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“If someone has a gun and is trying to kill you, it would be reasonable to shoot back with your own gun.”
― The Dalai Lama, Seattle Times, May 2001
Yes, with 1:20" twist you should be good with the 300 gr. heavies! I'd still recommend that you slug the bore. Not sure what specs Chiappa uses for land and groove diameter but the SAAMI spec for rifles is different than for handguns so "standard" 0.729"/0.730" bullets are too small and will lead.
if you are looking to download some then H110 is not a good powder. 2400 should be more flexible. I like IMR4227 for my .44 mag. Marlin. Unique will likely be fast for a 300 gr. bullet and may not give you the velocity you want. I can't comment on the other powders.
Good luck!
Longbow
This may be of some benefit, or it may not. I have the Lee C430-310-RF mold and it actually drops +or - 310 grains; as weighed after powder coating. I shoot them in an old Marlin 1894 (1:38" rifling) using 20.5 grains of H110. If you believe QuickLoad, this should yield about 1,400 fps at about 24,000 psi chamber pressure from the 18" barrel.
I attached this comment to Longbow's post because I see the same performance at distance. At 75 yards, I get pretty good accuracy and maybe you can see a little bit of yaw on the target paper. At 100 yards, you can definitely see yawing, but they are not keyholing yet. Still grouping fairly well. No data beyond 100 yards.
If one sits in thundering quiet the soul dies slow instead of yell to the heavens for all to hear and behold the righteous and upstanding and ones of which should be held with tales of woe. By C.A.S. <--- Thats Me lol.
I believe the new Marlin 1894 44 magnums will have a 1-20 twist.
Political correctness is a national suicide pact.
I am a sovereign individual, accountable
only to God and my own conscience.
Are you familiar with gmdr levergun performance web site? Loads for your situation there.
Yes, I have been told by Ranch Dog that his 300 gr. boolits stsay stable out to something like 300 yards from 1"38" tweist if pushed to max velocity.
Another fellow rold me that the WFN's will also stay stable from the 1:38" twist if pushed hard. I was unable to get stability past 100 yard using any 300 gr. boolits I had or "J" bullets but in fairness they were not WFN style.
Regardless the OP has 1:20" twist so not an issue for him.
Longbow
BP | Bronze Point | IMR | Improved Military Rifle | PTD | Pointed |
BR | Bench Rest | M | Magnum | RN | Round Nose |
BT | Boat Tail | PL | Power-Lokt | SP | Soft Point |
C | Compressed Charge | PR | Primer | SPCL | Soft Point "Core-Lokt" |
HP | Hollow Point | PSPCL | Pointed Soft Point "Core Lokt" | C.O.L. | Cartridge Overall Length |
PSP | Pointed Soft Point | Spz | Spitzer Point | SBT | Spitzer Boat Tail |
LRN | Lead Round Nose | LWC | Lead Wad Cutter | LSWC | Lead Semi Wad Cutter |
GC | Gas Check |