18 grains of what?
You can go up to around 19.5 grains of 2400, in a 44RM cartridge, under a 240gr bullet.
What is your goal for these loads? Fun plinking rounds, effective deer-hunting rounds, or just reliable function and accuracy? If 18 grains gets you what you need, well...that's probably plenty.
Well, if you want to hunt with it, don't be afraid to try another grain or two, particularly if the accuracy is good as you work up. I have a Model '92 in 44/40 that is pretty much the most popular gun in the cabinet. Everybody wants to try it out, except the guys who are hung up on AR's
First: What is the twist rate of the Henry. My marlins are 1 in 38 so it basically limits me to 265 and under grain bullets. 240 is what i use with my twist rate. I perfer JSP for accuracy, JHP i am experamenting with for hunting, (I could use some extra blood on the ground for tracking but I will not overlook accuracy for it) and Lead cast for plinking/close range loads non hunting season use.
Maybe herny uses a 1-20 or 1-24 twist rate. then you should be able to use the very largest grain bullets made. Many like the 300 grainers for large hogs to get into them well. I think the high grain for caliber issue was solved with the better bullets of today so for me 240 grain stuff is perfect. Look to Olin/serra for JSP and Hornaday for the XTP hollowpoints.
Second: Have you slugged you barrel to determine bore sixe. Marlin is big on oversize bores. Mine are Over size and right at correct size between the two. Having the bore slugged and using that info to order your bullets (Mainly lead cast stuff) Will improve accuracy tremendously.
Third: Powders and speed. My microgrrove barreled marlin loves fast speed and does not respond as well to cast bullets.(I think i have just not found the correct shaped bullet yet for it) My Ballard grooved marlin will shoot cast if I get the hardness correct to the spped I am shooting them at. To fast on to soft a lead bullet will give me some leading issues and the same is true for the opposte extreem. A good cronograph is a great idea. I load with H110 and lilgun for JSP and JHP and step down to trailboss for soft lead cast bulets. I find unique a bit dirty, and I like powders that I can't throw a double chagre with.
Fouth: Handgun verses rifle loads. Look at the rifle loads for rifles. Yes it would be a perfect world if my Smith 29 and my marlin shot the very same load the best but I have not found that to be the case. My handgun like SWC and the marlin does not like to load them. So for me it is easy the swc are for pistol and the rest for rifle.
Try those four hints and see if you get better groups. Open sights should not affect 50 yard groups.