I have accidently put on lay away a cz 550 .300 win mag...because it was at a good price and I love cz's

I was thinking about re-barreling to .338 LM but am now considering a re-chamber and putting a 4.5-14 leupold on it.

Is the .300 RUM what the .300 win mag should have been and is the .30-378 such a bad barrel burniner that it should not even be considered ??

Should I leave as a .300 win mag or re-chamber to .300 RUM or 30-378

I suppose I have a lot of action to work with seeing as it'a a magnum sized action !!

Your best bet is going to be to go with the .300 RUM. If you do that, then the only thing that has to be done is to rechamber and modifiy the magazine to feed the fatter cases. The 30/378 etc. are going to require modifications to the bolt face, plus the brass is too damned expensive.

The only way I'd step up to the Lapua or 30/378 would be if you were starting out on the 416 Rigby sized action. Not only is the action larger but the bolt face is the right size.

You are looking at gaining 120 fps (give or take a little) in the 300wby over the 300WM. Thw 300 RUM will give you about 300fps over the Winnie. I plugged the 180gr Partition into my Sierra Infinity program to give you a comparison of trajectory. Zeroed at 200 yds.

300WM - 3100fps muzzle velocity - 500 yard drop = 32"
300WBY - 3230fps mv - 29" drop
300 RUM - 3400fps - 26" drop

You are only looking at 6" less drop at 500 yards. But you are getting a huge increase in recoil, muzzle blast, extreme throat erosion, and the list goes on. Not to mention it requires HUGE amounts of powder to feed the RUM compared to the Winnie. Barrel life with the Super 30's is rated around 500-800 rds before the barrel is shot out. A winnie will give you 3000-5000. (depending on variables of course)

And the 6" trajectory difference is negligable in real world hunting situations. If you are taking those 500 yard shots you arent truly hunting. I believe in getting as close as possible to ensure proper shot placement. At extended ranges that isnt always possible.

Not to mention the cost in brass is extremely expensive in the Super 30's compared to the Winnie.