Fls cases at barrel change

I change barrels a lot, using fired cases, an RCBS Precision Mic to measure case headspace, and plenty of time. I resize cases with a Lee Collet Die, and almost never FLS. Some case dimension can/does change such that a FIRED .000" headspace case will NOT go in a barrel headspaced with a .000" headspaced FLS case.

After firing 223 cases in a Stevens 200 bbl, and changing to a M12 HV bbl, the .000 headspace cases would NOT go in the M12 bbl. headspaced with fls .000" cases. FLS the ,000" headspace fired cases keeping headspace .000", and the cases fit just fine in the M12 bbl.

I suspect that the M12 fired cases WILL fit in the Stevens 200 bbl. I don't know WHAT dimension increases in the Stevens bbl.


Headspace with fls fired cases only!

I do the same. Neck size only my 223 using the Lee collet die. Neck runout is virtually zero, and rarely have to trim. Occasionally anneal. Never a split neck.

When I removed my 22-250 bbl and spun on my Criterion 223 for the second time, I used one of the fire formed cases from previous loads to set headspace. According to my measurements, my cases have grown .001 to .0015 from the previous loadings of these cases. So I guess I got it pretty close when setting the barrel up.

There is another possibility with regard to using brass from one chamber in another and that is diameter of the case just above the head. Is one slightly bigger than the other?

Every chamber cut is different even with the same reamer. A FLS case is not a head space gauge. Head space gauges are undersized on the body to make sure the shoulder to bolt distance is maintained.
Because of spring back a fired and full length sized case may be larger than one fired in another chamber, even after being fls. That alone is enough reason to cause a non fit.
I only change 20 or 30 barrels a year. Use headspace gauges when available and a few dedicated fired cases in my wildcats. I have cut several chambers with the same reamer and these have all worked with the same dedicated "case/gauge" but I would not even expect they could possibly work in a chamber cut with a different reamer.

Yep^ the shoulder diameter is probably different.

I quit using head space gauges a long time ago. Way too big and when sizing brass, you have to move the brass too much. Moving brass only shortens it's life. The less movement the better. I only shoulder bump and use a body die as needed.
99.99% of my rigs are Wildcats so I couldn't use factory loaded ammo even if they made it. I've tried. I only full length size for my gas guns.
If I were to use gauges, I would consider setting the head space on the snug side. Again, less movement, longer brass life.
Whatever works for you is best. :cool: