44 mag

I am trying to shoot a Marlin 44 with 20.5 grains of 2400 over a hornady xtp 240 nad have had cases come aprt twice now.

Could it be i need new brass?
what about the fact that while the load is good in my Ruger redhawk, the longer barrel makes a difference?

everything is for pistol loads. where do I get rifle information?

What do you mean "come apart", head separation? If things like that are happening, no wonder you said in your other posting that you "weren't impressed". I assumed you meant balistically. My error. Sorry. The problem is either the ammo or the rifle - logical examination of the evidence may shed some more light on it. The more information you can provide, the easier it will be to help figure it out, but here are my first thoughts.

The Sierra manual has pistol cartridges for rifles listed in the rifle section, I assume many of the others do, too. That load is well below listed max and shouldn't be the problem, but....

If it might be the ammo, then start with the basic parameters.
What kind of brass, how many firings previously, etc? Does it happen with new brass, too, or just reloaded rounds?
What is your OAL and are you crimping enough to keep the rounds from telescoping as they are fed? (It takes a heavier crimp to deal with the feeding in a lever action than to keep the bullets from lengthening in a revolver from recoil.) If you load the rifle then unload it by running the rounds through unfired, is there a change in the OAL?
What do the primers of the fired cases look like, both the ones that come through OK and the ones that don't? Telescoping will drive pressures way up, (it KABOOMs semiauto pistols) and if that were causing the brass failures, the condition of the primers would tell you that.

If it might be the rifle, it's time to get out your measuring tools. Compare the dimensional measurements of fired cases from both the Redhawk and the Marlin. It is possible the Marlin could have a poorly cut chamber. If so, the comparitive measurements would tell you that right away.

to narrow it down use the same ammo in your redhawk if you get no problem
i would think marlin if both redhalk and marlin give you trouble then it is ammo
this is just my thoughts