YOU CANNOT DEPEND ON FACTORY AMMO
It is blind foolishness to expect all ammo to meet specs and produce correct headspace. Here is a perfect example.
Take the measurements and you will KNOW what the headspace is, then make corrections.
This gentleman complained he could not cock the hammer with a loaded .280 Rem. barrel and wondered why.
Taking a few simple measurements he could have saved himself a lot of aggravation and money paying someone else to tell him the ammo stuck out of the barrel too far and was preventing the barrel from locking up safely so the hammer could be cocked. The frame was simply doing what it is supposed to do.... protect the shooter from firing a barrel that is not locked up adequately to be safe.
Barrel-to-frame gap measurement was essentially zero. Ie, the barrel hits on the firing pin bushing.
With the barrel off the frame, Winchester ammo stuck out of the barrel from .005" to as much as .010," and it is no wonder the barrel would not close!
One might think on first impression that the chamber is too shallow. NOT SO!
Here you can visually see that the .280 Rem. "GO" gauge is below the end of the barrel. Actual depth below the end of the barrel is .001" which is perfect.
In this less than perfect photo, you can readily see that the ammo is sticking up WELL above the end of the barrel.
The reason is the cases are stopped by contact ONLY at the bottom of the shoulder!
I try and try to get the point across that case shoulders, datum lines on steel headspace gauges, and chambers often simply do NOT match at all. Note the ink is marred ONLY at the bottom of the shoulder where it meets the body of the chamber.
When it comes to these break open guns especially, forget about steel headspace gauges! They are meaningless when you can take direct measurements from the barrel and very easily calculate the headspace.
Going to Federal ammo, the results are better, but even the Federal ammo is a bit too long.
The barrel will likely close on this Federal ammo, but with no space at all between case head and firing pin bushing, vertical stringing may result. As this is being written, no range testing has been done yet, but will follow when Oregon weather permits.
So, which component in the system is off..... the chamber shoulder shape and location or the ammo shoulder's shape and location?
It could be either, but it really matters little. The important thing is to be able to recognize the problem first, then choose a corrective measure.
In this instance it is likely simply a different brand or different box of the same brand may solve the issue entirely. It makes no sense to ignore headspace and assume it is right.
And it makes no sense to rely on steel gauges when the ammo does not always resemble the gauges, and it is so easy to tell if the headspace is off.