Thank you Frank! I appreciate your input and it suggests some possibilities that would explain the discrepancies in field reports.
I am an end-user not a gunsmith so you will forgive me for attempting to relay second hand information as a layperson in this area. I just asked for end-user feedback hoping it would be available from the shooters who shoot more and therefore burn out barrels faster. As you can see, the feedback is a bit variable and inconclusive so far, with limited information on the difference between the earlier and later 400MOD runs. I provide the following in response to your questions and comments.
Anyone do a metallurgical analysis or some sort of a hardness test? How or what is someone basing this statistic on? Just by the way they feel it cuts with a reamer or an insert on they're lathe? With out hard data to fall back on.... you need to question this. Reamers can have inconsistencies in how they are ground and even when new might not cut good etc... or how many barrels has the reamer chambered? That's just one variable.
There were no metallurgical tests as far as I know. My understanding is that the best chamber cutting equipment uses sensors for cutting pressure and cutting torque in order to adjust cutting speed to achieve a high quality finish and concentricity down to a hundredth of a thou and even less. As I understand it, the machines tell the smith quite a bit about the material properties because the pressure and torque for a given cutting speed generally go up for harder steel. The information I was given is that at least some of the earlier 400MOD runs had higher cutting torque and cutting pressure values for a given cutting speed
as compared to standard 416 barrels and that the initial 400MOD barrels seemed to shoot accurately with limited velocity change for more rounds than a normal 416 barrel, albeit with not quite as much % increase in round count as initially indicated. I also understand that at least some of the subsequent 400MOD barrel runs, many months later, had chamber cutting pressure and torque at a given cutting speed that compared more closely to normal 416 barrels and this reduction in the
relative cutting pressure and torque seemed to correlate with barrel life measured in round count that was not as long as the earlier 400MOD runs mentioned directly above. I can't be sure but doubt that it had something to do with reamer wear because the pressure and torque values were compared at each point in time vs standard 416, not in absolute value. Pressure and torque certainly vary as the reamer wears, and my understanding is this is why the smith continually adjusts cutting speed to get quality results as a reamer wears and to account for differences from barrel to barrel, manufacturer to manufacturer.
So I went back and looked at the material certs/inspection reports as well as what we double checked with the RC tester we have. Now keep in mind our RC tester usually reads a point lower than what the material inspection reports say. I've seen this across the board and pretty consistent with other lots and types of steel we've checked. So I don't believe the mill is skewing the inspection reports at all.
That all being said we are on our 3rd lot of material. I have a 4th lot that is close to being done. Looking back at the numbers the first lot of material RC wise was technically the softest lot. The 2nd lot was about the same to a point and a half harder.... the 3rd lot basically mimic's the 2nd lot... maybe and this is a maybe a point higher.
And right now I'm just looking at one piece of the criteria and that is the RC of the material. The chemical composition make up can vary a smidge to and effect how the material machines as well.... but that doesn't mean it's softer or won't last as long etc...
If I look at the average number of the BB material vs the avg. number of the standard 416R material at a quick glance...I'd say the BB material is at times about 2.5 points harder than standard 416R but we've had lots of 416R that have been just as hard when you only look at the RC scale numbers.... but this is just one thing to look at. It's also the chemical make up of the material itself that is different.
That's like guys arguing the LW50 material is harder and more difficult to machine so it should last longer which isn't the case and most guys agree it doesn't machine nice but nothing that goes towards helping it last longer because they think it's harder etc...
So.....?
I don't know what the correlation of cutting pressure and torque values is with the RC value of the steel, nor do I know if RC is the sole indicator of Barrel life. As you point out, in addition to RC value, the way a barrel cuts and the barrel life may also depend on other material properties such as chemical composition, perhaps along with other properties. As you also point out, the properties of different runs can vary, both for the standard 416, with runs some being "harder" than others, and for the 400MOD runs.
It could be that the data I received was from a moment in time where the standard 416 was running a little "harder" and the 400MOD was a little less than nominal "hardness". Perhaps subsequent comparisons between 416 and 400MOD at a later/different point in time would have been closer to the initial comparisons. This is quite possible, I don't know.
This is all the information I have. Please forgive me if I have ventured into conjecture that is not warranted given my limited material science experience in other fields.
Still trying to decide how to answer one of the gunsmiths I am working with regarding which barrel material to use...
Thanks again!
