FNM 84-10 headstamp, processed to 300BLK, won't chamber

[rbstern]

I've been making some subsonic 300BLK rounds using the Lee 312-155. Processed a bunch of 5.56 brass, cutting them down, forming and then trimming to length. All of the Lake City stuff worked out fine, shot well (4.0 grains of 700x, stays subsonic, nice accuracy). But I had a handful of FNM 84-10 cases, processed exactly the same way, and they won't chamber in my Ruger American Ranch 300BLK.

I'm about to go back over them with my digital calipers to see if I can spot the difference, but so far, I'm mystified.

[Mk42gunner]

Age or work hardened brass causing springback? I'm just guessing here, but you might try to anneal some of the FNM brass and run it through the last die again.

Good luck,

Robert

[rbstern]

Age or work hardened brass causing springback? I'm just guessing here, but you might try to anneal some of the FNM brass and run it through the last die again.

Good luck,

Robert

Could be. Gonna measure them with a micrometer and see what that says.

[bruce drake]

perhaps that brass is thinker than the LC and they have to be outside turned to proper diameter.

[rbstern]

I just measured one of my processed examples of FNM 84-10 brass: It has a neck diameter .009" larger than the Lake City brass I compared it to. It's either springback, as Robert suggested, or a thicker case wall, which, after bullet seating, causes the case to be too wide for the round to chamber.

I'm guessing thicker case wall. Hard to tell because I've chamfered the brass, and I don't have my calipers here with me, but my eyeballs are saying it looks thicker.

[Geezer in NH]

They do have thicker case walls as in reloading 223 they may need a compressed load vs. LC cases.

BTDT but I load starting loads to begin with. Never formed one to something else.

[runfiverun]

I'd have been mystified all the way to the scrap brass bucket.
there's plenty of LC brass around, just use what works.

[BK7saum]

GRUMPA had a list of different headstamps that worked for the blackout conversion and those that didn't. I've used LC and R-P with 0.3095 bullets in a 300 whisper. My guess is that the case walls were too thick.

Brad

[Pee Wee]

BK7saum got it. LC is all .1105 you can use up to .1200 all else is to thick. I stick strictly with Lake City only and I anneal all of them.

[rbstern]

Thanks, all, for chipping in with comments. Indeed, the FNM brass is on the "too thick/don't use" list. Will stick with Lake City and WCC, both of which worked well.

Pee Wee...what does your annealing process look like?

[BK7saum]

FYI, you seem to have found the list, but for anyone else looking, here is the thread that Grumpa lists neck thickness according to manufacturer.

http://castboolits.gunloads.com/archive/index.php/t-279513.html

Brad

[root]

I'd have been mystified all the way to the scrap brass bucket.


Thanks for the chuckle runfive. made my night.

Rich

[GONRA]

GONRA suggests MAYBE rbstern needs a Big Robust Press
to form the necked case reproducibly without any regard to fine details
(hardness, etc.) of the parent brass?