Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
In the end, it only matters if one's crimping. I find the Montana Gold to be just right when cases are trimmed to 1.750 and OAL is 2.255" The military tech manual indicates that the cannelure should be about .510 from the meplat of the bullet when working backwards off their drawings for M-193 round.
I crimp so I am going to have to do some die adjustments. Still, why isn't the cannelure standardized????
OR, I could just forget about the cannelure and go from there.
Reading perhaps more into your question than what is actually there, I truly think the answer to your question is that if you change bullets, you definitely need to change a lot of other things (or at least examine them). Your good scrutinization of components (and resulting discovery of differing cannelure placement) is a healthy trait to continue to cultivate. nrc is absolutely right with his drawing of attention to the ogive.
With a good safe load with the bullet that seats more shallow, no horrible problem may erupt if you willy-nilly switch to the bullet that seats deeper, but pressures may indeed change, point of impact may indeed change, group size may indeed change, etc., etc., etc. The axiom here is that changing bullets requires that you begin again with load development for the new bullet (and that would include adjusting of the equipment, including seating depth). This "back to the drawing board" rule is not only beneficial from a safety standpoint, but also conducive to accuracy pursuits.
You will also find manufaturer-to-manufacturer inconsistencies with primers (one may seat more firmly than another, etc.), brass (capacity, rim thickness,etc.). We cannot just go "mixing and matching" to our heart's content if we want the best ammunition from our little factories at home.
Finally, I will restate my practice of not bothering with crimps and cannelures and such with my AR. Nor would I oppose the practice for those that choose it. I have simply found it not necessary for my purposes: my gun is a sporting weapon, not a battle gun. I have tried every way I can think of to make my bullets "walk" out of (or into) the case neck, and short of putting the cartridge in the Quinetic, it just doesn't happen. My crimping activities therefore remain restricted to revolver loads, and my blackpowder cartridge guns. For those operations, I share with others a devotion and trust for the consistency offered by Lee's "factory crimp" dies. Probably the best product Lee has ever developed.
Abandoning any thought of crimping AR cartridges allows for an almost infinitely vast selection of "cannelure-less" sleek-sided bullets to choose from, and allows me to choose seating depth without the dictation of crimping concerns. My criteria become magazine operation and throat placement.
The two bullets differ in taper, and therefore differ in cannelure placement.
IMPORTANT NOTE: Reloading data published by SPEER are for SPEER bullets. Many of our bullets are of unique construction; there is no such thing as "generic loading data" any more. Other bullet makes may produce significantly different pressures and velocities. We make no warranty that our published loads are safe with another make of bullet. You, the reloader, bear the ultimate responsibility for knowing your firearm, loading equipment, and techniques.
WARNING: Because of variations in powders from lot to lot, and variations and differences in components, the handloader should use the data in this manual as a guide. Do not start with the heaviest load shown. Start low, and try successively heavier charges carefully, watching for signs of excessive pressure, before increasing the charge. Many individual guns will safely handle greater charges of powder than shown here, but don't start at the top. Another reason for caution is that loads safe at normal temperatures can have excessive pressure if shot in either very hot or very cold temperatures.
. But, pressure is a lot harder to measure.