The other factor nobody has mentioned is case make and capacity. There is a fair variation in case capacities and that in turn affects pressures. In 308 for instance, there is a grain weight difference between a safe H. VarGet maximum charge in Lapua and Winchester brass give or take a smidgen.
Throw in a noticeable pressure difference between the very 'hot' Rem 9 1/2 standard primer and a mild example such as the S&B or Russian PMC/Murom and you've now potentially got a 1.2-1.5 difference in maximum loads from these two factors combined.
And ..... 150gn bullets are not necessarily the same across all makes. Jacket hardness and thickness, variations in lead core alloys can change the amount of resistance or inertia two identical weight and dimension bullets produce on being forced into the rifling. More inertia means slower 'engraving' and also potentially greater resistance once in the lands/grooves. Time in this context = pressure. Once it starts to burn, the charge produces gas in accelerating quantities multiplied by elapsed time from ignition, and the only thing that stops the resulting chamber pressure going past the red line is forward bullet movement into and down the barrel which increases the volume of the combustion chamber.
Also, gas production v time is not linear - ie we don't see X cc gas produced by Y gn of powder Z per millisecond, as powder combustion 'feeds on itself'. If something slows the bullet movement / combustion chamber volume increase thereby producing higher pressures early in the burn, the rate of burning of the powder charge and hence gas production also increases as a result of the higher pressure environment. This creates yet more gas, yet higher pressure, and a yet higher combustion rate creating a potentially dangerous positive loop.
And that's before we take into account bearing surface length.
Taking a selection of 150/155gn bullets they range over:
155gn Sierra MatchKing (#2156) ........0.254"
150gn Sierra GameKing ................... 0.255"
155gn Sierra TMK ........................... 0.260"
150gn Lapua FMJBT ........................ 0.270"
150gn Sierra MatchKing ................... 0.271"
155gn Sierra MatchKing (#2155) ...... 0.273"
150gn Hornady SST Savage ............. 0.279"
155gn Hornady HPBT Match ............. 0.289"
155gn Nosler Custom Competition ... 0.290"
155.5gn Berger BT Fullbore ............. 0.293"
155gn Berger Classic Hunter ........... 0.326"
155gn Berger Hunting VLD .............. 0.337"
150gn Hornady BTSP ...................... 0.351"
150gn Hornady FMJBT ..................... 0.355"
155gn Hornady ELD-M .................... 0.357"
155gn Berger Target VLD ................ 0.359"
155gn Hornady AMax ...................... 0.376"
150gn Hornady SST BT ................... 0.382"
150gn Nosler Ball Tip ..................... 0.391"
155gn Hornady TAP ....................... 0.403"
150 Berger Target Flat Base ........... 0.408"
That's a 60% variation between the shortest and longest bearing surface lengths which will change the amount of bullet to barrel friction and hence pressure build-up markedly. FWIW, Hornady bullets tend to be at the longer end of the list and when I've used the old 155gn AMax in load development testing, I'd always found it would produce pressure signs earlier than equivalent Bergers or Sierras, so much so that I'd use a full grain less powder as my initial top charge.
AND !!!! ............... all this is before we get into individual rifles' freebores as they leave the factory, some makes (in particular Remington 308 Win 700s) now notorious for huge jumps into the rifling. At least that should be a constant in the powder and bullet companies' ballistics lab testing as they will use SAAMI standard dimension barrels - but that doesn't stop it being a major issue as between two individual rifles from different makes or between factory and custom.
