I assume that "The Book" data was using a 22 inch barrel as well? Being equal in barrel length across the board...... ? At this moment I am not even sure I have a Speer #11, and even if so I have not looked at a load manual in 20 years I don't think....... so I really don't know the answer to my question. Example, if the book was using 18 inch barrel, that would explain some of the increase in velocity, 100-125 fps or so ........
Unfortunately I have not tested a flat base 400-405 conventional in recent years. I have an 18 inch gun set up to test pressures, but I am way behind on 45/70 data. The last tests I did was with the 400 CEB Lever Solid and we can use 45-46 gr of RL 7 (Depending on the BLEND of RL 7--discuss this next) until we hit 45000 PSI which about as much as you want to push a Marlin on average. In a 2014 test and Blend of RL 7 we used 46/RL 7 and the 400 Solid for 1821 fps at 47000 PSI, and 48/RL 7 for 1895 fps and 52000 PSI, both over max pressures. But, keep in mind, this 400 CEB Lever Solid has to be seated very deep to meet overall length for the Marlin action, there is more bullet in the case than a 400-405 conventional flat base bullet, which makes it a tad more difficult to load.
Even with your longer 22 inch barrel I would say at 52/RL 7 and the 400 Speer you were somewhat over max just by the velocity..... 4 inches more barrel should put you around a gain of 100-125 fps
Also, all lever guns are not created equal.......... I have had Marlins and WInchesters that would handle much more pressure than another just like it. I have had loads in one gun that worked perfect, and take them to another, same make, model and cartridge, and it would lock them up tight. In the Marlins if you keep them at 45000 PSI or less, then I have not seen any of them have issues.
The Reloader, RL powders are notorious for serious differences, and it can be from one container to the next, regardless of Lot # or date of manufacture. RL Powders are great, but you have to be aware of these issues or you will eventually find yourself in a bind. Other powders can also experience differences, but I have not found them as excessive as the RL's. For this reason I normally blend several pounds of RL together, then do some pressure tests with various cartridges to see where this Blend is. Now, obviously everyone does not have pressure equipment to work with. What you can do is just keep an eye on your velocity. If your velocity is lower, or higher than the last blend you checked, then you can adjust for that. Remember, there are no free rides, and it is very rare you will find a magic powder that will give you MORE VELOCITY and LESS PRESSURES......... Magic powders are very rare and elusive. If you are getting higher velocity than expected, you can bet your ass you are also getting added pressures.............
You take all your RL 7 powder, pour it in a clean dry 5 gallon bucket, and mix that..................
One well mixed, you repackage and label the Blend...... I use the date that it was blended..... and then you test that against known loads from the previous blend............
Remember, all RL 7 is not created Equal. All Lever guns, same make, model, and cartridge, are not created equal. And the list goes on...........
For those with Winchester 1886 and M71s that have been coverted to larger caliber, these guns are stronger than the Marlin action and can easily handle at least 50000 PSI, perhaps a tad more. I more or less left my 45/70s behind many years ago, using my 50 B&M Alaskans, true .500 caliber. Caliber makes a difference.........