The "Hummer" barrel
It is my opinion that the “Hummer” barrel exists in only one harmonic condition and here’s why.
If 100 identical barrels are identically chambered and mounted to the same rifle then we will get a few that will perform much better than the rest of them so we call them “Hummer” barrels. This is most likely because only the few “Hummer” barrels hit the absolute ideal harmonic condition on the first go.
But what happens when we place those “Hummer” barrels onto a different rifle? 99.9% of the time the barrel accuracy will not transfer equally even when using the same action. This happens because the harmonics were changed.
Harmonics are a very important aspect of accuracy because accuracy is all about repeatability.
So what makes rifle barrels have different harmonics and why do some barrels hit absolute ideal harmonic condition while others can’t?
One of the biggest factors that effects barrel harmonics is residual stress and how your barrel maker deals with this stress within the manufacturing process. Most button barrel makers perform a stress relief cycle as part of the manufacturing process and most cut barrel makers rely on the steel mill.
This is from my own experience and I will share what I have learned.
I have tested barrel steel from France, Germany, Slovakia and the USA.
I have concluded that all barrel steels have some amount of residual stress in it from the mill. It is a product of the rolling process at the mill.
Yes, the steel mills run the material through stress relief cycles but this is normally done in a production environment…. 40,000 lbs at a time in a rail bottom furnace.
Production means that they probably shorten the cycle any time they can to increase productivity.
40,000 lbs at a time means that each bar will have inconsistent results within each bundle. The bar stock ends are yet another story.
This residual stress is why cut barrels are normally contoured prior to reaming/honing and rifling. Think about it… would it not be easier to rifle and lap everything as a cylindrical blank ahead of time and then just contour and ship? It’s not done this way for a reason.
As a button barrel maker I have developed my own pre-machine stress relief cycle on the material followed by a long soak post-machine cycle. From what I have learned I wouldn’t make a cut barrel without a stress relief cycle.
The less residual stress that is in steel the more that it will ring. I have heard of old time gunsmiths listening for barrels to “ring” but I never really believed in it until I experienced it myself by accident.
I was stacking pre-machine stress relief barrel material onto a pallet one night with the shop quiet and the machines shut down. Every time that I added a piece of material to the pallet the whole pallet would ring. I never heard this ringing sound before I started doing a pre-machine stress relief cycle on the metal.
Another thing that I have noticed is that the rack in my stress relief furnace rings. I know that it had stress in it at first because I welded it up myself and I had to beat it into position with a sledgehammer because it twisted after I tacked it up. Now that it has been through many stress relief cycles it will ring for over 5 minutes. It sounds like a big church bell if I tap it with a small brass hammer.
I know that it is old school but the “ring” test still works especially on bar stock material.
I can cut 3 pieces of barrel stock out of the center of a 12’ or 17’ bar from any steel mill and then drill a small hole perpendicular through the ends of them so they can be hung up like a wind chime.
Piece #1 Hang it up just as it came from the steel mill with nothing else done
Piece #2 Material ran through my stress relief cycles
Piece #3 Material ran through my stress relief cycles and then a cryo cycle
When I tap these pieces with a small brass hammer I can hear three very distinct music notes.
The idea of ringing cryo material came from my cryogenic contractor because he was running brass music instruments for the Indianapolis Philharmonic Orchestra. The musicians can hear a tone difference after cryogenic treatment because cryo changes the crystalline structure of the metal.
This may be more of a factor than we think as we all chase tune and harmonics.
