There are 16 drams to an ounce so 1 dram = 27.34375 grans (for practical purposes your 27.3 is close aplenty). Industry standard for the volume of 1 dram of Black Powder is .115 CuIn. Black powder weight can, & Will, vary a bit according to garanulation (# of F'S) & by lot, depending primarily upon the charcoal use. This is really of no great concern, the volume measures are fine. Generally speaking a 1 oz shot measure will hold about 2˝ (68grs) drams of black. You can multiply the wt of your shot dipper by that 2˝ to get an approximate volume for the desired load of other weights. Note that this is a Volume to Volume or Equal Volume load. This has been a long standing practice for BP Shotguns & usually will give good results. This will in fact give loads very close to what was for years considered the "Field" loads for shotshells.
A "Square Load" for a shotgun is defined as one in which the column of shot in the Bore has a length equal to the bore diameter. This will in a 10 gauge give approximately a 1Ľoz load & approximately 1 1/16oz in a 12 gauge. A Square load in any other gauge is virtually non-existant (Take note you 28ga fans, they don't shoot a Sq load by any defination).
