What is the difference between L.U.P VS PSI?
P.S.I. (pounds per square inch) and L.U.P. (lead units of pressure) are used to measure chamber pressure in the various shotshell gauges. L.U.P. is calculated through the use of a lead crusher and was used for many years, but has been replaced by the more accurate P.S.I. arrived at through the use of the piezo system.
The rest of the article goes further in depth.
The L.U.P., also known as lead crusher method, cannot indicate the peak pressure created in many shotshell loads.
The P.S.I., or piezo method is the updated, industry standard way of seeing the quick pressure peaks of steel and heavier lead shotshell loads.
Pressures and Recoil:
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, because there are many great lead loads that were developed using this system, and are still perfectly safe.
(Get a recoil buttstock pad to make your shoulder feel better during recoil.)
If you are 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 Scientific Fact:
Lead units of pressure is an exact number, not a formula, not a guess or otherwise. LUP is a physical number generated by the actual pressure generated by that particular 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 particular velocity and gauge by doing so.
Higher L.U.P Loads Recoil Harder And Will Wear You Down:
Many times, shooters who reload and shoot a lot of shotshells may argue they can or cannot tell the difference. The difference may be ever so slight but in the end, shooter fatigue will always tell the difference.
Let’s look at this in a slightly exaggerated 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 let’s 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 equivalent to 131 of shooter (2)’s reloads. On that premise it’s more than clear that there is a case to be made for loads of equal velocity with less felt recoil as dictated by lead units of pressure.
Simply put the laws of physics state that 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 that is 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 stated before the older method of Lead Units Of Pressure could not measure peak pressure for the simple reason that 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 that in the absence of pressure there will be no velocity or recoil, either.
Confusing Misuse Of The Word “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. It never has and never will for the simple reason there is no measure of acceleration, resistance, time or distance involved.
Pressure Fact (L.U.P.):
As stated before the older method of Lead Units Of Pressure cannot measure peak pressure for the simple reason that a certain length of time, pressure must exist to completely crush metal.
This same time frame directly relates to acceleration and length of acceleration which translates into felt recoil.
Editor’s note: The above is a re-written piece from deceased master gunsmith, Kim Lockhart. The facts remain his, but it has been re-written for clarity. Original publish date: Circa 2001.