I've tried bullets from five sources...........
One final question...... I have not seen any rifle bullets with gas checks while some of my revolver bullets do have them.
Are they not effective even if velocities are kept in the range I've mentioned.?
I suppose it's been covered adequately already but piling on. The Marlin Micro Groove rifling form can do well and even very well with cast bullets. That said the successful cast bullets will be
fit to the Marlin Micro Groove and likely enough those that work with the Marlin will not work very well with other rifling forms. Both bore and groove dimensions are different for Micro Groove. As noted Marlins have shipped with a wide variety of rifling over the years. The 444 generally has a very slow twist and is picky about what can be stabilized.
Expecting to vary bore and groove without changing performance falls in the category of never was and never will be.
That means cast bullets that perform well in the marketplace likely fit the majority of rifles but do not fit Marlin Micro Groove rifling forms. That can reasonably lead to a decision not to bother as it can be more trouble than it's worth.
Temper and gas checks will need optimizing for desired speeds or vice versa speed can be optimized for temper and gas check forms on hand.
This implies ideally getting a feel for throat dimensions and bore dimensions and groove dimensions with lead slugs or preferably chamber casts. Followed ideally by varying dimensions by alloy and sizing on your own bench.
supposed to be is a recipe for disappointment.
When you get it down then try plastic coating or paper patched.