Note ............ I specified "Match Grade Ammo."  -  NOT the cheap stuff. I have no intention of sorting by rim thickness, weight, etc. My query stands ............
Once I have chosen the ammo. best suited for my rifle/barrel/conditions by extensive range testing, do I need to re address my headspace in order to achieve best accuracy?

A lot going on.

Headspace on a straight wall, rimmed cartridge can be achieved three different places.

On the rim as designed
The bullet if it is jammed into the rifling
On the case mouth.

How far the case moves when hit with the firing pin depends on where the headspace is achieved. Then fit of the case and bullet. Is it easier to move the case in the chamber, bullet down the barrel, or overcome the crimp, than set off the primer.

The only way a fire formed, straight wall, rimmed case will decrease headspace, is if it stretched long enough so that it is now headspacing of the neck in the chamber like a rimless pistol cartridge. This can lead to crimping the bullet where the chamber tapers from where the case ends and throat begins. Generally causing accuracy problems, as well as pressure and velocity spikes. Or diameter has increased to where it sticks to the chamber wall.

Keep in mind, that jamming the bullet into the lands by closing the breech block, will put the headspace on the wrong side of the rim, unless a mechanical cartridge seating tool is used. In this way you eliminate the firing pin’s ability to move the cartridge before the primer is set off. Should result in better accuracy.

Headspacing off the rim, as designed falls somewhere in between. A consistent load can be found, accuracy will depend on the fit in the chamber and repeatability.

Anything that can be done to decrease variables should, increase accuracy. A custom sizing die to minimize slop in the chamber. Custom fit of barrel and breech face to match rim thickness, or custom brass to match the existing chamber.

Will be interesting to see your findings.

hepburnman wrote on Apr 20^th, 2023 at 10:04am:


Depends on neck tension, crimp, and or powder charge, against force required to seat the bullet into the lands. When seating the cartridge, if there is not enough resistance the keep the bullet from being pushed back into the case, your seating depth, both into the lands and into the case will vary.

If you really want to know if the force of the firing pin will move the case or cartridge, remove the extractor, insert an inert case with an oil soaked primer with the anvil removed, close the action and carefully measure the outside rim to barrel distance. 
Close the breech, drop the hammer and remeasure. You will know not only if, but how far.

If you’re using a powder compressed load, you can fill the case with a substitute. In theory actual powder with an inert primer should be safe, but I can’t suggest that.

If it helps, I can confirm that a 45 ACP case, with a 200 grain .308 bullet dropped in to provide resistance, will set off a large pistol primer in a Model 94 Winchester chambered in 45 Colt. It will also set off the primer on an empty case on the third or fourth strike.

Hardness and diameter of the bullet, fouling in the barrel, lube qualities will again factor in to any cartridge movement.

It doesn’t take much to set off a primer. That’s why they are shipped in single compartment trays. Plenty of verifiable story’s of ejected empty brass hitting the primer on a box of ammo sitting on the shooting bench and setting a round off. Or a primer stack in a tube feeder having a primer ignite do to the drop. 

What I have to add to this is that a primer that is not fully seated, may not have enough support to fire because it moves in the seat. So somewhere in between the resistance of the force needed to seat a primer, and a 200 grain weight is what is needed 

Redneck science, sometimes is pretty accurate. But stay safe.

Here are my results from today. Definitely the BR2 primers look better than the Fed 215M. Increasing the rim thickness also looks to produce some interesting results. I still need to work with these and increased/decrease the powder and see what happens.

All shots were made at 200 yards with BP and 429g tin/lead alloy grease-groove bullets

more results

Not making any firm conclusion from this but will hold on it

I am very curious about how this works.

In your other thread, the way you increased rim thickness was to move the outside of the rim forward.

Above you explained that you seat the bullet on a compressed powder charge and seat the cartridge with the breech block. Seating the bullet into the lands.

Headspace is the void between the case head and breech face. You have none. The cartridge will not move, you have effectively “headspaced” off the bullet instead of the rim.

The only function deforming the rim may serve, would be to keep the case from moving forward with the firing pin strike. With a compressed charge and jammed bullet, you need to either be able to further compress the powder, or drive the bullet deeper into the lands. Or the thicker rim serves no purpose.

Partly why I suggested checking if this was actually happening earlier.

Am I completely missing something?