I have no idea if this is common knowledge but I couldn't find any data on how weight of the weapon affects how long you can hold your weapon up till full stamina drain in comparison to ergo, so I decided to test it myself.
What I found out was surprising as I've been under the impression that weight of the firearm has a little to no impact on the duration your arm stamina drains, compared to the ergonomics but as it turn out I was dead wrong. and I made those specifically so that the M1A would have higher Ergo but also higher Weight while the RSAAS was made the other way around. My testing method was to take a stopwatch and measure the time it took for the whole arm stamina to drain, do it 10 times and calculate the average for each weapon.
To my surprise RSASS ended up draining stamina in 37.56 seconds on average while M1A took 33.70 seconds to achieve full drain on average. This means that the weapon with lower ergonomics but lower weight will achieve better results than the weapon with higher ergo and higher weight which makes sense a lot more sense as in real life it's easier to hold up lighter objects even if those are less comfortable to hold.
Don't just take my results though, test it for yourselves and your preferred weapons. I might also just be wrong as there might be other variable that I'm unaware of, this is why I tag this as a discussion because I'd like you share your own thoughts on the topic or point out flaws in my reasoning/testing method. Like I also said that the beginning this might be already well known in the community but since I couldn't really find any data on the topic I made this thread so it might prove helpful for other new(-ish) players like me, who are unaware of said mechanic.
