Need a logic check --- help me TresMon

[Bob 964]

I have been struggling with my attempts to accurately measure length to the ogive using my Hornady L-N-L overall length gauge and bullet comparator. If I measure the same bullet 10 times, pushing the rod lightly until I feel ANY contact, I may get several different measurements with considerable variation. If I lightly push the rod until the bullet STOPS, I get a much more consistent set of measurements with much less variation.

Most, if not all, of the standard cartridge diagrams show an OAL for the .308 cartridge of 2.800 inches. This, ostensibly, is a "safe" length designed to keep the bullet a safe distance from the lands. Measuring to the ogive allows bullets to be seated closer to the lands, resulting (hopefully) in more accurate rounds.

This suggests that the overall length of cartridges with bullets seated closer to the lands should be greater than 2.800 inches. Using one set of measurements I get cartridges with overall lengths greater than 2.800 inches which would move the bullet closer to the lands. Conversely, using the other set of measurements I get cartridges with overall lengths less than 2.800 inches which would move the bullet further away from the lands.

Does this make sense? If so, then I am feeling more comfortable about how to use this gauge.

 

[Teggy1]

Re: Need a logic check --- help me TresMon

I don't think safe distance is the reason behind the 2.800". It is more likely due to magazine restrictions. I don't think there is necessarily a "safe" distance from the lands, provided you adjust your loads accordingly.

Yes, a measurement longer than 2.800" will get you closer to the lands, and vice versa.

Regarding differing pressures on the rod, use what you believe is consistent.

 

[crewchef]

Re: Need a logic check --- help me TresMon

Consistency is key to knowing you have accurate measurement. You don't really care about OAL at this point unless you are loading for magazine use. The ogive is what touches rifling first and what matters. Seat your bullets using length to ogive and you'll know exactly how far off the lands they are. When I first got started the guy who was helping me learn suggested seating HPBT bullets .025 off the lands to start. Once you get a good feel for proper seating and find a good load you can move it in/out to tweak a load. Significant pressure changes should only occur if you seat it long enough to actually touch the lands or seat it significantly shorter and create a compressed load.

If you're loading for magazine use figure out OAL for magazine and then measure those loaded rounds to ogive to know how far off lands your are. If loading for magazine length I would think you'll be quite a ways off the lands but measuring it will tell you for sure.