It's not an either or question
I can have AR receivers forged from any number of wrought alloys (2024, 6061, 7075, etc) at various tempers. I can also have them machined from wrought bar or plate (what non-machinists/non-engineers call "billet") of the same alloys I mentioned at various tempers
Your question is two-fold
- Do I have a receiver made from a forging or from barstock
- What alloy is my receiver made from
The two are completely independent.
Now, it's a safe bet to assume that if your receivers were forged by one of the several foundries that pump them out for everyone; then they are 7075 T6 (or T651) as that is by FAR the most common material large volume runs are made from.
PS, if one orders custom forgings with more stock than what is commonly forged for "mil-spec" receivers then a whole lot of custom work can be done and it becomes very hard to tell whether the receiver was milled from a forging or from wrought stock.
The reason I mention all of the above is for education only. There is a TON of misinformation and misunderstanding about metallurgy surrounding the AR-15. Most of it originated at AR15.com