Shallow headspace in a rolling block?

[bradley.moss72]

I bought a Rolling Block on Gunbroker. It has been rebarreled to 45-70. I suspect that it was not completely chambered. It has a .053 rim cut according to my best measurements with a dial caliper?

I have read in Loading and Shooting Black Powder Cartridge where Paul Mathews had his chambered with a rim cut to accept brass with no more than a .066 rim thickness, which itself is less than the standard dimensions. Mathews talks of trimming brass to fit this.

I have found a few cartridges that would fire, but the hammer still binds up on the block.

My thoughts are to finish ream until it will chamber any cartridge.

What are your thoughts?

BW

Sent from my SM-G900V using Tapatalk

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[country gent]

I would cut the rim thickness to sammi standard ( on rimmed cartridges this is the headspace) - .002 -.003 this allows for some rim variance and light wear of the tooling marks to take place. Another route done on rimmed cartridges ( mostly with 22 rimfire) is to cut to a large batch of ammo cases rims and use only that in the firearm then. Cutting the rim slightly shallow by - .002 - .003 will allow for brass to fit while losing very few to to thick. This will also have the benefit of a slightly short case length in the chamber allowing cases to be trimmed to exact length needed to fit the chamber.

[marlinman93]

I'd start by cutting the rim recess to allow standard spec brass. Then try your sized cases in the chamber. If they have trouble chambering from the chamber being too tight, then you can finish ream to allow them to chamber. It's better to sneak up on it, than go at it with a reamer and find out you have a loose chamber, but a good fitting rim.

[MostlyLeverGuns]

I would measure the rims of the brass I intend to use and go from there. The SAAMI range for the CARTRIDGE rim thickness is from .070 to .055. The CHAMBER rim thickness is .070 to .077. A minimum cartridge in a max chamber can be pretty sloppy. I have encountered a lot of brass that is on the low end of SAAMI specs. Very Old (purchased 1980) Winchester 45-70 brass I checked measured .063 rim thickness, old Federal boxed brass (again 80's) averaged .064, a max chamber rim (.077) gives .013/.014 'slop' or headspace. Rolling blocks do need the cartridges to be an 'easy' fit or hammer fall can be affected, but too much slop can affect ignition while the firing pin moves the cartridge into the chamber. If you need to shoot any cartridge anywhere than SAAMI minimum CHAMBER specs will always work.

[Dan Cash]

Measure the rims of the brass plus make sure the brass is not stopping with the case mouth on a too short chamber throat. If brass is at SAMI spec, investigate the cutting of the chamber. rim recess. Suggest that you confer with a quality, capable gunsmith.

[John in PA]

Add in a chamber cast to determine if the entire chamber is cut too shallow vs the rim recess only. You don't want to choke the cartridge at the case mouth, as that might increase pressure (though with a roller's inability to cam in tight cartridges, this may be a moot point) But overall short chamber might still inhibit easy loading of otherwise properly prepared ammo.

[bradley.moss72]

I had considered throating the chamber for heavier boolets or long seating standard weight boolets along with whatever it takes to get the rim straightened out.

I believe chamber casting is good advice to start with.

BW

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[EDG]

this is the solution


I would cut the rim thickness to sammi standard ( on rimmed cartridges this is the headspace) - .002 -.003 this allows for some rim variance and light wear of the tooling marks to take place. Another route done on rimmed cartridges ( mostly with 22 rimfire) is to cut to a large batch of ammo cases rims and use only that in the firearm then. Cutting the rim slightly shallow by - .002 - .003 will allow for brass to fit while losing very few to to thick. This will also have the benefit of a slightly short case length in the chamber allowing cases to be trimmed to exact length needed to fit the chamber.