Once again I'd like to refer you to the December 2012 issue of "Handloader" magazine. There are a couple of articles related to chamber pressure that are well worth reading if you roll your own cartridges. One thing that caught my attention here was references to a bullet's "J factor", i.e. the bullet's resistance to being pushed down the bore. The J factor is determined by the bullet's jacket shape and materials. An example given is the comparison of the .243 Hornady 100 grain RN and the pretty similar looking Speer 105 grain RN. The Hornady has a J factor of 1.19 while the Speer has a J factor of 2.35! What that means for us is that if you substituted the Speer bullet in a load you developed with the Hornady you could have a very unpleasant surprise when you pulled the trigger! I’ve always known that switching bullets would make some difference to a load but now I see that I have underestimated the possible effects. Luckily I’ve never had any problems related to this but I’ll be more careful from now on.

This months issue of Handloader you say? Curious if they have any form of a calculation that the general population can use to determine how they derive at their numbers for the "J" factor. I have known alot of this principle has existed for many years insofar as cetain bullet manufacturers had many different "glideing metal" alloys they used which all related to the chamber pressures and bullet speeds as well. This was one of the reasons I bought a chrony many years ago. Extreme examples for me that proved this principle were the use of moly coating on bullets and at the other end of the spectrum the original Bares X bullets when they came out. Moly coated bullets although slicker than greased Owl stuff tended to run slower , pressures were reduced and as a result powder increases were needed to match the speeds of the same bullets that were uncoated. In addition to this adding a moderate crimp to uniform "starting pressures" were the ticket verified by the chrony in greatly reduced ES and SD factors. At the opposite end was the extremely harsh Barnes X solid copper bullet. Barnes was aware of this extreme drag issue from the load bearing surface of their bullets and the effect it had on load data insofar as pressure was concerned . They addressed this in stages with first trying the XLC blue coating that was a flop due to notso hotso bonding of the coating to the bullets and eventually came up with the grooved bullets we all know as the TSX line. This was such a pressure drop for them when they first progated these bullets and load data was scarce they informed folks such as myself you could use other posted data from other manufactures and expect to add 1 1/2 to 2% increases in powder charges to achieve same load performance . This again proved out to be correct for me in the 3 calibres I loaded for with the TSX with 2 of the 3 shooting a good 2 grains of powder higher than most published data at the time from the major powder manufacturers. This pressure drop I believe as well was responsible for their reccomendation to seat the bullets farther off of the lands and to expect best accuracy results at or near .050 setback. Here too in my load workup for the 3 different calibres this proved to be true. Just recently as well my curiousity got the best of me on trying to figure why there was so much desparity in the .223 with the never ending bickering on what twist rate to use for what grain weight bullet. Everywhere you look folks are constantly breaking the rules for the faster twist= heavier bullets and slower twist = lighter bullets rule of thumb. I took to using my Stoney Point ( now Hornady ) bullet seating comparator to just measure the dozen or so jacketed bullets I had on hand. I wasnt ready for the extreme disparity in some of the bullets insofar as to how much or how little of the total length of the bullet was actually load bearing in bullets of same grain weights. I will be noting this into my future load proofing data for sure in the future and trying to see if the Handloader article can help me make better sense of it all. Being borderline OCD can be a PITA