I've been loading .300 Win Mag and I've had a few cases split just above the belt. I assume it's because I was full length re-sizing. If I neck size only, can I expect to get more case life by head spacing off the shoulder instead of the belt, or will it not make a difference? Brass is hard to come by and I'd like to make it last as long as I can.

More load details will help.

Don't do a neck size only. Simply set your die to allow you to headspace on the shoulder.

As the Guys said, give us a break.

To forensically investigate the corpse we need background, history per loadings, powder, quantity , type.

Tell us the history of all aspects and we will figure it out.

Use the shoulder and ignore that darn belt and you should greatly increase brass life

Just push the shoulder back 3 to 5 thousands

Unless you have a situation like graybird had....an badly overly deep chamber

This situation will kill brass on just a couple firings as he found

Ted's got it right. I've got 7STW brass on its 10th firings and have not had an issue. Until something proves my process wrong I'll keep doing my normal which is right in line with Mr. Ted's suggestion.