Bob,
Leading with cast bullets can be caused by many factors. Lube breakdown, improper size for barrel, improper hardness for the operating pressure of the cartridge, etc.
I have used the 280gr. WFN Gas checked LBT design in the 444 Marlin. (same offered by Beartooth and Cast Performance) You can have a problem fully chambering in certain rifles with shorter throats due to the wide driving band ahead of the crimp groove on this bullet. I shorten my cases .1" and use loads for one of the jacketed 300gr bullets such as the Hornady XTP. Bullet seating depths are the same ie., the 280 in shortened cases compared to the 300gr. jacketed in the full length cases. Hence I use load data for the 300gr. jacketed bullets. With IMR 4198 I get 2185 FPS avg and an extreme spread in the high teens. AA 2200 gets about the same. To me, the 280 is an ideal weight for the 444 Marlin. In the shortened cases it feeds excellent also. In effect, I've lengthened the throat by doing so.
You will also find that with sized bullets with gas checks, the checks themselves can run .0005 to .001 above sized bullet diameter due to spring back of the cup after it's trip through the sizing die.
The key is to slug your barrel and determine the groove diameter dimension. In actuality you want to determine the throat diameter and have your bullets sized to that dimension or just under. This aids in bullet support so it starts square into the rifling upon firing. I believe .432 is about right for the Marlin but check for yourself. Micro Groove rifling is more critical here with regard to proper sizing but will shoot exceedingly well with cast bullets if they are sized properly.
Head over to the Beartooth Forum and check out the wealth of info on shooting cast in the 444.
Sorry for the "book". Have fun with that rifle!
Regards, Ray
Disclaimer: The above referenced practice with regard to shortened cases though proving quite safe and satisfactory in my rifle may or may not be in yours. Your milaege may vary.
