It depends on several factors. Generally a 300gr in a .44 mag will be slow enough that the velocity wont cause leading without a gas check but if your bore is on the larger side or is rough you can get leading whether you have a gas check or not.
You want the bullets .001" to .002" over groove diameter which varies from barrel to barrel even in brand new factory firearms. My Marlin 1894 with Micro-Groove rifling slugs at .4315" which is quite large for .44 mag. Micro-Groove have special considerations for cast as well and pretty much always need a gas check.
If you have a rough or pitted, uneven bore you can also get leading.
If you have a really generous throat/forcing cone in your rifle/revolver you can get pretty bad leading there if you don't use bullets sized for that instead of the bore.
I had leading near the muzzle in a .45-70 that I shot plain-base cast through when I approached 1700fps. If I stayed under that I was fine.
In short, the answer is "it depends". A lot more detail is required to make an educated guess. If you haven't slugged your bore yet there is no way to tell even if other factors are known.
Where you can run into issues with hard cast is in medium loads where the lube is not enough to seal the gasses but the base of the bullet isn't filling the rifling fully allowing the gas to slip by.
The lube doesn't seal the bore at all. Wax can't seal 30,000+ psi. The lube just lubricates the interface between lead and steel so the lead doesn't gall in the barrel. Without lube you would definitely have leading but not because of a gas seal. The bullet should be large enough (oversized .001-.002") to seal the entire barrel on it's own. The bullet should be swaged down to groove/bore diameter in the throat/forcing cone. Gas checks are used generally when the pressures are high enough to damage/deteriorate the lead alloy used. It's essentially a condom between the hot, high-pressure gasses and the (relatively) soft, malleable lead.
