How much room in a chamber for an unfired round?

[Kck]

In talking to a guy about cutting a chamber with a boring bar on a NC lathe (which I’ve never done or plan to do) I was asked how much room or wiggle an unfired round should have. I really didn’t know the answer as I’ve always cut my chambers with a reamer. Is there a spec that details how much room an unfired case should have in a chamber. I’m guessing no more than a few thou total but just enough to allow the brass to expand and then contract enough to eject?

 

[RWO]

Which unfired case? New cases from different manuf. can vary several thou., not to mention wildcats. You should look at the chamber dwgs. published by SAAMI and abide by those chamber dimensions.

RWO

 

[joshb]

What round are you chambering? You could follow SAMMI specs but you might get “a rat turd in a violin case”! More info from you is needed!

 

[gunsandgunsmithing]

Are you going to go the route of custom dies? If not, the die will dictate the chamber dimensions. Yes, really!
Actually, I've spec'd chamber reamers based on the die I want to use for years. It's no different than spec'ing a custom die that works with a given chamber reamer except the other way around and does away with the need for custom dies to properly size your brass.
As others have said, really need more info for the best answers.
You could certainly work from a reamer print and then worry about dies afterwards.

 

[Shawnba67]

The advice about the sizing die is solid. Especially since a boring bar chamber will cost the same no matter what numbers you go with! Think what you will save in brass cost with a perfectly sized chamber!

 

[Dusty Stevens]

Make it a tad small to be able to polish out the machine marks

 

[Ggmac]

In talking to a guy about cutting a chamber with a boring bar on a NC lathe (which I’ve never done or plan to do) I was asked how much room or wiggle an unfired round should have. I really didn’t know the answer as I’ve always cut my chambers with a reamer. Is there a spec that details how much room an unfired case should have in a chamber. I’m guessing no more than a few thou total but just enough to allow the brass to expand and then contract enough to eject?

Saami publishes maximum cartridge size and min chamber

 

[Kck]

338 Lapua was the chamber in question and brass would be Lapua as well. I’ll take a look at the saami specs for the chamber dimensions. I don’t think he’d go with custom dies but my concern would be the first shot with virgin brass and if the case may have the ability to stretch too much and separate.

 

[tobnpr]

Is the intent to cut the leade with the boring bar as well?

I'm not a pro machinist...but I would question whether a boring bar-cut chamber could be done to the same exacting tolerances as a precision ground form tool. Regardless of the rigidity of the machine holding the tool, a boring bar of the small size needed to cut even a .338 Lap chamber would seem to have "some" lack of rigidity.

Plus, no way to actually measure/verify the tolerances of the end result. I'm not clear as to what is the intended "advantage" of this approach...

 

[RWO]

338 Lapua was the chamber in question and brass would be Lapua as well. I’ll take a look at the saami specs for the chamber dimensions. I don’t think he’d go with custom dies but my concern would be the first shot with virgin brass and if the case may have the ability to stretch too much and separate.

Why would you worry about it? If the chamber is cut to SAMMI specs, factory ammo will work perfectly well. That is why SAMMI specs exist. Why you would consider boring rather than reaming is a mystery to me. Seems to me you are asking for trouble. I doubt there is a single commercial gun maker that bores their chambers and all of them have CNC machine tools.

RWO

 

[Doug Beach]

I’d be interested to see what the throat looked like, given the long stick-out, and interrupted cut.

 

[Kck]

Why would you worry about it? If the chamber is cut to SAMMI specs, factory ammo will work perfectly well. That is why SAMMI specs exist. Why you would consider boring rather than reaming is a mystery to me. Seems to me you are asking for trouble. I doubt there is a single commercial gun maker that bores their chambers and all of them have CNC machine tools.

RWO

Not worried about it as it ain’t my gun or my build. I questioned it because, other than pouring a mold of the chamber, I told him I wouldn’t know if it’s cut to sammi specs as he’d have no way to check ALL dimensions of the chamber down to the throat. As stated, I’d never bore a chamber due to push off and tool marks but the question was raised to my by another fella.

 

[SS GO DEVIL]

Wow. This boring bar is gonna be sticking out quite a bit to make it in the neck portion of the chamber. Not to mention its gonna be long and slim. Deflection is gonna be a problem. If it works I would love to see a video of chambering session.

 

[rardoin]

Wow. This boring bar is gonna be sticking out quite a bit to make it in the neck portion of the chamber. Not to mention its gonna be long and slim. Deflection is gonna be a problem. If it works I would love to see a video of chambering session.

Yeah.....it will be singing like a prison snitch! Wear ear plugs....

 

[SS GO DEVIL]

Now that i think about it, its possible. I would single point cut the body and shoulder. Use a standard neck reamer. Use hand throating tool for freebore. Possible. This is an interesting project. Then fire a couple rounds of virgin brass and have sizing die made to exact chamber dimensions.

 

[Shawnba67]

I had formed a plan to cut chamber necks using a boring bar. I planned to grind the bar from hss in the relative shape of the chamber so as to have as thick a shank as possible.
Then i measured runout on some chucking reamer-ed holes and ordered a $15 reamer.