You are doing it wrong. Your datum zero point is wrong. It is an AR gage, designed for use in AR’s. Most AR’s I gage with that are at zero, or less. Heavy worn barrels are maybe like 4-6, etc. Even “used” barrels gage really close to zero typically (most people don’t shoot that much). And it depends on if there is chrome lining or not as well, for example Colt chromed vs Colt ban era un-chromed.
I believe a rule of thumb (at best) is that 1000 rds or so gets you approx 1 measurement of movement in some match barrels. Military rifles might be rejected when they read at 10 or higher. Some shooters consider 5 to be worn out for competition needs.
None of that means anything in a SCAR. The gage, even in an AR is just for monitoring wear. Most new Colt AR’s will be less than the zero line in my experience. Basically at -1, so 10 units of wear would be at the 9th mark.
So if you are reading 3.7 against whatever location you decided to read against when NEW, you need to figure that “worn out” is gonna be basically 13.7 or beyond- in other words, past the last line quite a ways. If you use 5 units of measurement, worn out is at 8.7. I get the feeling that you somehow think that when you read “5” your barrel will be worn out? Totally wrong... when you see 5 units of change, then it might be too worn for competition. When you get 10 units of movement, then a military armorer might shit can the barrel even if it still shot OK.
You have seen it go from 3.7 to 4.1 (also I call BS on reading 3.7, 4.1, etc- it is more like 3, 4. Give me a break- there are no in between hash marks)
But if we assume you are correct: .4 of measurement per 1200 rds. If we extrapolate that to say a nominal 10 units of measure (13.7 reading), that means you have a barrel life of 30,000 rds- assuming constant wear, and that a movement of 10 units means “worn out”. Of course neither of those are likely true for barrel wear, but it is as close as we can get. If we are using the 5 units of measure= worn out, then it is still 15,000 rds to get there.
Quit using that gage wrong. Monitor how it shoots, and how much movement you get with the gauge, but you can’t assume that because it it in a SCAR it now is a 3.7-10 gage vs a 0-10 gage. 3.7=0.
Basically- the measurements don’t mean shit. If 3.7 is new, see how it is doing at 4.7, 5.7, etc etc. Let us know how it shoots when it is at 8.7, because that it the number you seem to think equals “worn out”. The way you are doing math, you are calling a barrel with only 1.3 units of wear as worn out!
Again- no way you can legit say 3.7 anyhow with that gauge! If you were saying halves, or quarters, I could let it ride...but geez.
Basically- seems like kind of normal wear. Quit freaking out that your barrel is going to wear out based on that gage.
Not addressing the photos, just the gage usage.
TLDR- that gage is designed to be read in an AR thru the bottom of the upper receiver, SCAR’s are not AR’s, so you need to understand that how it initially lines up is different!
Info from Fulton Armory on throat erosion gauges