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Old February 5, 2013, 09:07 AM   #8
Join Date: March 4, 2005
Location: Ohio
Posts: 13,715
Scottish Highlander,

You seem to be describing a load moved to the lands. Note that touching the lands as compared to as little as .7 mm off the lands can increase pressure 20%. That causes a substantial difference in velocity and barrel time, which can easily move you off a barrel time sweet spot.

Any time you change a non-powder component in your load recipe, be it primer, case, or bullet, reduce the charge 10% (5% is OK for the primer) and work it back up in 2% steps, watching for pressure signs. That's just six rounds total shooting in a quick check, so not much of a burden for safety. I recently read about some older testing in a pressure gun in which same-weight bullets were used and all worked OK until they tried a Speer bullet that drove the pressure up substantially. Speer said their jacket was thicker. I suspect their ogive radius was shorter, too.

Try locating a best seating depth for your bullets. It is not uncommon to find more that one and sometimes the second one is a lot further back behind the lands than you'd expect. Read this. Also read item 3., here.

If you would like a good general approach to identifying accuracy loads, take a look at Dan Newberry's OCW method. We can always add more tools and toys to your load development list for further refinement later. As you are shooting .308, you might also enjoy this.

The reason different cartridge overall lengths (COL) are needed with different bullets has to do with profile differences. Below is an exaggerated example of two different bullets loaded to the same distance off the lands.

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