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Rocky’s Reloading Room |
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Seating Bullets to Best “Off-lands” Distance
How often have you read “I get best accuracy with the bullets seated “x” distance off the rifling?” What does that mean and how the heck do we measure it?
What we're trying to do is seat a bullet so that the curved portion of the bullet nose sits just short of the start of the ramped area of the lands (off the lands) by a certain but very small distance. Why? Because a bullet that's crammed into the lands requires a tremendous amount of force to start it into those lands (rifling), where it must be distorted a certain amount (Remember, we're forcing a bullet that's .308" diameter in, but the tops of the lands are only .300" apart!) It would be like trying to force a splitting wedge into a log by placing the wedge on the log and pushing. Possible, but it takes a heckuva lot of effort.
By having the bullet just a tiny fraction of an inch off the lands, it gets just a bit of a running start, and the effort to engrave it in the rifling is much reduced. How far is best? Ah, that's the rub. Some guns are much more accurate if the bullets are almost touching, and others work best when the bullets are back as far as .050-inch. The only way to tell is to experiment. Hint: start at .010-inch. Complicating things, every bullet has a slightly different curve or ogive (pronounced {i}OH-jive[/i]) and has to be seated just a bit differently than another kind of bullet to achieve the same "off the lands" distance.
So how the heck do we measure that distance? The three most common methods are the dummy round, the cleaning rod technique, and using specialized tools. With the dummy round, you slightly squeeze the neck of an empty case just enough that a bullet can be pushed in with the fingers but is held tightly. Set the bullet just barely in the case so the overall length is very long. Carefully chamber the round until the action is closed, then carefully extract the round into your hand. In theory, the bullet will press against the rifling and be seated in the case to a zero fit. Then you measure the overall length and subtract whatever distance off the lands you choose. Repeat with each different bullet you shoot to account for different ogives.
But there are problems with this technique. If the bullet fit in the case is snug, you may force the bullet several thousandths past the start of the lands instead of stopping at first contact. The resulting overall cartridge length (OAL) will be too long. Conversely, if the bullet is looser in the case, the “grip” of the lands may pull the bullet out of the case a bit before it releases it. Again, the OAL will be too long.
The cleaning rod technique uses a flat-ended (not threaded) rod and a bullet. You drop the bullet into the chamber and hold it against the lands with a pencil or similar tool. Insert the cleaning rod into the barrel from the muzzle until it gently touches the bullet. Mark the rod exactly at the muzzle. Now push out the bullet and close the action. Push the cleaning rod back against the breech and again mark the rod exactly at the muzzle. Measure the distance from mark to mark and you supposedly have the “touching the lands” OAL.
This system too has flaws. First, the soft tips of bullets are the most likely to be damaged or be formed with slightly different profile. So no two bullet tips are exactly alike. Second, it’s difficult to make the muzzle marks exactly in the same place. Third, measuring to the breech face may not accurately represent the measurement to where the base of the cartridge actually sits. The dimensions of the extractor cut in each case, paired to the gun’s extractor, may or may not hold the case precisely against the breech. All told, there are many ways for this method to be off.
There are several specialized tools that purport to measure the off-lands distance. Some use a generic bullet shape and some use modified actual cases and bullets. But all such tools are subject to some of the same errors noted above. I’ve tried several. In ten consecutive tests, I’ve often come up with six or seven different “measurements” of the in-contact dimension. It may have been operator error, but it was still erroneous.
How do I “measure” the off-lands dimension? I go about it backwards.
Instead of trying to determine the best seating depth before shooting, I measure it AFTER I find the best seating depth. Using a plastic-tipped bullet to minimize tip distortion, I start with the maximum OAL listed in the reloading manual and a known accurate load recipe. I load four batches of five rounds, each one with the bullet seated .005” shorter than the last. I carefully shoot those four batches over a chronograph off a good rest.
I record the group sizes of the best four shots in each group (allowing myself an inevitable flyer) to determine the most accurate length with that brand of bullet. Now, here’s the unusual part: to transfer that ideal seating depth to other bullet brands.
For seating depth, the critical part of the bullet is not the tip, but the exact spot of the ogive that actually touches the lands. That occurs, naturally, where the diameter of the ogive matches the land-to-land diameter of the bore, or the caliber diameter.
Please allow me a short digression here to explain that. A rifle barrel is reamed to caliber size before it is rifled. For a 30-caliber bore, that hole is .300” in diameter. The rifling is cut or formed deeper than that, to give a final groove bottom-to-groove bottom diameter of .308-inch, which matches the bullet diameter. But the tops of the lands remain at caliber diameter, or about .300-inch. So the critical diameter on the bullet ogive is the first place that reaches caliber diameter. Clear?
I determine that point by setting the jaws of a good stainless steel caliper at caliber diameter (.300” for a 30-caliber round). Holding one of the most accurate-length rounds in my fingers, I spin the bullet between the sharp portion of the caliper jaws to leave a faint line inscribed on the bullet jacket. Now I measure the distance from the case base to that scribed line. That measurement is the loaded round datum length for best accuracy in that rifle.
To load any other brand or style of bullet, I first scribe another line on the new bullet at the .300” diameter, again using the calipers. I back out the seater stem, and then adjust it downward, seating that marked bullet slightly deeper until it is seated at the datum length. At that point, it will be off the lands at the most accurate point, no matter what shape or curve that bullet has.
Very few bullets are longer or have sharper ogives than the plastic-tipped ones, so almost any other bullet will result in a shorter overall cartridge length. So if the datum-length round with a plastic-tip bullet fits and function through the rifle magazine, everything is copasetic. Test-feed rounds with the new bullet anyway, just to be sure. That’s it. Assuming your rifle is most accurate with a given off-lands distance for any bullet shape –and most are- you now have a way to load any bullet to that distance.
At this point, I still have no idea exactly how far off the lands that bullet sits. Here’s the shocking truth: I don’t need to! I know it’s at the most accurate length for that rifle. It makes not an iota of difference whether the bullet is .005” or .025” or .25”!! As long as I can determine how deep to seat ANY bullet for that rifle’s best accuracy, it doesn’t matter at all what the actual off-lands distance is.
In summary, you first find the seating depth that your rifle likes best. Then, measure what that seating depth is at the caliber-diameter datum line. Seat all other bullets to the same case head-to-datum length.
Ahh, the pleasures of reloading! In addition to an almost infinite combination of bullets, brass, primers and powder choices and charge weights, we have seating depth to judge and set. It's a wonder we manage at all, huh?
Rocky Raab © Copyright Rocky Raab, 2002 |
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First, find the seating depth your gun likes best. |
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Then, find the caliber-diameter datum line on a round loaded the same way. Duplicate that depth with all other bullets. |