Hello shotgun..
I run my .375 and .380 -.384 boolits thru regular Star lubrasizer dies to lube them and install the gas check...Then I run them thru Lee type push thru dies to the size I need...Lee will make custom sized push thru's but there is a fellow on the castboolit forum that goes by the screen name of Buckshot that makes these dies also..I have him make my custom sized dies because his dies are better (longer bearing surface), cost less and he can have the custom dies to you in a short while as Lee is swamped and they have a long waiting period..Fact is Buckshot just put some sizers in the mail for me today...He also makes custom expanders for Lee, RCBS and Lyman "M" expander dies...
My CZ550 is the full stock model...here are the specs.
Groove .367"
Bore .361"
Throat .371"+ tapered to .368" at the lands
Throat length is .370" from case mouth to start of Lands which is quite long..
I can hand seat a .369"/.370" boolit in a unsized fired case...So I size my boolits .369"
If you order a mold before you make a throat slug I suggest that you make sure the boolit is going to be at least .369 in dia. with your alloy...If you use a a Star lubrasizer there is another fellow that on Castboolits that makes dies for them and at better price than Magma does...I am not sure but I think Buckshot makes custom sizing dies for the RCBS and Lyman sizers...
I have had this rifle for 3 years now but haven't shot it much because of other projects and work..I do have some rounds with lwt. boolits, 238grs loaded up for the next range trip...This boolit is from a 38-55/.375 Win. group buy at the castboolit site and started out at .380" in dia....
My long term goal with this rifle is is 2200 - 2300fps with cast and at least 1 1/2" MOA ...I am also working with a .35 Whelen, 8X57, and newly acquired .308 and .405 Win....lot's to do
As an after thought you may want to go to tthe castboolit site and offer to buy some boolits to try or contact Bullshop there and have him make you some boolits sized the dia. you want...I use his lube "Greenspeed"...His link is at the bottom om the web pages there..That way you'll have something to shoot while waitng on your mold..I would send you some of mine to try but I am sure there would be border issues with U.S customs..
Good luck...
One more trick..order Hornady reloading dies as they have a sliding bushing insert for their seating dies that you can hone out (a pain) or you can replace with a .375 bushing... That way you'll be able to easliy seat these fat boolits..These bushing are easliy removeable from the die..I use Hornady's for all my cast boolit reloading now...It has taken me along time to figure all this crap out!..