The BB 38Spl and 38+P loads are ALL tuned for 2" barrels. In fact with a gain of only 60fps going to a 4" barrel, that would indicate that the powder charge is getting mostly burned up in the first 2" - normally I'd expect more boost out of a 4" over a 2" (closer to 100fps difference).
BUT: while Pythons are accurate, they're not known for being "fast spitters" like the newer S&W tubes and most Rugers. So the test out of a 4" Python may not mean that much...
What really matters is "how fast is the slug going, and what speed does it need to expand?"
We know that 158gr all-lead hollowpoints by Remington expand well at 850fps, which they usually get in a snub when loaded by Remington at +P. Assuming the Buffalo Bore is ballpark similar a slug, then the 850-870 range it gets in standard pressure should be fine, and the 1,000fps they get in +P and a snubby will be great
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OK, so what if you have a 6" barrel 38Spl gun? Not a common setup but they do exist...they tend to be older models like the Colt Officer's Match 38 (the Python's ancestor) or various old S&Ws. If you shoot BuffBore +P in these, first, you're accelerating the wear of a classic gun and second, you might have been better off shooting the BuffBore standard pressure version.
Any hollowpoint can be "overdriven" - move it too fast on impact and the nose will expand but then "shred" and you're left with a shortened wadcutter that's maybe 30% lighter than the original slug and only slightly fatter than original if at all. This isn't optimum, although it'll still hurt
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The BuffBore +P variant might end up overdriven out of a 6". The standard pressure load (starting out 150fps or so slower) is more likely to work from a long tube.
As a bonus you put less wear on a vintage gun.
Another point:
It's easier to get this "long mild pressure spike" effect with a large-case-capacity shell. A lot of people successfully drive 44magnum energy levels with the same weight and speed of bullet in 45LC from a strong gun (Ruger full-size single action, or Redhawk, SuperRedhawk, Colt Anaconda, etc.). When you do, the 45LC+P setup will get that work done with as much as 10,000psi less peak pressure versus 44Maggie because the 45LC's case capacity is bigger. Firing these bigger-shell recipes gives you as much recoil as on an equivelent 44Mag load, but the 45LC's "recoil profile" feels "mellower". It's like the difference between your hand getting pushed on versus slapped - the total energy delivered to your hand might be the same, but the way you feel it is radically different.
We're seeing some of that effect with the BuffBore 158+P. Yeah, the energy is really there but because it's tuned to issue that power over a broader area of time (even if we're only talking tiny fractions of a second), the power delivery isn't as horrid as the raw numbers might suggest
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