The L.U.P. lead crusher method can not indicate the peak pressure created in many shotshell loads.

The piezo method is electronic and can see the quick pressure peaks of steel and heavier lead shotshell loads.

The L.U.P. pressure test method was always a superb indicator of felt recoil for a given load and not peak pressure as it was intended. This does not mean that data using the L.U.P. system is dangerous, there are many great lead loads that were developed using this system, and are still perfectly safe, if your lucky enough to have manuals that list units of L.U.P. and P.S.I. then cross reference them this way L.U.P. for felt recoil factor and P.S.I. for safety on peak pressure. However I would not trust any data for steel shot, bismuth shot or buffered lead loads that were not tested by a reputable source with the piezo system of P.S.I..

 L.U.P. is a science fact, not a guess!
 Lead units of pressure is an exact number, not a formula, not a guess or otherwise, it is a physical number generated by the actual pressure generated by that imparticular shotshell acting against a piece of lead in a pressure test gun. Always when loading shotshells look for the highest velocity with the lowest L.U.P. and you will have the lowest felt recoil for that imparticular velocity and gauge by doing so.

 They do recoil more and will wear you down!
 Many times shooters who reload and shoot a lot of shotshells may argue they can or can not tell the difference, the difference may be ever so slight but in the end, shooter fatigue will always tell the difference. Lets look at this in a slightly exagerated perspective, take 2 reloads, load (1) gives 11,000 L.U.P., load (2) gives 8,400 L.U.P. and both deliver a 1 1/8 oz. load of shot at 1,200 feet per second velocity, now lets take 2 shooters and fire 100 rounds of these loads, shooter (1) shoots load (1) and shooter (2) shoots load (2) after firing 100 rounds, shooter (1) will have absorbed the felt recoil equivalant of 131 of shooter (2) reloads, so on that premise I think its more than clear that there is a case to be made for loads of = velocity with less felt recoil as dictated by lead units of pressure.

 Recoil fact!
 Simply put the law of physics says for every action there is an equal and opposite reaction. In light of that fact it is more than fair to say that all shot loads of equal weight and velocity recoil the same.
 However there is an acceleration time frame chemically engineered into all reloading powders that cannot be ignored either and that acceleration time frame equates into what a shooter feels on his shoulder or felt recoil.
 (Did you achieve full acceleration of shot load to peak velocity in 6" of barrel or 26" of barrel as this equates into the felt recoil factor.)
 As I stated before the older method of Lead Units Of Pressure could not measure peak pressure for the simple reason a certain length of time, pressure must exist to completely crush metal, this same time frame directly relates to acceleration and or length of acceleration which translates into felt recoil.

 Pressure fact (P.S.I.)!
 Everyone knows and reads in almost all reloading books that pressure has nothing to do with recoil for the simple reason you can blow up a gun with overpressure and have no velocity and therefore no recoil, and this is true.
 However also true is the fact in the absence of pressure there will be no velocity or recoil either.

 Confusing misuse of the word, pressure, should be peak pressure! Simply put the word pressure is used all the time in place of peak pressure and this is wrong. Peak pressure (measured in terms of P.S.I.) has nothing at all to do with recoil never has and never will for the simple reason there is no measure of acceleration, resistance, time or distance involved.