You're correct - the SMK is smaller diameter than the Berger. All other things being the same, this will reduce both pressures and velocities slightly as it reduces the bullet's 'inertia' or resistance to movement in the barrel. Smokeless powder combustion behaviours aren't fixed, but are variable depending on both the application (ie cartridge and its internal ballistics) and differences between individual components in the mix in any given application - factors such as fireformed case capacity (a mixture of case build and chamber dimensions), bullet to barrel 'fit', length of the bullet's parallel bearing surface, whether loaded to make a jump to the lands v in the lands v 'jammed' and if so by how much.
It may be counter-intuitive as one would assume that with less bullet inertia, MVs would rise, but up to a point it has the opposite effect, likewise seating the bullet into the lands usually raises pressures / velocities over a jump situation. This is because smokeless powders 'feed on pressures' - as they rise, the burn becomes faster and more efficient. This is why a relatively small charge increase in an already over-loaded cartridge often produces a much larger rise in chamber pressures than the % charge weight increase, or the normal two to one ratio (ie + 1% on the charge = c.+ 2% PMax) leading to disastrous results.
The other factor here is the actual internal barrel dimensions as given to it by the barrelmaker. A half to full thou' difference in bullet diameter between makes or models may make a design better suited the barrel, or the reverse. This an issue that is very well known in the 6mm calibre with many threads on it in these forums and features on the Daily Bulletin. There is an unusually large range of bullet diameters to be found in 6mm match bullets, and as a result, barrelmakers offer two different bore (ie land) dimensions. IIRC the 0.243" 105gn Lapua Scenar is a thou' or so smaller diameter than its Berger equivalents and therefore often shoots better in a barrel with the 'tighter' dimensions. As soon as I got hold of the 183gn SMK for my 284, this diameter reduction concerned me as it is not only down on rivals such as the Bergers but also on the older SMKs that I currently load in the cartridge making me wonder how it is going to perform. I've not had the chance so far to get an optimal loading for the 183, so don't know yet whether this is going to be a factor or not. (And anyway, any change in results could be due to other factors with such a significant change in the bullet design over its 175 and 180gn predecessors.)
The final factor is that all other things being the same, the 3gn weight increase will see a small MV reduction. The standard way to get ballistically comparable performance values for different weight bullets in a cartridge is to compare MEs the loadings produce in ft/lbs. So, a 180gn bullet at a typical 284 match rifle MV, let's say 2,825 fps, generates 3,190 ft/lb ME. The MV that produces this value with a 183gn bullet is 2,802 fps - a larger drop than one would expect.
