H110/W296 Load Variations
Of course everyone is right to state in this thread that H110 cannot be downloaded - and then again, they are wrong.
While many load data sources only list a single max load for these powders, and state flatly that they must be loaded exactly as published with NO starting load, other sources list a range of loads with the same powder and bullet combo. Furthermore, the max load published in one particular manual is often quite different than the max load for the same bullet in a different manual. So clearly, some download is possible from the full max data. For example, Speer #9, 1974, gives a range of H110 with a 240g JSP from 20.5g - 22.5g (with the lower load generating 1274 fps). But if you look in Hodgdon #23, 1979, you will find just one maximum load for H110 with a 240g JSP, and it is way up at 24.0 grains! That is a pretty wide spread for a powder that "CANNOT" be downloaded.
I am NOT suggesting that you should just ignore the published data and try to download H110. Just know that by checking multiple data sources, you might find a published load for the same bullet that is in fact a "download" that you were thinking about, thus making it legit to try. In fact, I always try to check multiple sources for new load data I am considering, and if I find that most agree in general ranges, but maybe one is way off, either high or low, then I tend to avoid the strange one and try to keep loads in line with the common consensus from the other data sources.
I absolutely DO agree with the caveat that these are powders where you should be careful to not go below the minimum published in whatever source data you are using, and even then, be very cautious of any changes is sound or recoil when testing the lower loads so that you never get surprised by a stuck bullet.
Now to the OP's original questions - how to get a 1200 fps load with H110 and 240g plated bullets in a .44 mag -
(BIG storm blasting here right now - gonna save this and go back to edit with more info before I loose it all!) WOW - that was close - almost instantaneously when I hit the enter, we got knocked off line - couldn't have been even a second later when the power went out and monitor turned black! But the save made it though...
Anyway, back to the question - My first suggestion would be to consider the minimum load from the Speer #9 manual for a Speer 240 gr Soft Point with 20.5 gr H110, producing 1274 fps in a 7 1/2" SBH.
Secondly, do consider a .44 Spl load - I know you said you do not want to drop to the .44 Special level, but really, what is the difference if you get the same speed from the same weight bullet (other than saving some very expensive powder)???? And I am
not talking about overloading the Special - I am using published data that should produce less than 15,000 CUP.
I am loading 15.0 grains of W296 (same as H110) behind a 240g hard cast SWC bullet with a 3/4 turn crimp on the seating die, and I get measured 1156 fps from my Ruger SBH with an average deviation of just 8 fps.
NOTE THAT THIS LOAD IS FOR .44 SPL and requires correct .44 Spl cases, NOT .44 Magnum! Here's the background on that load:
H110 data for a .44 Spl is not particularly easy to find, and especially not for cast bullets. The Hodgdon #23 manual does show 14.5 gr H110 with a 240 gr JSP at 1192 fps and 14,600 CUP. It also shows 16.0 grains H110 with a 250 gr lead gas checked bullet for 1202 fps and 13,200 CUP. So I decided to substitute the 240 gr hard cast LSW and start with 14.5 grains W296. While I did get consistent ignition and good accuracy, the speed variation was quite wide, averaging almost 100 fps. This indicated to me that the load was probably just hanging on the ragged edge of almost too little for this powder and particular bullet combination in the .44 Spl. As soon as I upped the powder charge to 15.0 grains, everything smoothed out perfectly.
Basically this load produces muzzle energy equal to a typical .357 Magnum spitting 158 gr JSPs, but does it while staying within the very low pressure levels of the .44 Spl. Bigger holes in the paper with a nice 'medium' level of recoil from a big gun. And the estimated low pressures seem about right - the cases drop from the cylinder under their own weight.