Where the 350g SMK is concerned, I wanted to see how fast I could push this bullet from a .375 H&H with several different powders. I was looking for a less expensive practice bullet that would offer similar flight characteristics to the high bc Cutting Edge bullets, allowing for more long range practice for my shooting dollar.
A few months ago, I e-mailed Sierra for load information using the 350g SMK in the 375 H&H using H4350 and H4831. A Sierra tech replied with the following:
68-72g H4350
72-76g H4831
I really expected H4831 to be too slow to work well in .375 H&H, but I thought that the heavier bullet might perhaps work better with a slower powder. Either way, I felt that trying H4831 alongside H4350 would tell me something about what the optimum burn rate range is for heavy bullets from a .375 H&H.
Along with H4350 and H4831, I also tried RL-17. Conventional wisdom has it that RL-17 is close enough in burn rate to H4350 to use the same starting load for a given bullet/cartridge combination. Most reports I have seen indicate that RL-17 tends to reach its max anywhere from about one grain below max for H4350 to about two grains above book max for H4350. With that in mind, I loaded my test rounds for RL-17 using the H4350 data that I obtained from Sierra.
For my velocity run up, I loaded five rounds at one grain increments from min to max for each powder, with the bullet seated .040" from the rifling (with a COAL of 3.9"+), and fired the rounds over a chronograph into a dirt bank about 50 yards away.
With H4831, velocity topped out around 2300 fps. As I expected, the rounds loaded with H4350 were faster, topping out at 2406 fps with no pressure signs.
RL-17 delivered a bit more velocity along with a couple of surprises. It was obvious that RL-17 was building pressure more quickly than H4350 in the first couple of rounds as the starting velocity was higher and climbed more quickly from shot to shot. The third shot delivered 2436 fps, already above where H4350 had topped out. The fourth shot showed about 50 fps LOWER velocity than shot #3 and the chronograph did not register the fifth shot. Interestingly, a friend who was observing the test stated that the fifth shot sounded noticeably "hotter" than the others.
Based on what I had read in a link I posted earlier in this thread, I was watching for a velocity drop off between increased powder charges as the warning sign to back off when using RL-17. Truthfully, I had not expected to encounter the drop off so soon in the powder charge run up. I had expected to encounter that at a much higher powder charge. As it turns out, actual max appears to be somewhere between 70g and 71g of RL-17 with a 350g SMK. In this instance, RL-17 appeared to behave as a faster burn rate powder than H4350.
In order for the 350g SMK to work for my purposes (long range or ELR), I needed to be able to launch it at 2550 or above. I am pretty sure I could get it to 2450 with H4350, but that is about it. However, the 15 SMK's that I put into the dirt bank dug a 6" diameter hole that went several feet into the ground and impressive dirt clod showers accompanied every impact. None of my 300g loads produced such results. The 350g SMK appears to pack quite a punch at short range.
One other thing of note was load density with the three powders tested. H4831 produced 100%+ load densities with every powder charge. I was crunching powder every time I seated a bullet.
Though I did not crunch any powder with H4350, I could not hear powder moving in the case when I shook the loaded rounds, which leads me to believe I was achieving near 100% load densities.
In contrast, using the same powder charges with RL-17 as I did with H4350, there was a lot of empty space in the case after seating a bullet. I could hear a lot of powder movement in the case when shaking a loaded round. Had I needed to seat the bullet deeper to fit an action shorter than the full length Magnum Mauser action that my CZ550 is based on, I believe I could have done so using RL-17 without a performance penalty. That is something that may be worthy of note for anyone shooting the .375 H&H in actions like the Remington 700 or Winchester Model 70.