38-55 Hardness vs Velocity vs PC

[Dom]

Shot my Marlin Cowboy today with with my PC LBT 260gr GC bullet, sized at .380. BHN 10.5 from scrap WW & PC. As long as I stayed at 1400 fps ( chrono said 1406 ) it shot great ( 100 yd target shown ). Used IMR- 4198 powder. If I added just 1 gr more powder the group grew to over 2"with an occasion flier, way out. The same bullet cast from my BHN 16 alloy, sized & lube with LBT soft blue will shoot this same size ( small group ) at 1688 fps. So why? It was my assumption PC on a bullet would allow a higher velocity with the softer alloy. Does the PC bullet also have to be of the harder alloy to achieve accuracy at the higher velocity ? This Marlin has what Marlin calls the " Ballard Deep Grove " rifling. Not Micro Grove. Picture of the two bullets, one with PC, the other with traditional lube. What am I missing here.

[mnewcomb59]

That is a relatively long unsupported nose. No cast bullet is ever perfect. The soft alloy has more nose slump and whenever it hits 80000 rpm or whatever in a few thousandths of a second it slumps toward one side of the barrel because it is not perfectly balanced. Kind of like riding in the center of the merry-go round.

The harder bullet likely has slight voids in the nose too because no cast bullet is perfect but it slumps less.

These are 10 and 12 bhn bullets fired about 1600 fps (same load for both) but recovered down range in water where they slowed beyond their expansion velocity. Look how far the nose squashed into the rifling. This mold's nose is shorter and would slump less under the same acceleration as your rifle mold.

You can see the fired tumble lube 12 bhn bullet has even rifling engagement about half way up the nose. This load was accurate. You can see the softer PC bullet has rifling engagement 2/3 of the way up the nose AND that the side you are looking at has rifling engagement further up than the the back side. The off center slumped bullet was not an accurate load. The target tells you the limit of your alloy and velocity combo. When accuracy goes south you are either stripping the rifling or nose slumps a few degrees off center, not straight back any more like 75 fps ago.

[Dom]

I have harder alloy I am going to try. Perhaps it will retain it's shape better. Will take me a couple days to cast , size & PC. Would really like to hit 1600 fps which I have done with this bullet & BHN 16 bullets. I know it is possible. Just have to keep trying until I come across a solution. If needed I can go as hard as BHN 21.

[Dom]

Cast a handful of BHN 17 bullets today. PC ed. Got my load up 1 more grain pf powder to 1484 FPS. 260gr .380 dia. bullet. Decent group at 100yds ( shown ). Tried to up another grain & the group went to 3". I'm want to develop a 150yd load. If I can get close to1600fps I believe that will work. With the 260gr FP bullet I believe this level of velocity will give me the momentum needed for deep penetration on game out to 150yds. I have a very ample supply of old type metal ( lynotype ) available. BHN of around 21. This will be my next & last alloy to try.Attachment 285034