my bother in law has a Marlin 1894 44 mag, he is looking for a factory load that give good groups.

About the only thing he can do is accuracy test all the different ammo available locally. All rifles are different and it's almost impossible to predict which factory ammo will shoot best. I'm sure yall realize that it's not going to give same accuracy level as a bolt action on average. The marlin micro-groove rifling is a funny critter as to which ammo it'll like best.

As with any question of this type the final answer will be to use the loading that gives the smallest groups, and is suitable for the game. The only real way to find this answer is to purchase as many loadings as possible and shoot all of them. At the end of this process you will know your rifle better and know which loading to use.

I have the same rifle and use two loadings for it. I use only jacketed bullets in this gun, but lead cast may be in the future after deer season is over and I get time to play with it. I use Magtech 240 grain Semi Jacketed Soft Points. These give me acceptable accuracy given my limitations. I also have a box of Winchester White Box 240 grain Jacketed Soft Points.

I use the Remington 240gr SJHP. They do a nice job!