Were those weapons (guns specifically) used by ISIS made by USA or Russia or some other known entity?
As is often the case, the “terrorist” label creates more confusion than clarity. It can cover many different organizations, with widely different weapons and sources of support.
For example, the weapons used in the recent attacks in and around Paris were sourced locally (possibly from Belgium), it's the kind of things bank robbers or drug traffickers occasionally use as well. Most of the weapons mentioned by the press (Kalashnikov, Tokarev pistol, Skorpion submachine gun) were designed and produced in large numbers in the Soviet Union and its satellite states. Many have been used during the civil war in Yugoslavia in the 1990s and from there apparently found their way to various criminal circuits. Some of them simply get stolen from the armies there. The Telegraph published an interesting article on that with more details and nuances.
Daech is a completely different beast. Various press reports suggest most of their weapons were captured from their enemies, including Iraqi army armories. I found a report from some British organization tracing the source of ammunition found in the field. A few results:
- Daech was able to source ammunition from very different countries, including the US, China and Russia (some of it so old as to have been manufactured in the Soviet Union!).
- What was found depends on the specific area. In Iraq, it's mostly US-made (by way of US allies in the region). In Syria, it's older China or Soviet-made ammunition that Syria's army and security forces would have imported earlier.
- Recently produced ammunition only accounts for 10% of the finds. It comes from Bulgaria and China, which still manufacture ammunition for Soviet/Warsaw pact weapons. There is some from Turkey as well, for weapons from NATO countries.
- A few spent cartridges come from Iran, possibly through the Syrian security forces. It's not the most important source but the report highlights this because it seems that any export from Iran (even to official Syrian forces) would have been in violation of a UN security council resolution.
- Some ammunition comes from far away countries like Kyrgyzstan, Albania or North Korea. There are also a couple from Sudan. The number is small so there is no evidence that Sudan plays a great role in all this but this illustrates the complexity of the supply routes.
- Some ammunition for Soviet weapons was produced in Russia but for a US company. It was then distributed to US allies in the region, another strange twist in the story.
The article mentioned by PointlessSpike in a comment also provides some details on heavier weapons.
You might find this article useful. It details the tactics of terrorism and kinds of weapons used.
The question of where terrorists get their weapons from is very broad, possibly stemming from a misunderstanding of the nature of terrorism. Terrorism, for a start, doesn't have a single definition- mainly, I think, because one man's terrorist is another's freedom fighter. So I'll instead answer the question "where do small, belligerent organisations incapable of creating sophisticated weaponry get their weaponry from?".
From friendly, larger organisations such as countries. Often the success of belligerent organisations is dependant on larger ones supporting them. This could be in the form of money to buy weapons or the weapons themselves. They'll then be able to procure sophisticated weaponry that they can use against others. Of course, there is no guarantee the belligerents will do what the larger organisation wants- Al-Qaeda being a good example.
From enemy, larger organisations. Often belligerents will steal weapons or money from larger organisations- for example, IS captured much US weaponry left by Iraqi forces and regularly attempts to extort ransom so they can get more.
Improvised. Many organisations with few resources create primitive weaponry themselves, such as Improvised Explosive Devices. An example would be the Taliban use of IED's in Afghanistan.
From anyone willing to sell. This could be larger organisations with a surplus who don't oppose the buyer, small suppliers who serve as middlemen, or even individuals selling weapons without authorisation. Of course, this requires money or other tradeable resources, so generally this is almost beside the point as they would have to go through others to get these things.
The main source of ISIS weapons is the Syrian regime who has continues support from Russia. ISIS take these weapons from corrupted army officers or by attacking military bases.
other sources are not continues as ISIS is surrounded by non supporting countries.