Haven't been able to find the information assembled in a way that worked for me- wanted to confirm if my understanding is correct or if I am missing something obvious. Basically, I want to build an AR-15 similar to my foxtrot mike AR-9. The gun has a left side charging handle and right side ejection port, which is a style I like.
My understanding is that there are two "styles" of side charging ARs.
Handguard Side-Charge: My understanding is you need a complete upper designed to support this. I would need to find a complete upper with a left handguard charging handle and right side-ejection port. Not much more to it than that.
Receiver Side-Charge:My understanding is that I would need two things
A. A bolt carrier where a handle can be attached on the left side. this can either be left specific, or ambidextrous. B. An upper receiver which has as opening on both the left and right.

Is my understanding here basically sound/correct? If so a few a question
1. Am I correct that handguard side chargers are basically non-existent? After lots of googling I can't really find anyone that makes these. Maybe my google is weak or I am not using the proper terms.
2. Am I correct that 95% of uppers only have one opening, thereby forcing the charging handle/ejection port on the same side?
3. Is it just "which hole is available" that determines ejection side? I have seen specially designed uppers like pictured in the receiver side-charge video where the charging handle side is just big enough for the handle to work, so it makes sense to me that the bullet would eject to the right. But if it is open on both ends like this example here, wouldn't it be able to eject from either side?
4. Based on all of the above and still wanting a left handle and right ejection... would it be smarter/cheaper just to get a normal right side ejection upper and/or bolt carrier group and have a gunsmith drill the needed holes so I can charge from the left?

Hm, hadn't ever seen a hand guard (forward mounted) AR charge handle.
If I new it had been done before, my 308 would probably been done that way.

If you go with a RH charge handle, the ejection port will still need to be machined more due to BCG travel distance, so you are not saving any machining costs.

Seen a few pictures, of LH receivers done, but they all had handles that reciprocated with the bolt, as do the RH ones.

There are to companies that do a side charge set up that is an extension off the stock position. They don't travel with the BCG when firing.

I did a LH side charge, using the FAL assembly, that does not reciprocate when firing.

Ok you also need to decide reciprocating or non-reciprocating.

The FM 9mm upper w/ the forward positioned side charging works because it doesn't use a gas tube, so there is a rod that runs where the gas tube would normally be.  Has a small spring to push it back forward, and it simply presses on the face of what would normally be the gas key.

Side receiver set ups can be two ways - reciprocating and non-reciprocating.  For the love of god, why you'd want reciprocating support side on a semi auto (SCAR style... appropriately named) I don't know.  For a manually operated gun like a Solo 300 or similar ? Youbetcha.

But yes, for reciprocating you simply need to drill a hole in the bolt to put a handle and cut a notch in the upper for the handle to run back and forth in.

For non-reciprocating (the way it should be god bless the FN-FAL and D. Seive ) you need a channel to hold something that can push on the bolt/bolt carrier but not interfere with the gas tube/gas key, etc.  Don't need a return spring - the bolt will send it forward but not pull it back, or you can manually slide it forward.  The way the Gibbz patent receivers do it (Gibbs, Spartan, NFA) is they use a special bolt cam pin with a extra ledge sticking up from it, this gives that side charging handle a place to push on that is both strong enough and doesn't interfere with gas tube/key.  Other than the special cam pin the BCG is a standard milspec style BCG.

You MAY be able to do something like the HK/G3 cocking tube and push a rod against the same type of bolt cam pin that the Gibbz style use.  You'd need a forearm with a rail in the right place (or missing one) to attach it along and to drill the needed hole in the face of the upper.

One member here in the past few months posted details on how he attached a side charging handle unit from a FN-FAL to a standard AR upper.   If you are a McBendy/McGrindy type, that would be where I'd start.

Personally I just bought a Spartan from Joe Bob's.  Best $250 I've spent.  That and the $400 on a FM 8.5" 9mm forward side charging set up :)

If you need pics lemme know - wife's asleep and the beep beep of the safe opening wakes her up.  She's a redheaded Puerto Rican, and I'd like to live through the night please.

That was probably my thread.......That vanished

This is a link to a relevant thread that is now anonymous. I'm still working on mine thanks to that posters inspiration. I'm doing a radiused base which has caused some issues.
https://www.ar15.com/forums/Armory/Adding-left-side-charging-handle-to-AR-platform/49-510437/

Yup, that's my thread, thought it was gone forever, having never saved the address.
Thanks !

I had thought about cutting the base the base to fit the radius of the receiver for about 5 minutes, and said screw it, and put the flat on the receiver instead.
The easiest way to have done it would have been to buy a rather expensive ball end mill, but the big ones are expensive.

Could have also used a smaller, say 1" standard end mill, and tilted the head, but that's a lot of setup time.

Example of forward-mounted side-charger: Forward-Mounted Side-Charger