Lou,
Sorry for the late reply but, I just joined the forum tonight. I too have one of the original .44 carbines and LOVE IT! Mine is stamped "Made in the 200th year of American Liberty"...so that puts mine as being made in 1976. Anyway, I _have_ loaded for this rifle and would HIGHLY recommend a Hornady reloading manual, since their 44 rifle loads we worked up FOR the Ruger carbine! My rifle seemed to like 24.5gr of H110 in Winchester brass using Federal 155 Match primers. It was shooting ~2" groups at 100 yards and averaged 1794 fps. Cycled without a hitch!
With your current load of 23.5gr H110, the Hornady book puts you right at 1700fps. One thing to consider when loading for these rifles is they do NOT like other brand bullets very much. Hornady bullets are 0.430" while other manufacturers measure 0.429"...and the Ruger barrel measures 0.430". I found this out the hard way when trying to load Barnes bullets...would not group AT ALL and key-holed like crazy. I call Barnes and talked to the owner of the company...and he just so happened to HAVE a Ruger carbine so he brought it in and tested/measured. His results were the same and apologized for the problem...even offering to exchange the bullets I bought for some I COULD use in my other guns. Top notice customer service if you ask me!
Back to the Hornady XTP's....in talking to Hornady they didn't really recommend anything lighter than 240's at the time since they don't handle the added velocity well (even though they do have load data for them). I also loaded some 300's but stuck with the 240's since I could push them faster and they packed more energy muzzle AND 100 yards! Needless to say they are down right NASTY on whitetail!!! I recently picked up some of their 225 FTX's to see how they do with H110 and Lil'gun. Only time will tell on the FTX's but, they are going to have to do miracles to beat the 240 XTP's!!!
-Phil
(edit/ps....A couple of things NEED to be noted! Hornady list 24.8gr H110 as a MAX load, so if trying 24.5, you would want to work your way up slowly just to be safe! AND...double check the rilfe MAX loads with the pistol load data. Some of the rifle loads are slightly hotter than what's listed for pistol so if using MAX loads, make SURE you check and/or keep them separate!)