OAL gauges, modified cases, seating depths for accuracy etc

[Timmis2]

I've read that reloaders will use a OAL gauge, modified case, bullet comparator, and projectile (or other means) to measure distance to the lands to determine their seating depths for accuracy tests. The recommended OAL/COL starting point is .002" less than where the bullet touches the lands (or the portion of the chamber/bore the projectile is obstructed). When reloaders use this COL to begin accuracy tests, will they usually keep the same .002" COL and adjust powder charges to test for accuracy? Or will they keep the same powder charge and reduce the COL incrementally to look for differences in accuracy?

I have read that pressure increases as seating depth is extended closer to the lands. I assume this is because less gas can escape out the barrel before the bullet obturates and seals the chamber. But it is also said that pressure decreases as your seating depth is set longer. Is the "pressure decrease from longer seating depth" theory usually just a measure of pressure difference inside the case? If it is a measure of overall pressure difference in the chamber, then that contradicts the theory that pressure increases as bullets are seated closer to the lands. Perhaps most reloaders are seating their bullets far enough from the lands that there is usually an overall decrease in pressure with longer OAL's, and as your OAL is set closer and closer to the lands, the pressure difference may shift from a decrease to an increase.

Also, isn't it true that when using a fast burning pistol powder in a rifle cartridge with reduced powder charges (for example Hodgdon publishes 3.6gr of Titegroup for subsonic .223 loads), pressures increase and may spike dangerously if COL's are set longer due to the possible reaction of pistol powder detonation inside a case with that much empty case space? This contradicts "pressure decrease from longer seating depths" too, but I imagine there's a whole different set of rules and reactions when dealing with fast powders and empty case space.

 

[K22]

The answers to some of your questions depends on your shooting discipline and accuracy requirements. I can only share my process where my goal is varmint grade accuracy (about .5 moa or so)

Point 1: I think you mean .020" from lands. At least that is my starting point assuming at least one bullet diameter (minus the Boat Tail portion) is inserted to the case to provide adequate bullet tension (grip on the bullet) and the cartridge will fit the magazine. I never seat closer than .010" from the lands due to variations in bullet ogives because I do not want the bullet either touching or jammed into the lands.

Point 2: When I conduct load development, I test various powder charges keeping COL constant for a given bullet. I only change one variable at a time. In my experience, bullet selection along with the powder chosen and resultant charges are the most significant factors affecting accuracy. Seating depth can make a significant difference in some rifles. I have at least one where this is the case. Whereas I have others where, within reason, there is little to no measurable effect on target. Once I find a bullet / powder / charge combo that meets my accuracy requirements or close to it, I may try some different seating depths if necessary.

Point 3: Pressure - stick to published data from reliable and lab tested sources such as Speer, Lyman, Sierra, Hornady, etc. Jamming bullets into the lands can increase pressure. I am not a fan of doing that. I never had trouble finding a serviceable load without resorting to jamming the bullet. Most manuals provide the COL that the load was tested at. If you adhere to that with some reasonable variation for testing for accuracy you should be fine as long as you do not jam the bullet into the lands.

Point 4: Use reliable published data for reduced loads. The issue of denotation due to reduced charges was tested by White Laboratories many years ago and it was never conclusively proven but my information on this issue is quite dated. There may be new testing that sheds light of this issue.

I consider safety the most important element in reloading. In almost 60 years of reloading, I never found it necessary to deviate from published data or the safety guidelines stated in reloading manuals. Keep in mind that if you change components from those listed in published data, pressure can change. So be conservative in your powder charge selection. Learn how to read pressure signs. At the first sign of high pressure, stop, reassess your load.

 

[JMayo]

In rifle It's base to ogive - BTO not col. The tip doesn't matter except in magazines it's gotta fit.

Most i know do "the wheeler method" . Take FP & ejector off the bolt, to get shoulder and touch established.
Start at touch and go.002 at a time which ever way you want to go.
Allot of r&r had been done shorting the learning curve, imo