Question for the Gurus. I'm reloading .308 Win cartridges to be shot on a Rem700P. I decided to fine tune the book data so as to have optimum seating depth. Using a Sinclair seating depth gauge, I get an overall cartridge length of 2.920 (multiple tries changing cases and bullets (matchkings 168 gr). The reloading manual gives 2.800 as the COAL. Quite significant difference. Also the SAAMI lists 2.810 as the max length. Where did I went astray? Or is it possible that my rifle have a non-standard chamber?
AJ
My first question would be how are you defining optimal seating depth? I load well beyond max length for my 600 yard ammo (as do most Highpower shooters) but I'm not sure what you're looking for in loading them longer. Let me add I see no inherent problem with it, but you do need to be more cautious when loading bullets that are jammed into the lands, and of course reliable function is paramount. So i would ask you to tell us more about what you're doing and I'm sure good advice will be forthcoming.
What Agrivere says!
Quote from: Agrivere on June 02, 2015, 09:32:46 PM
My first question would be how are you defining optimal seating depth? I load well beyond max length for my 600 yard ammo (as do most Highpower shooters) but I'm not sure what you're looking for in loading them longer. Let me add I see no inherent problem with it, but you do need to be more cautious when loading bullets that are jammed into the lands, and of course reliable function is paramount. So i would ask you to tell us more about what you're doing and I'm sure good advice will be forthcoming.
x3
Thanks for the info. What I'm trying to determine is the length at which the bullet (I guess the ogive) just touches the lands. Then back up in small increments (.0055) until I discover what length gives me the best group. I do not intend to load beyond the lands. Right now to me its more of a theoretical exercise. Just learning.
AJ
Okay, that certainly makes sense, and from what I can see you're most of the way there, but not quite. Your Sinclair tool, used correctly, should give you a measurement that tells you when the bullet is "touching" the lands. The tool must be used carefully, as pressures increase quickly when bullets are jammed into the lands, so you'll want to make sure you've correctly measured "touching", which it sounds like you have.
For what it's worth, I have the Hornady tool which is designed to do the same thing, but I don't use it as I find it to be less accurate than I would like. What I do to measure "touching" is use a thin wooden dowel, inserted into the muzzle, and simply seat a bullet long and place the dummy cartridge in the chamber. Using the dowel to push the bullet away from the lands, and my finger to push it toward the lands, it's easy to feel when the bullet is sticking in the lands. I decrease the seating depth in small increments until I can feel the bullet no longer sticking. For me at least, this method is far more precise and repeatable than any tool I've tried to use. Every other method I know uses some unknown amount of pressure on the bullet into the lands, and I find my measurements to vary by as much as 0.01" every time I measure. Using my method, I can repeatable measure "touching" to within 0.001" every time.
In any case, now you've got a measurement, and hopefully a dummy cartridge, that indicates where the bullet is touching the lands. Now, unfortunately, you need another tool. Measuring cartridge overall length simply isn't very useful for this. The overall length of the bullets varies way too much. Measure a box of bullets and see for yourself. You'll likely find bullets to vary in length by at least 0.01", and often more (sometimes FAR more). What you ideally want to measure is the base to ogive length, which is generally a FAR more consistent length, and is actually the measurement you care about, since that's the part of the bullet which will first touch the rifling. So instead of measuring the OAL of your dummy cartridge, pick up a tool to measure base to ogive (the Hornady one is the one I use, though the Sinclair tool is fine also), and use that measurement.
Note that you'll want to save your dummy cartridge, so as the barrel wears you can measure throat erosion. That's right, the measurement you just made will change over the life of the barrel. Save that dummy cartridge and you can measure your throat erosion. How fast the throat erodes depends greatly on the cartridge and the load, but you can figure from 0.01" to 0.02" per thousand rounds as a good estimate.
Once you have the base-ogive measurement that indicates "touching", simply back off of that measurement to determine the bullet "jump". Generally you'll find best accuracy between 0.010" to 0.020" of jump, but some bullets will like to jump a LOT, so don't be afraid to try jumping your bullets even more. My current NM AR-15 shoots best with 82 grain Bergers jumped 0.040", which is common for these Bergers. Most Sierra Match Kings will shoot best around 0.015" to 0.020", though it can of course vary.
