Critter;
I was in the development stage of building a 408cal on basicly the same case as the 416 Barrett. Both bullets are close to the same weight, 400-419grns. 3300fps is advertised from Barrett, you can get 3000fps with a long barreled 408C-T with much less rifle weight. I put my project on hold waiting for a better bullet, 45cal seems to be a good choice for the case size. I also wanted a long range lead cored bullet, not just a "lathe turned solid".
The Barrett case has a head size of .800 vs the 408 case at .635 witch translates into a much smaller action, the Barrett action minimum dia 1.900" vs the 408case action size minimum dia of 1.580" Both actions are about the same length. Barrel sizes, the barrett bbl needs to be at least 1.625" dia at the shank and a 408cal is 1.350" at the shank.
There are better wildcats out there based from the 408 case, 338cals and the 375cal, both will out perform the standard 408 C-T, exceptional accuracy with the 338 versions. Single shot rifles from 13#... Tactical rifles built for the wildcats run around 16# on up... vs the barrett style guns, 28# on up.
Balistic wise, there is not a whole lot of difference, i dont know what the Barretts balistics are, they have not released any pertant info, i asked and all i got was, its still in testing. I have built several 408C-T rifles, they shoot very well, but not to advertised load speeds, i found 2850 to be about tops in a 29" barrel. You are stuck with one type of bullet with either gun, monolithics, "lathe turned solids" Not a good choice for hunting or punching paper, they are made to shoot extreme ranges and hold a certain MOA tollerance, mainly to penetrate hard targets.
With a 338 designed 408 wildcat you have the choices of the range of High BC match bullets, the 300 Sierra match king bullet, with bc's in the low .800's, speeds in the 3400's will really excell for long range shooting and hunting. If and when Sierra makes a match bullet for the 375cal, it will also be a good choice based on the 408 case.
The 416 Barrett is a shortened 50BMG case, about 3.2" long. The 50 Spotter case is 3" long. The 408 case is a re-designed 505 Gibbs case, the case was strengthened to handle the 65,000 chamber pressure of the 408 C-T rifles. The 408 case is 3.030" long. One big difference is the primers, all 50cal cases take the big BMG primer, the 408 case takes std large rifle mag primers. Case capacities are as follows: (416 Barrett 200grns, 408 C-T 130grns)top loads.
In my opinon, i would build a 408 based wildcat or the 408 itself before the Barrett. If i wanted a "bigger rifle" then maybe the std 50 BMG or a wildcat on the 50 spotter case. A 50 spotter improved will have a case capacity around 200grns, couple this with a 750Amax bullet and you will acheive speeds "close" to that of a 50BMG rifle. The BMG rifle i had, top load was 210grns of powder, Lilja bbl, 34" long. rifle weight 32# Personally i like the lighter carry rifles, 16# range.
I hope this helps
Dave
