line movement of the boring head movement to it spindle shaft, the cross line is the line of the spindle of the boring head that you can see the head has been advanced past this spindle center line, the V arrow is the direction that the head will be advanced outward to cut a larger radius bore each step, with  the boring bar cutting insert set with a slight positive rake to the  heads center line advance  of the boring head movement each step outwards( the leading edge of the insert is always the leading edge on the heads cutting radius circle movement).




With the major negative rake you have the boring bar inset set up now to the movement line of the head leading edge,  the boring bar tip cutting edges has not support, and your going to just crack the cutting leading edge off the carbide tip instead.
Note, think cutting edge of the insert and direction of cut pressure, and the amount of material the insert edge has behind in when it touches the stock and starts to cut it.



To add, the way you have the bar stock set so high above the vise, you going to make a mess of it, Even if you can get the bolt channels EDM cut afterwards,


The through bore in the receiver is best done in a lathe,  even if you are trying to save edm wire from not having to cut the center bore channel as well.  

Hence true the round stock up in a spider, drill the through channel a few thousands smaller than needed, the run a  finish reamer  for a perfect through channel instead.  Simply with the reach of the boring bar, and the smaller  deep channel it would need to bore, even with a bunch of final passes to bring the channel up to spec with a single point cutter, the boring bar is going to have too much flex with it trying to reach that far out from it support.  

Now you can either broach or EDM cut the bolt lug channels.  
From there, go to town on boring and threading the barrel channel/setting the initial locking lug face surfaces in the lathe, then you can finish the rest of the receiver on the mill.

I recommend not using "PH" in conjunction with 4140 or other carbon steels as it has a specific meaning in metallurgy.  

PH means precipitation hardening.  Not "pre hardened".

That's a thousand tenths, and even the machinists I've been around don't talk that way!

What I really like are advertisements for "1000 millionths accuracy", or some such nonsense.

On the heat treating, I'm referring to writing the condition as 4140 PH, instead of 4140 HT (HT to what?!).  PH13-8M0 is a proper use of "PH", and it's even part of the alloys name in this case.

Something interesting I ran across this spring was whining about heat treat of carbon steel splines and gears with the usual process, and a guarantee of mechanical properties over a 20 ksi span.  I don't know if the guy ever got it, maybe they plan to reject every part that didn't harden to the upper end of the range for the heat treat condition specified.

Your working it backwards.

Start with the barrel in a lathe since IIRQ, your going to be using one already threaded, and see what needs to be cleaned up on the barrels threads and shoulder to get them concentric to the bore/chamber line on the barrel.  Once you have that cleaned up, then you know what to threaded the receiver  and set the threaded depth to get you close for the  barrel isntall .


Also, until you get the bolt channel completed, you have nothing to square the receiver  to, to start truing the barrel channel and receiver lugs to center line of the bolt channel.

As for running a threading tap on anything, short of say the scope mount threaded channel, the barrel channel needs to single point lathe cut to keep the threaded channel square with the bolt channel.

Hence on a production action to clean it up, you are first squaring the receiver up against the bolt channel on the lathe, then you are squaring the threads (ending up with them over sized), facing the end of the receiver to the center line of the bolt channel, and them squaring the face of the receiver locking lugs up to the bolt channel.  This now sets what the new barrel will need to be threaded (to match the new threads on the receiver), and even if the bolt handle will need to be removed and re-brazed back on (if the receiver bolt lugs had to be faced way back to get them squared to the bolt channel center line, and now the handle no longer fits correctly to the bolt handle slot in the receiver).

As for a receiver from scratch, you really never want to start with your round stock at final OD.  Hence when the bolt channel is created, it not always going to be concentric to the OD of the stock piece, and with it being over sized in OD, this allow you to center the piece on the bolt channel in the lathe, then turn the OD down to the final dimensions  so the OD of the receiver is square to the bolt channel.  Again, on a production 700 receiver, 99.9% the bolt channel is not square to the OD of the receiver, so that is why you never, never trying center the piece in a lathe off the OD of the receiver (center line of bolt channel always driving the bus).

I think my point about "tenths" blew right past; I know how it's used in the shop.

My point about "HT" is that there are many choices, I can specify heat treats of 4140 from annealed and normalized to 220 ksi condition.  (Last winter I was hunting for a piece of pre heat treated plate to use on my blacksmith anvil and I was surprised by the prices, plus I would like to use a harder heat treat.)

We are illustrating the different ways we think about these materials.  You have a notion about what 4140HT means for your uses and needs, and I have another one.  I think in terms of the strength of the material in ksi, and have to use a hardness table for translation if I'm faced with a material that is presented as having some Rockwell hardness without the rest of the story.

Each heat treat specification produces material heat treated over a 20 ksi range.  In other words, if I specify heat treat to 140 ksi per process spec "1", the resulting material will exhibit 120 to 140 ksi ultimate strength, or hardnesses of Rc 26 to 31.

If I chose spec "2", the next one in the list without any other control, the result would be 125 ksi to 145 ksi (Rc 27 to 32.5).

What I would do in real life for a part that let's say I required to be 140 ksi, is specify, "Heat treat to 140 ksi minimum (Rc 31.5) per process spec 4." , knowing that I might have to live with parts that came out of tempering at 160 ksi.  As part of the analysis, I would take a look at other properties than the ultimate strength, mostly the ductility and yield strengths.  I probably would not specify mid range properties without a remarkable reason that justified the scrap rate, and an expensive materials testing project to develop the allowables for that essentially unique material would be necessary to justify choosing a particular strength.  That ain't gonna happen.

A nice feature of working 4140 is that heat treat processes are available that produce overlapping ranges in the strength.  4340 and 6150 are two others.  The rest process in steps such as 120 to 140 ksi, then 140 to 160 ksi, then 160 to 180 ksi, and so on.

It's too bad carbon steels love to rust, they can be used to produce really great parts with smart choices.

I welded a glider fuselage in the living room of an apartment.

I put particle board with a plastic sheet on the floor to protect the carpet.  That mostly works, except when a hot part bounces off the particle board onto the plastic on the carpet.

I fixed it with a pocket knife and comb when I moved out.

An 11/16ths' drill is more than 10 thousandths of an inch smaller in diameter than .702.

But the wobble in your set up will probably take care of that.

A couple of articles about a rail gun built by an Australian -

http://bulletin.accurateshooter.com/tag/rail-gun/  Scroll down the page.

http://www.benchrestbulletin.net/drupal/sites/bulletin/personalprojects/action%20details.htm

A little light on the bloody details, but still interesting.