Thanks for the comments guys, I'll try to answer a few of your questions.
I took an Alaskan moose last year with the 50 Alaskan shooting .510 lever gun safari solids from Cutting Edge Bullets. Those 400 grain CEB's were traveling a little over 2000 fps from the 20" barrel of my Marlin 1895. I shot the bull twice in his left shoulder and both bullets exited. The bull turned to face me and I shot him in his left shoulder again at a steep "quartering to" angle and that CEB penetrated about 8-9' of moose and lodged just under the hide of the bull's right hind quarter. The bullet was recovered and showed no signs of deformity other than rifling marks; it looked like it could be loaded and shot again. If you're interested in reading more about that hunt it is featured in the September-21 issue of GUNS magazine.
I received the 50-110 from Turnbull in February of this year. With the need to obtain pre-approved gun permits just a few months away, I had little time for load development before deciding on a load and practicing extensively in an effort to become proficient with my new rifle. Based on my experience with the 400 grain CEB solid in the 50 Alaskan, I felt confident in the bullet's ability to effectively take a cape buffalo if fired from similar velocities from the 50-110. I also wanted to try the 450 grain Swift A Frame based on my prior large animal experience shooting A Frames in the .348 Winchester, 454 Casull, and 44 Mag.
My 50-110 was built on a Miroku Browning 71 action and I felt that it could be loaded to higher pressures than the older antique rifles. However, I wasn't trying to push the envelope with the rifle / cartridge combination. My goal was to drive a 450 grain A Frame to 2000 FPS which would make it legal for dangerous game in African countries such as Zimbabwe. I did a google search on modern 50-110 loads since and I stumbled across an old post from gunner500 here on 24hourcampfire. I sent him a PM and he informed me that he had had great results with RL15 in his modern 50-110 conversion. I started with 75 grains of RL15 and worked my way up slowly until I was pushing the 450 grain A Frame to a little over 2000 FPS. Being a monolithic solid, the 400 grain CEB is actually significantly longer than the heavier A Frame and the same powder charge completely fills the case. Shooting both loads over the chronograph revealed that the heavier A Frame left the barrel about 30 FPS faster than the lighter CEB solid. I suspect that this has something to do with the fact that the A Frame is .509" and the CEB is .510? At any rate, both loads provided great accuracy and shot to the same point of impact so I was set.
My PH was unsure about the penetrating ability of the lever gun so he asked me to load up completely with solids; suggesting that we do a penetration test with the 450 grain A Frame "softs" post mortem. My first shot on the buffalo was at a steep "quartering to" angle with the 400 grain CEB. The bull collapsed at the shot, fell over on his right side and immediately started bellowing. The bullet broke the left shoulder, penetrated the lungs, stomach, and intestines, coming to rest just under the skin in front of his right hind quarter making a golf ball sized lump. That bullet was later recovered and (as expected) looked like it could have been loaded up and fired again.
After the first shot, I worked the lever and and my PH and I ran forward. He stopped and asked me to put a "soft" in the chamber so with trembling fingers I extracted the CEB load and somehow managed to get a 450 grain Swift load in the chamber. I shot him from the front hitting him high in the chest. That bullet penetrated the chest cavity and made it into the stomach / intestines where it was not recovered. With the bull still lying on his right side, we swung around to the bull's left and I put another solid low through the left leg, penetrating the heart, lungs, and lodging in the spine.
Hope this helps answer some of your questions, thanks for taking the time to read my ramblings.