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Posted: 1/8/2011 4:28:45 PM EDT
| We always get questions about why people can't get a consistent length when they use SMKs. I understand that the meplat is slightly different bullet to bullet causing slightly differing lenghts. So then how do you know when the bullet is properly seated? I assume the ogive position is what is important, so how do you meausre that? Is there a tool that will measure from case head to ogive or something like that? |
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For loading to magazine length, a comparator isn't needed. Once the seater stem is set, it contacts every bullet at the same location, so they are all seated to the same depth in the case - that is important for uniform combustion.
The fiddling part comes from getting the seater adjusted so the longest bullet produces a COAL that functions in the magazine. Seating .223 Rem cartridges to 2.25 inches pretty well guarantees that the longest bullet will clear, without a lot of checking and readjusting. Seating to 2.26 inches means a little more work; that's generally where I seat when I can, and I let a few cartridges sneak out the 2.265 inches, but no longer. So far I have never had a feed problem. If you want a comparator, go to the hardware store and buy a nylong bushing to put over the bullet. I have one that I think has a .20X ID; don't recall, but it's a perfect diameter. 3/16th's ID is a little small and if that's all you can find, get one with a thick wall then drill it out. Mine cost twenty five cents and it gives up nothing in precision, they just seem like three hands are needed until you get the hand of it. |
| Now that you mention it, I have seen the drawings of the insides of dies. Since they are all contacted on the same point on the bullet it doesn't really do any good to measure them, right? As long as the die is set so they are under max length, the bullets should all be seated the same, correct? Hope that makes sense. |
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Quoted:
Now that you mention it, I have seen the drawings of the insides of dies. Since they are all contacted on the same point on the bullet it doesn't really do any good to measure them, right? As long as the die is set so they are under max length, the bullets should all be seated the same, correct? Hope that makes sense. A comparator is useful when we're seating near the rifling lands. In that case, we want to know the distance between the rifling lands and the place where they contact the bullet ogive. Even that was worked around for decades by measuring to the meplat, and still works with bullets that aren't pointed up like a HPBT style bullet, but it's not much good with HPBT's unless the meplat has been uniformed so that every bullet is identical in length. |
| Sinclair also has a comparator shaped like a hex nut. On each "wrench flat" is a caliber specific hole for measuring in conjuction with a dial micrometer. It's a fairly old school tool. I think they also have a version out now similar to the hornady comparator that attaches to the micrometer. |
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FWIW, the SMK's (minus the 80grs) are very insensitive to seating depth. I've got a comparator, but I don't use it much. Here's how I set the seating die to a desired bullet jump distance. It's quick, cheap, and good. First, find the lands. 1. Make a dummy case w/ the bullet seated very long––you want it in the lands 2. Measure the OAL and then stuff it in the chamber with your thumb 3. Seat it deeper in 0.005'ish increments till it will fall out of the chamber on it's own *when it drops out of the chamber from gravity alone, it's no longer in contact w/ the lands *measure it––this OAL for this individual bullet is where the lands are *the seating die is now set to put the bullet more or less exactly at the lands And at this point, you set your jump. If you want 0.020" jump, you now tweak the die to seat that bullet 0.020" deeper. Save the dummy round. You can duplicate the die setting at a later date by backing out the seating plug, running the dummy into the die, then moving the plug down till it contacts. Lock it and you're done. |
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Quoted:
FWIW, the SMK's (minus the 80grs) are very insensitive to seating depth. I've got a comparator, but I don't use it much. Here's how I set the seating die to a desired bullet jump distance. It's quick, cheap, and good. First, find the lands. 1. Make a dummy case w/ the bullet seated very long––you want it in the lands 2. Measure the OAL and then stuff it in the chamber with your thumb 3. Seat it deeper in 0.005'ish increments till it will fall out of the chamber on it's own *when it drops out of the chamber from gravity alone, it's no longer in contact w/ the lands *measure it––this OAL for this individual bullet is where the lands are *the seating die is now set to put the bullet more or less exactly at the lands And at this point, you set your jump. If you want 0.020" jump, you now tweak the die to seat that bullet 0.020" deeper. Save the dummy round. You can duplicate the die setting at a later date by backing out the seating plug, running the dummy into the die, then moving the plug down till it contacts. Lock it and you're done. Good deal, thank you very much. How much of a difference in accuracy will this make? Is it just something the benchrest guys will notice? |
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All modern seating dies seat the bullet with respect to the ogive (or very close). For a given bullet type, the seating die will seat the bullet consistently to the desired base to ogive dimension. The consistency of seating can be easily checked using the RCBS Precision Micrometer.
OAL will be variable because the ogive to meplat dimension of a given bullet (HP bullets) varies. If this is of concern a good meplat uniformer (Tubbs) can be used to make this dimension more consistent. OAL after meplat uniforming is checked using a good caliper. Meplat uniforming will slightly decrease the ballistic coefficient but make the coefficient more consistent. The practical impact of OAL and meplat uniforming will not be easily seen unless using a very good rifle over 500 plus yards. Shooting in a semi-auto rifle requires loading to a magazine compatible OAL so most of the concern for OAL consistently is of little importance. When a necked cartridge is loaded into the chamber it seats at the shoulder. Therefore the bullet distance to the rifling (jump) is determined by the datum to ogive distance. This distance must be calculated for a specific rifle, brass sizing and bullet type. The OAL will be the result of the base to ogive dimension and the variability of the bullet's ogive to meplat dimension (assuming the base to datum is consistent). |
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