Re: 850 Lapua
Take a look at the 325WSM with its approx 51mm case length and similar diameter, this would be in my opinion the nearest commercial cartridge to what you are proposing.
One of our administrators IanM has one and if remember correctly he has killed Elk at 800yrds plus, so your proposed cartridge would certainly be a potent killer especially with an extra 10grs or so of powder.
I think its a great idea, especially if you enjoy the process of case forming and preparation, load developed etc.
Most reamer manufacturers will make you a chamber reamer on reciept of a dimensioned drawing and can also offer advice on its developement, go for it.
"I meant to shoot the pike but the duck got in the way"
PS. Where would we be if Experimenters took the word of everyone who doubted their ideas. Ooh, and a more Quirky name might be required.