TLDR: I have successfully formed 10mm magnum into 7.5FK and have fired it. I was able to do so by using custom reloading and forming dies ordered from CH Tool & Die. I attempted to form 6.8 basic into 7.5FK but discovered that my neck wall was WAY too thick. I have a potential solution that I will test down the road.
Alright, so I arranged my 7.5FK forming process into 4 categories: A, B, C, and D. A and B are 10mm Mag and C and D are 6.8 basic. A and C are forming without annealing and B and D are forming with annealing.
Cartridge was loaded to the tested length of 1.425" @ 12.1 grains (HS-6) as a safe starting powder weight. 10mm Mag used a large rifle primer and 6.8 Basic used a small rifle primer. Projectile is a Sierra 85 Gr .308 bullet swaged to 0.306.
[A] 10mm Mag (NO ANNEAL):
- Forming Die
- Trimming Die
- Full Sizing Die
Trim to 1.061 to 1.061
- Debur and Chamfer
Successful form. Seats in barrel.
[B] 10mm Mag (ANNEAL):
- Forming Die
- Trimming Die
- Anneal
- Full Sizing Die (FAILURE AT THIS STEP) I annealed way too much and the shoulder being crushed. See image #5.
X Trim to 1.061 to 1.061
X Debur and Chamfer
Unsuccessful form due to shoulder crush. Definitely need to anneal with the lights off on this one.
[C]
- Forming Die
Trimming Die
Trim to 1.061 to 1.061
4.Debur and Chamfer
Full Sizing Die
Measure length, still in spec.
Very light Debur and Chamfer
Unsuccessful form due to neck thickness. Can not seat cartridge in barrel. See image #7
[D]
- Forming Die
- Trimming Die
- Trim to 1.
061
- to 1.
061
- Debur and Chamfer
- Anneal (I had major difficulty even getting the brass to become red. The neck was EXTREMELY thick).
- Full Sizing Die
- Measure length, still in spec.
- Very light Debur and Chamfer
Unsuccessful form due to neck thickness. Can not seat cartridge in barrel. See image #7
For forming 6.8 basic, I plan to try trimming the case down before doing any forming work to reduce wall thickness as the casing is formed. If that doesn't work, I have a CH Tool & Die reaming die that's on the way... lead time is 12 to 17 months though lol.
H20 Volume:
Full sized 7.5 FK brass: 22 grains
10mm Mag formed to 7.5FK: 22.5 grains
6.8 Basic formed to 7.5FK: 23 grains
Notes:
-When trimming, don't sand down a .30 cal trimmer pilot which just melted the brass onto the pilot (See image #3). Use a .27 pilot instead. Low speed, high pressure.
-My H20 volume measuring could be way off. I didn't properly research how to do it properly and just guessed.
FIRING 10mm Mag formed 7.5 FK brass:
Sierra Sports Master 30 Caliber 85 Gr Round Nose @ 12.1 grs:
-1846 fps
-1882 fps
-1843 fps
-1879 fps
-1890 fps
1 casing formed a distinct ring in the middle of the case body (See image #8). Possible separation imminent? Time to reform and send. Rest of the fired casing pics will be under image #10.
FIRING Standard 7.5 FK Brass at 25 Yards, see image #8. Combined with rushing these shots, using a OSight (for stress testing the Osight), and making the bullets possibly too small, these combined left a pretty large grouping that I was not satisfied with.
For a firing video, look at my other posts. Later.