The 2 7/8" (100gr.) case is about max for good work with either BP or smokeless in my opinion(for low pressure guns). That comes from working with Sharps and rolling blocks in 2.1", 2.4", 2 7/8" and 3-1/4" cases.
: The 3-1/4" case was terrible working with BP, and also has too much capacity with smokeless too. What I mean is, it has too much powder capacity for either powder for easily working up accurate low pressure loads. I am talking about accurate loads that stay under 1-1/2" at 100 meters off the bags, iron sights black powder or smokeless. Fillers became necessary for smokeless due to excessive powder space.
: The 2 7/8" case was much easier and Ken Waters data developed up in a 2-3/4" rolling block are is useable for virtually identical ballistics. This means a 400gr. bullet, cast or jacketed can be pushed out at just over 2,000fps with low pressure. I would not use his W748 data- too many hangfires. As well, magnum powders work best, as middle burning rates become slow in such a large capacity, high expansion ratio case. The bullet moving 2-7/8" doubles the case capacity, so the powder pressure curve has a difficult time keeping up. I've used John Buhmiller's double primer experimentation with good results, but don't try it without consulting me or having knowledge of how he did it. You may have a blowing up accident.
: The 2.4" case, normally called the .45/90, in an NEF single shot is probably about all the recoil you'll want, and with it's available pressure limit higher? than the rolling blocks and Sharps, you should be able to run 2,100fps+ with 400gr.cast or jacketed bullets very easily. In a .45/70, with a pressure limit of 32,000PSI, 400gr. jacketed bulelts can be given almost 2,100fps or perhaps a bit better. That's from the 2.1" case. The 2.4" case will allow higher speeds yet. They will belt, giving you the feeling of good power, which they possess of course.
: Up to you, as the same velocities come from a larger case with less pressure yet.
: There are few bullets capable of handling velocities over 2,000fps with normal expansion. The Barnes 400gr. Spitzer and the various Barnes "X" bullets are about it, unless Woodleighs and Hawk bullets are available to you. My present choice in my .458 is the 400 Barnes at 2,250fps and 350gr. Barnes "X" at 2,350fps. I suspect the 2 7/8" case should do that in the NEF. I could easily do away wih the 400gr Barnes and stick mainly with the 350 "X" as they are both cloverleafing bullets in my rifle & to the same POI as. A 400gr. "X" at 2,200fps would of course, do the job here in B.C., as would any of the available 350gr. of heavier .458" bullets at appropriate velocities, be they 1,800fps(Reminton and Winchster) or higher with Hornady or Barnes. Speers seem to max out around 2,000fps and seem too soft if driven faster than that. Of course, some people's experiences differ. Depends on what you hit and how deeply you want them to penetrate.
: If the NEF is capable of handling same pressure as a .458 Win Mag, it will deliver the same velocities at reduced pressure. In other words, it should also match .450 Lott ballistics if you use the 2-7/8" case. Your choice, - is it worth it?
: Hope this helps.