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Old 05-22-2012, 08:36 PM   #11
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5 Shot groups and cleaning every 20 is perfect. But youre not completely correct about the pressure corresponding to length. Actually, the deeper the bullet is seated, the more pressure inside the case will increase. The benchrest guys also are not backing off their loads with the bullets touching the lands. In fact, we do the opposite. Once we have length where we want it, we increase the powder load until either the groups start to open up or we see signs of too much pressure: sticky bolts, primers backing out, primers piercing, etc. Your powder load you already have will be perfectly safe with the bullet seated touching the lands. (Ohh yeah, another point, you are seating to the lands, not the grooves.) If you think about it, the bullet has to hit those lands eventually when it's fired. Whether it has to jump or is already touching, the pressure in the chamber wont change. Pressure increases when air volume inside the case decreases. That air volume is decreased either by increasing powder (duh) or seating the bullet deeper.

Now you made an initial. 015 change to find that last group. Go back to that point and try another load. 005 longer and another .005 shorter. Keep powder the same as what you used before. If you find that the .005 longer than before works a little better, then write that down. Then go another. 005 longer than that. Keep taking notes until you see groups open up. When you find a good length that youre happy with, you can change powder load plus or minus a half grain at a time to really fine tune it

If you change ANY of your components, bullet, powder brand, brass, primer, etc, you will have to start over from the beginning.

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Old 05-22-2012, 09:27 PM   #12
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Actually, the deeper the bullet is seated, the more pressure inside the case will increase.
Don't know about pressure "inside the case" but chamber pressure decreases with a decrease in OAL.

Here is some actual pressure tested data.





And this from John Barsness of Handloader Magazine.

It decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.
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Old 05-22-2012, 10:49 PM   #13
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if your going to experiment with beyound book oal you absolutely need to have a chamber gage and bullet comparator such as hornady lnl system or similar device that tells you the exact chamber length to the lands.

you must leave at least .010 from the lands for minimum safety. go below that your taking your life in your hands.

just guessing is a good way to have your gun go boom in a bad way.

i tailor my loads for my 308 to fit the chamber. a good method is take your measure start at .010. for this to be effective you need to have the ogive measurement not the measurement of the tip of the bullet, you also need the length of the case and ogive to the lands. bullets can have great differences in length when measured from base to tip and great accuracy fron base to ogive.

anyway start at .010 and load 20 then another 20 at .015 and so on. this gives you 4, 5 round groups for each measurement.

if you experiment with bullet lead you need the proper tools for safety. the lnl system is decent and doesnt cost an arm and a leg

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Old 05-24-2012, 01:22 AM   #14
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Quote:
Originally Posted by JonM View Post
if your going to experiment with beyound book oal you absolutely need to have a chamber gage and bullet comparator such as hornady lnl system or similar device that tells you the exact chamber length to the lands.

you must leave at least .010 from the lands for minimum safety. go below that your taking your life in your hands.

just guessing is a good way to have your gun go boom in a bad way.

i tailor my loads for my 308 to fit the chamber. a good method is take your measure start at .010. for this to be effective you need to have the ogive measurement not the measurement of the tip of the bullet, you also need the length of the case and ogive to the lands. bullets can have great differences in length when measured from base to tip and great accuracy fron base to ogive.

anyway start at .010 and load 20 then another 20 at .015 and so on. this gives you 4, 5 round groups for each measurement.

if you experiment with bullet lead you need the proper tools for safety. the lnl system is decent and doesnt cost an arm and a leg
The next item on my list is a bullet comparator. Measuring over all length is good for having a "rough" idea of where a particular bullet should be, but it is a pain having to go through all of that for every type of bullet. I also completely agree with your comment about the overall length of each bullet varying a lot. Right now, I wouldn't go below .020" off the lands for my loads just because of the potential error in my measurements. I would also advise anyone else to be extremely careful if using a dummy round to determine where the lands start. I've literally made a career out of making precise measurements and I'm aware of how imprecise this method is. By the time you see marks from your lands on the dummy round you're already past were the point of contact is by several thousandths because what you're seeing is the drag marks. It's nothing for those drag marks to be .010" or .020" wide.
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