Some may or may not agree with me but i have proven it, been there and done that. Education can be expensive when you learn the hard way.
My original batch of the CSR brakes was made of standard 416 bar stock that has a hardness of R20-21 in it's delivered form. I put one on a friends 300win mag. After about 275-325 rounds of 190 smks on H1000 Greg noticed a fall off in accuracy that would then come and go, and then just go. Luckily Greg shoots alot and this took about 2 months to develop. That brake was bored .022" over bullet dia, allowing a .330 pin gauge to slide thru the bore hole. At the point that he brought in the rifle, only a .315 pin gauge would slide thru.
(The next week, all my existing brakes went out for heat treat, the 416 stock can be hardened up to about 29-31 rockwell scale.)
The thru holes had closed up .015", caused by the displacement of the leading edge of the clearance hole being punished by pressure and residue, rolling over a bur into the leading edge of the hole. This bur was unconcentric thus causing a disruption in accuracy. (Would the change be noticed on a factory rifle that shoots over moa? may-be not.) But this turned his legit sub 1/2 moa gun into a 1 1/2 moa gun.
SS is a gummy sticky material and the little burs want to hold on, after impacts, pressure and work hardening they can fracture off, in other words the clearance hole is in a constant state of change.
Shortly after this discovery I had a 6.5wsm, built by Kirby, come thru the shop with accuracy problems 1-2 moa with about 175 rounds fired. (Now this had a brake that Kirby used before he developed the Painkiller line of brakes, so please don't take this as anything other than an example of this issue being discussed.) A 3 port with straight thru ports no angles. Same issue bur rolled into thru holes and clearance contact issues with the unconcentric bur and bullets.
I replaced the brake and it was back to 1/2moa performance. To the gunsmith that is aware of this potential problem it is easy to spot with or with-out a bore scope, and is the first thing I check on any gun with a brake that comes thru my shop with accuracy issues, because it is that prevalent and common.
I have inspected a radial ported brake with holes around the circumfrance, each little hole showed a formed bur, which thus interfered with concentric clearance.
These brakes all had one thing in common, you guessed it. Now I start with pre-hardened 416 which tests out between 29-31 depending on the lot. This material cost more and takes longer to machine. This does not 100% elliminate the problem but it greatly reduces and slows it. I have an additional step I do in the process of boring the thru holes in the brakes during instal, that now virtually eliminates the formation of any bur in my brakes. Any-one who has ordered a brake from me in the last 2 years know what I am talking about because it is included in the installation instructions.
Now a bur in your brake dosn't mean it needs to be trashed they can be removed/cleaned up by the user, and if watched can give reliable service.
I just don't need the headache of a product that isn't what I feel is the best I can offer.
Hardening does matter, often the brakes "failure" goes unnoticed because it is not on a rifle capable of sub 1/2 moa accuracy.
How many guys have had a good shooting rifle go south and blamed throat erosion when it could have been brake "failure".
I'm not the only custom rifle
builder to be aware of this issue, I have discussed it with a few of them. I sell brakes to at least 10 gunsmiths across the country, and they receive my instructions for the install mod to prevent issues and none of them have called to debate my findings as misleading or false.
Like anything in this business mileage may vary, a soft brake on a 26" 308 may show no problems for 1000+ rounds, add a magnum, overbore, short barrel, slow burning powder. and things change in a hurry.