I’m not talking about the rise and fall of the bullet path versus the line of sight... all of the ballistics doodads report that.

Are there any ballistic programs or charts showing the drop of a bullet fired from a horizontal barrel without respect to the sight plane? Better yet, is there a program or set of charts, etc. that will report the angle a barrel has to be at in order to strike a target at a given distance? Ultimately, that is what I am after.

Empirically, I suppose I could go to the range and use a laser boresighter to get the line of sight and bore parallel at 100 yards (assuming he boresighter beam is truly VERY concentric to the bore) and then use trig to measure the impact drop... but that’s a hassle.

Alternatively I could plot the velocity decay curve of a known round to get a best fit equation (easy to get data points from normal ballistics program) in order to to determine a time of impact value for a given distance ... and then just use that time to impact figure to determine the vertical drop due to gravity (@ 9.8 m/s²). But is this really this simple ... will it get me close enough? or are there other aerodynamic forces effecting vertical drop rate?

Set the zero range to zero. Set height of cross hair over bore to zero. That should give you drop from level. Then drop and path should be exactly the same.  
And..most programs give you both drops anyway. Drop and path given what data you used in settings.
Edit: I do not know if the program will even run with those specs. It might..?

Close enough for what ? Perfect ? No. The drag coefficient tables are never 100% perfect. An engineer made a decision based on several factors when picking which drag coefficient table to use with a particular round. 87% accurate.

Any good ballistic program will give you "time of flight" (TOF) values for any given range.  If you take the time of flight (or "fall") and plug it into a free falling body equation (link), you will get the vertical drop for "common" ranges.  Air limits the rate of fall (terminal velocity) so if the fall is long enough, the free fall equation (used alone or unmodfied) fails miserably...  but for short-ish ranges if should work just fine...  There is also a vertical component to the Coriolis effect which is direction of fire dependant which might effect your vertical deflection with shots with TOFs greater than about 1 second or so...

Don't forget, since the earth is curved you'll have to include the rate of curvature in your calculation too which is starting to make an unusable mess of the whole thing since the earth is an oblate spheroid and not a true sphere.

OP:
I'm not sure I want to ask why you want this...….but:
-most? some? ballistic calculators will allow you to set the zero distance at 0yds, so yes, you could get the bullet drop that way.  
-as far as your angle question...……. your sights provide a measure of adjustment from one zero to another.  (ie. 4 1/2 MOA clicks from a 200yd zero to 300yds).  You could convert that mathematically to an angle.  (Those angles, though, are caliber specific BTW).
-I would guess the time/gravity math approach would get you very close thought the Ballistic Calculators out there would be more realistic.
-The B Calc would (should) take into account the "decay" of the bullets BC as it slows down.
-Yes there are forces that could affect the drop but they are truly minute so can be ignored.  Unless you "what if" this...… ie I'm shooting across a canyon and there is an significant updraft.

To get drop from a horizontal barrel, JBM Ballistics will let you set sight height to zero and zero range to 1.

We may have a failure to communicate here...

How high is your barrel from the ground?

Drop and time a bullet from that point, or do the math at 9.81 meters/sec^2

Say it's .2 seconds.  Estimated.

What's the muzzle velocity?

Let's say it a 20" ar, shooting 55gr military ammo. 3250 ft/sec

Your bullet impacts ground at 650 ft. ( discounting curvature)

Txl

This.

A projectile fired horizontally, ignoring that the world is round, is going to hit the ground at the same time as one dropped by hand from the same height.   The vertical component can be calculated entirely independent of the horizontal component.

One of my programs called quickload has this feature (bullet drop from the muzzle) I haven't played with it and I don't think it has an angular representation to hit a defined distance.  But like many have mentioned there are far to many variables to accurately gauge this. You would also have to add the elevation in which the round was shot and the elevation of the area where the bullet will strike. IE. your atop a canyon and the bullet hits in the canyon. it's going to travel a fair distance before impacting vs a perfectly flat terrain (which doesn't really exist) or if you were shooting towards a mountain/hill.