I corrected my earlier mistake about the bore of the MPA-7 (brain fart, sorry, its 7/8", not 7/10" as I initially said). The rule of thumb for cannon design is that the cross-sectional area should be constant or decrease as the air moves through the cannon. Start from the end of the gun, three 1/4" diameter barrels, and calculate the total area: 0.125^2*pi*3=0.147 square inches. The beginning of the air passage must be at least equal to this, but greater is better. A 0.5" bore valve will give you 0.196 square inches, which is greater than the area at the end of the barrels, and so is sufficient for three 1/4" barrels. The rule of thumb for accumulator size is I personally like to be oversize compared to the rule of thumb, both on accumulator volume and valve bore, but that's just me and I'm a bit of a monster sometimes.
Here is a thread talking about some of Kotori's and my ideas on cannon efficiency. The good part is near the bottom of the first page. There is also a thread talking about Kotori's and my general rules of thumb for cannon design, but I can't seem to find it. If we do find that other thread, I'd like to consolidate the two and get them stickied so they are more readily available to see, since they cover some important ideas on big gun cannon design.