even though a shotgun is a self contained unit, not like a CF bullet. i think its due to the residual pressures / gas volume equilibrating to outside pressures. my experience is only on shotgun. i do get what the other poster is saying ie different pressures / same burn rates etc. but the peak pressures are just that, the maximum recorded pressure upon firing. it is not total volume of gas produced.

i like my subsonics high pressure, and low speed. it gives ultra reliability. the only way i get high pressure and low volume of gas is with ultrafast powders (in 12gauge). i even did a 21gram subsonic load but pressures were seriously low (but consistent).

the mod on a shotgun are chambers to bleed off gas / knocking off the residual pressure from the peak chamber readings. now this will vary because depending on A, the original pressure of the load and B, the gas bleed off rate. shotgun silencers are really huge, but then shotguns burn anywhere between 15grains to 40 grains of powder.

reading a burnrate chart is like the equivalent of reading the daily mail. utterly pointless. (i stand by that statement whole hartedly) especially when different calibre, different cases, different MAP are brought into the picture.