I've been reading the fma wikia, and I'm working from a number of it's claims though I can't honestly figure out what sources in the series were used to come up with them. Let me know if you have a source for the claims I'm going to list below, or disagree with them and why, etc...
1.Father created a barrier of philosopher stone material (psm) under amestris, which is how he was able to block alchemy when confronted by ed and the gang.
2.Alchemy used in amestris was technically through the use of fathers psm.
3.Homunculus tried to do the same thing he did at the end of the series, absorbing "god" as he called it, in xerxes but failed because he lacked enough power at the time even with the vast number of souls harvested into a philosopher stone.
4.Alchemy and alkahestry require one has a soul, and access to the gate of truth in or with it.
For whatever reason father didn't understand alkahestry, and throughout the series there is very little exposure to what it's capable of, and how it fits in with amestrian alchemy (kind of weird, you'd think as part of the gate he'd understand everything, but apparently not...).
*Come to think of it, it's been a while, what exactly is may even shown to be capable of with alkahestry?!? Examples please.*
Much like alkahestry the full extent of xerxian alchemy is also unclear, at least to my recollection, especially what was known before they pulled out the homunculus (who would later become father) from the gate.
I'd have to guess that all those techniques follow from the 'truth' behind the gate, and so I don't like to think of them as separate, but to refer to both of them, whatever was practiced in xerxes, and whatever other similar techniques unique to the fma universe "exist" so to speak, I'll use the phrase 'truth-gate techniques'.
Question: What is the capacity of truth-gate techniques without the use of philosophers stones?
My guess: is that even beings like Hohenheim after the xerxian philosohper stone event, and homunculus, don't fully understand everything in the universe, and thus not every possible aspect of truth-gate techniques. Given that at the height of their society they are about as scientifically advanced as late 19th century or early 20th century earth, it seems possible that very much more than was shown in the entirety of the series is possible. But it's clear that there are some very steep limits, from the person performing truth-gate technique; both their knowledge and chi, and with the limits of equivalent exchange. But I get this weird feeling that the real limitations of alchemy without the use of a philosopher stone, in particular fathers psm in amestrian alchemy, is much steeper than with it. Could people like the iron blood alchemist even transmute a series of cannons without fathers psm?