Revisiting my mildcat .
Posted: 16 Mar 2024 17:40
Some yrs ago I commissioned a build . I had a Carcano with about .05 too much head space and an inexpensive 7mm barrel . I was torn between using the 6.8 case with a .007 neck up , or turning rims on 30-30 etc to make rimless 7-30 Waters . Well now 30&32 Rem brass is available so ........ anyway I went with the 7×6.8 SPC . Turns out the 6.8 reamer had a larger dia neck than the 7-30......
At the door and of it all I can freely chamber a .287 dia cast without turning or reaming the the 6.8 SPC RP brass . I shot a few 120 gr jacketed and gobs of 130-154gr cast and enough 7mm168s@178 gr to know that's not a happy bullet in the rifle . After much study I lengthened throat to .1 and have basically started over .
RP LRP brass
Win LRP
IMR 4350 22.0 ,22.3 ,22.5 ,22.8 to start .
Sierra Match King 180 BTHP
OAL at 2.475 with the top of the BT at the bottom of the neck.
This plan of action was derived from the 6.8 and 7.62×39 sharing case volume and an experiment many years ago in the x39 . I used 22 gr at mag length and 25 single fed with a 200 gr paper patch cast Spitzer . Literally a scoop it full to the bottom of the neck and shake enough out to get seating length . The 22 gr just made clean up , about 32kpsi in the .318 groove and .305 bore . At mag length it netted a whopping 1640fps . But I had a 24" and a .130 gas port . This rifle is only .284×.277 20" and gas port . It's also a long bearing length jacketed bullet . So I have a likely 40-42 kpsi and about 1gr of powder space available yet load it should be supersonic it may even make 2000 fps MV being "only" 180gr . I have zero data that supports any of this so it's possible I could have a pressure event .
The box says I need 1-8" twist , the old Mountain Mold design calculator said a bullet of 250 gr nearly 2" long was well inside the stability range . If it doesn't work no big deal, I think the box of 100 was a "I can't use this maybe you can" add on in a trade . There's 80 left and 2 boxes of similar bullets at 20 each . It's trigger time in any case .
At the door and of it all I can freely chamber a .287 dia cast without turning or reaming the the 6.8 SPC RP brass . I shot a few 120 gr jacketed and gobs of 130-154gr cast and enough 7mm168s@178 gr to know that's not a happy bullet in the rifle . After much study I lengthened throat to .1 and have basically started over .
RP LRP brass
Win LRP
IMR 4350 22.0 ,22.3 ,22.5 ,22.8 to start .
Sierra Match King 180 BTHP
OAL at 2.475 with the top of the BT at the bottom of the neck.
This plan of action was derived from the 6.8 and 7.62×39 sharing case volume and an experiment many years ago in the x39 . I used 22 gr at mag length and 25 single fed with a 200 gr paper patch cast Spitzer . Literally a scoop it full to the bottom of the neck and shake enough out to get seating length . The 22 gr just made clean up , about 32kpsi in the .318 groove and .305 bore . At mag length it netted a whopping 1640fps . But I had a 24" and a .130 gas port . This rifle is only .284×.277 20" and gas port . It's also a long bearing length jacketed bullet . So I have a likely 40-42 kpsi and about 1gr of powder space available yet load it should be supersonic it may even make 2000 fps MV being "only" 180gr . I have zero data that supports any of this so it's possible I could have a pressure event .
The box says I need 1-8" twist , the old Mountain Mold design calculator said a bullet of 250 gr nearly 2" long was well inside the stability range . If it doesn't work no big deal, I think the box of 100 was a "I can't use this maybe you can" add on in a trade . There's 80 left and 2 boxes of similar bullets at 20 each . It's trigger time in any case .