Tuning Old skool .45 big bore 3000psi tuning?

[mackeral5]

A few years back I traded by way into a beautiful Extreme Big Bore .457. Apparently this was one of the last made in Michigan before the sale of the platform to TX. it was a strange combo, a somewhat random 26.5" barrel. Perhaps it was all that was left to build that last rifle, I dunno? I didn't really care for the valve, hammer spring, nor hammer weight combination so in went a peek poppet and different hammer and springs. Ultimately this yielded more energy and tighter ES than as built in Michigan. Later it was found to have a badly machined muzzle and throat/lead, forcing a barrel swap. I purchased 32" of TJ's 45-70 barrel.

Fast forward 3 years and I am finally able to assemble the gun with its 32" barrel. The gun was designed around 4500psi fills, which I still find to be a bit wasteful, so my focus has been on a 3000psi fill. Making a gun built around max energy 4500psi to perform at 3000psi involved cutting weight from the hammer, reducing hammer travel, and reducing hammer-induced valve lift. There are other methods to accomplish this task, this is just the path I chose...

Currently I have a 4 shot 3000psi tune resulting in 925, 950, 920, 880fps when shooting bobs boattail 220gr hollowpoints.

Swapping back to the OEM hammer and OEM spring, a 4500psi fill produced 3 shots of NSA 302gr at 980, 921, 846fps, ending at 3000psi.

Next steps are to lug it down with heavy ammo and see how much energy it can produce at 3000 and 4500psi.

Those that have previously traveled this road, please share your experiences.

 

[mackeral5]

I casted up around 50 or so of an NOE mold 460-396-RF-AE2. Using the hollowpoint pins these are dropping around 368-370gr.

3 shots at 4500psi using the OEM hammer and spring. I can tighten this up through tweaking hammer travel and limiting valve lift, but this is at least a baseline.

913, 851,735fps

 

[ChrisS]

Though not the same platform, I went thru a similar contortion with my Cothran build. Started with a 24" barrel and airtube. That yielded like 300 ft/lbs on 3500psi with a 370 grain slug from an "Accurate" mould. Went to a 32" barrel and airtube and a 350 grain slug from a Lee mould. Now making better than 400 ft/lbs on 3000psi. Along the way I tried tons of different slugs all the way up to the old standard 405 grain slug for the 45-70 cartridge, even roundballs. 350 grains seems to be the best overall weight for energy in my rifle at least. And I have heard others make mention that they are using 350 grain slugs predominantly as well.

FWIW
Chris