Load data varies because the firearms and component used in testing vary. A load worked up in a universal receiver and a test barrel with minimum chamber and bore dimensions will give higher pressures. The chamber pressure measured on this test setup will give direct pressure readings from a transducer wired to a pressure gauge and give the most accurate pressure reading.
The other method used in manuals is a strain gauge glued to the barrel and then calibrated with a cartridge of a known chamber pressure. This is a close ballpark reading BUT it is not as accurate as a direct pressure reading using the transducer method.
What is also important to know what the H.P. White testing laboratories have to say about chamber pressure.
When you load "hot" you decrease the useful life of your cartridge cases, I have 30-30 cases that are over 30 years old and only die when the neck splits. This is because the chamber pressure of the 30-30 is 38,000 cup or 42,000 psi, at this pressure the case never stretches to meet the bolt face and the primers always protrude from the rear of the case.
The loads I posted with the Quickload charts are below the normal "rated" pressure of the .223/5.56 and used for practice. These lower pressure loads were selected for using mixed brass with varying internal capacity "AND" making the cartridge cases last "MUCH" longer.
Below is a scrap brass bucket that contains primarily factory loaded Federal cases that have oversized primer pockets after the first firing. Meaning these case were made from soft brass and were useless for reloading purposes.
Then we have someone here who tells us he hasn't used a reloading manual in 40 years and post photos of over stressed cartridge cases and has the audacity to tells us the percent the cases were overloaded. So be forewarned you can't test brass without knowing the hardness of the brass and the actual chamber pressure the brass began to flow.
Your car has a speedometer you are to use, you have a tire pressure gauge you should use, and you can "NOT" tell your speed by wetting your finger and holding it out the window or just looking at your tires and tell how much air pressure is in the tire.
Bottom line, taking bad advice from faceless screen names in forums can be hazardous to your health especially when these faceless screen names have egos bigger than their ability. YOUR face is resting next to the action of your rifle when it is fired and "YOUR" face will be rearranged if anything goes wrong. There is a reason why firearms are built with reserve strength and at least twice as strong as necessary. And there is a reason why the data varies in your loading manuals and "YOUR" firearms were "NOT" used in the manuals you are looking at.
In closing we have some Yo-yos in our forums who "THINK" they understand Einstein's theory of relativity, "BUT" fail to realize there was only "ONE" Einstein.
You will be much better off reading the front part of your reloading manuals with information from experts on reloading than listening to faceless screen names with egos much bigger than their ability. This includes the faceless screen name who can tell if someone has a match grade chamber by reading the dimensions of a fired case the OP posted in the forum.
Notice: No faceless screen names were hurt or injured during the filming of this posting, and Clark and 918v names were never mentioned.