To wrap up, let me offer a real world example with my rifle. I currently shoot 82 grain Berger bullets for my 600 yard Highpower load. The first thing I do is measure a handful of bullets to see what their overall length is. I want to select a bullet with an "average" overall length for my dummy cartridge. Once I've got a small pile of bullets that are all "average" in overall length, I'll measure the base to ogive length on that small pile. Once again, I'm looking for an "average" bullet from this lot. This part isn't strictly necessary, but I figure if I'm going to go to the trouble of measuring it, I may as well use a "typical" bullet from that lot, and avoid using one that's extremely long or short or whatever.
Once I have my dummy bullet, I seat it in an empty cartridge, and I'll seat it way long. Using my wooden dowel, I'll insert my dummy cartridge in the chamber, and I should feel it sticking in the lands when I push it into the chamber. It's easy to feel. I'll then decrease the seating depth in 0.005" increments and keep checking. Note that a micrometer seating die makes the MUCH easier. As you get closer to just touching the lands, you'll feel the bullet sticking in the lands much less tightly. At that point I'll reduce my seating depth increment to 0.002" each time I try. I'll repeat this process until I can press that dummy cartridge into the chamber and feel NO friction from the bullet in the lands. If you want to confirm it, give that dummy cartridge a whack in your bullet puller, re-seat it a 0.001" longer, and you should feel it touch. That's the measurement I want, where it JUST touches the lands.
From here it's easy. I'll simply measure the base to ogive length on that dummy round. In my case, in my rifle, the 82 Berger touches the lands at 1.944" base to ogive. Note that I do not care what the overall length of that cartridge is, as that measurement is completely irrelevant other than for fitting them in a magazine. It will vary so much from bullet to bullet as to be pointless to even measure. In my case they fall in somewhere around 2.50" in overall length, which is well beyond what will fit in an AR15 magazine.
Then I'll start testing. I'll load up some test rounds at 1.934", 1.924", 1.914", and 1.904". Jumping them ten thousands of an inch, and increasing the jump by ten thousandths each time. You'll almost find one of those will group much better than the others. If you like what you get, you're done. If you want to refine it more, you can try smaller increments of jump and see what you find.
For my rifle, I found jumping the 82 Bergers by 0.040" to shoot the best groups by far, so that's what I run with. I've shot 10 shot strings at 600 yards that scored as high as 98, so I know this load at this length will shoot as well as I can hold.
My final thought relates to function. Rounds loaded over SAAMI max length are absolutely safe, as long as you work up your load appropriately. Always remember that pressures increase fast when bullets are jammed into the lands, so be careful with neck tension and watch for pressure signs during load development. Also remember that cartridges loaded this way will often not fit in magazines, and much be single loaded. If you want to be able to use the magazine, you may need to load them much shorter (I certainly do in my AR), and you may want to use different bullets that are more jump tolerant.
Hope that helps, let me know if you have any questions! And if you want to learn a WHOLE lot more about all of this, look for a book by Glen Zediker called Competition Reloading. It's a tremendous resource if you want to learn more about this kind of thing.
The hornady tool, properly used will let you get a length for your chamber. You can then use that data to make up loads of varying lengths and determine which gives you the best accuracy. But some bullets are more sensitive to jump than others.
The OAL's in the manual aren't necessarily chamber dimensions, as a matter of fact, for magazine rifles they are almost never the chamber dimensions but rather the maximum length that will fit into a magazine.
Thanks Agrivere!! I sure appreciate you taking time to explain. Like I mentioned, I'm a rookie at reloading; I do not want to flirt with overpressures or anything that resembles it. Like you explained, once I find the "zero" bullet jump, I'll shorten that cartridge length so that I'll always have a positive bullet jump; load samples of various lengths and go to the range. Your explanation gives me a very good road map on how to proceed past the point I'm now at. Thanks again!!
AJ
1) rem chambers are notorious for a long throat - not necessarily a bad thing, just the way it is
2) if you want to "jamb to the lands" or close to it, the internal magazine will not work - you will need to single load
3) sierra match bullets are fine to "jump"to the lands - it is vld type bullets that commonly suffer from jumping