I have searched several times and can't quite find the info I am looking for. I just finished measuring case capacities between the 338 RUM and the 338-300 RUM. I am deciding which one I'm going to use, and the RUM cases are as large as I want to use for an M-70 action. I started by using my 33 caliber Sinclair expander mandrel on both a new 300 RUM case and a new 338 RUM case. The OD of the necks measured the same after I expanded them. I then primed each with an intact spent primer. I didn't do any trimming or sizing. The 338 RUM will hold 106.1 grains of H-1000 and 109 grains of water. The 338-300 RUM will hold 110.4 grains of H-1000 and 113.9 grains of water. I measured the powder charges five times each being as consistent as possible in everything I did. I measured the case of water three times each, also being as consistent as I could. The 300-338 will hold 4.3 grains more of H-1000 and 5 grains more water. Five grains more water is a 4.4% increase in capacity, which should yeild a 1.1% increase in velocity. Given the 300 grain SMKs typically chrono around 2800 fps, that would give the 338-300 a 31 fps velocity advantage. However, many guys claim a 100-150 fps increase. At least that is what I have found from all the searches I have done, though I have seen no actual chrono results for a 28-30" barreled 338 RUM with a 300 grain SMK. To get a 100 fps increase in velocity at around 2800 fps we need a 14% increase in capacity. Given I already know a 338-300 RUM holds 114 grains of water, that would mean the 338 RUM's capcity would have to be 100 grains. I suppose if we are comparing a 3.6" COL 338 RUM to a 4" COL 338-300 we might get there, but if both chamberings are long throated a similar amount there just isn't enough case capacity to get a 100 fps plus advatange to the 338-300 RUM. For the 338-300 RUM (Edge), I have seen Shawn Carlock's velocities of 2800-2950 fps, with the standard load being 2800-2830 fps, from a 30" barrel. Like I just mentioned, I haven't seen RUM data for the 300 grain SMK from a 28-30" barrel What does the 338 RUM with a 300 SMK in a 28-30" barrel usually chrono? Is the comparison as I suspect--3.6 COL 338 RUM vs a 4.0 COL 338-300, are the Edge guys just loading hot, or is there something else going on? Thanks.