Seatting Depth

Discussion in 'Ammunition & Reloading' started by TLuker, May 18, 2012.

  1. TLuker

    TLuker New Member

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    I tried some different loads out in my .308 yesterday. The first five were light on powder compared to what I normally shoot and the second five were a little hot. I really didn't see any change in groups between the two. Both were a little worse than I normally shoot and were about 1.23" at 100 yards. Next I tried five shots .015" longer than I normally shoot and then I tried 5 shots .015" longer than that, or .030" longer than my normal load. Powder charge was what I normally shoot which is in between the light and hot loads I tried to start with. The .030" longer bullet fired the same size group as the first two loads. The .015" longer was a different story. I had 4 shots in one whole. I saved the 5th round just to make sure I wrote the length down right!!! That then brought up some questions.

    I was really surprised to see the my group size change that dramatically with a .015" change in seating depth. So I'm wondering how many of you have seen that sort of change in group size from such a small change in seating depth?

    Second, I realize that just because I got one great group from four bullets doesn't mean a whole lot. That could have been just luck. So what are some different thoughts on how many shots should be fired when checking a load? :)
     
  2. Nickwashere

    Nickwashere New Member

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    I know long distance shooters actually change the lengths according to their rifles chamber leaving as little of head space as possible so there its literally no space for the bullet to travel before entering the rifling of the barrel..but I literally just started reloading so I have no actual experience..this is just information ive read on other blogs
     

  3. steve4102

    steve4102 New Member

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    I have a couple rifles that are very sensitive to OAL and a .015 could make a big difference. The rest of my rifles are much less sensitive and an OAL change of .015 would do little if anything as far as accuracy goes.

    Load up 10 more each with both OALs and test again. Shoot five of one then five of the other and see if the improvement continues. It would be great if you could have someone else load the rifle for you so you did not know which length you were shooting.
     
  4. tri70

    tri70 New Member

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    Make sure you fire 2 or 3 fouling shots from a clean bore, I usually don't get good groups till then but it is always important to know where the 1st shot lands. I was loading for my 223 target mini and I have great load for it with 55 gr Nosler and 21.5 gr RL7 col 2.260. The same powder and col in the Hornady fmj is terrible, I went back to what Hornady recomended at 2.220 and it shot great.
     
  5. ryguy00

    ryguy00 New Member

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    you said .308 but not what kind of rifle???

    for bolt rifles, try to figure out what length your bullet just kisses the lands and start there. make dummy rounds for testing: sized case with bullet, no powder, no primer. chamber the dummy round and then eject, look close with a magnifying glass to see if the lands left marks on the bullet. Not touching: no marks. Jammed hard: rectangular marks. Just right: the marks will be almost perfect squares. take notes on what is jammed, not touching, etc. Start out just touching and work up a powder load. (you seem to already have one that works well so use it) Only change one thing at a time, shoot 5 shot groups, 2-3 foulers before starting the first group, and clean after every 2 groups (10-13 shots) .015 is a pretty substantial change. you may wanna make changes in increments of .002-.005 at a time. It all depends on your definition of accuracy and what you're looking to achieve.

    you will initially see groups start to shrink with the changes and then eventually open back up. Refer to your notes to go back to where they were good and fine tune. Keep in mind that the throat will erode with use and lands move further away from the chamber due to wear. Eventually you will have to increase OAL to chase the lands and that "sweet spot"

    For semi auto's, I just load to the max OAL that will function in the magazine and go from there. But I also don't require any precision from my semi's. My bolt rifles are reserved for precision shots.
     
    Last edited: May 19, 2012
  6. locutus

    locutus Well-Known Member Supporter

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    Set depth by the manual and you're good to go. I think that far too many of us obsess about seating depth.

    OTOH, if you shoot benchrest, buy a Sinclair measuring tool and bullet comparator.
     
  7. 1hole

    1hole New Member

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    "The .030" longer bullet fired the same size group as the first two loads."

    I assume you meant to say "cartridge", not "bullet"? You seem to have proven that 15 thou can make a difference. But you know you'll need to repeat it a few times before you start to count on it; statisically, a single example of anything doesn't mean a lot. I often fire 20 rounds of what I really want to prove and use the results of that. Averaging a series of groups gives us a way to rationalise the occasional 'flyers' and pretend they don't matter but we shoot them too, don't we?

    If I limited my reloads to what's "in the book" for either powder or OAL, I'd quit kidding myself about custom tayloring my ammo to my rifles and just shoot factory stuff.
     
  8. fmj

    fmj New Member

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    I have nothing to add to this conversation...other than to say i am reading intently trying to learn.

    If your dropping 4 shots in the same hole @100 i would say its skill and not luck.

    Is the gun locked down in a "lead sled" or are you "free handing"?
     
  9. BlueTurf

    BlueTurf New Member

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    I have both a SA NM M1A and Savage 10FLP that shoots .308. I was experimenting with different bullet seating depths for ammo I was shooting through my Savage. I used my Hornady bullet seating measuring tool. I wasn't getting the results I had hoped for. I tried some loads I had developed for my M1A through it and was quite surprised at how well they grouped. I have two loads that will cloverleaf four shots at 100 yards every time. These cartridges had an O.A.L. of 2.80 because that is the maximum for smooth feeding from the magazine for the M1A. Now I seat all of my .308 loads to 2.80 and get great results from both rifles.
     
  10. TLuker

    TLuker New Member

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    I'm shooting a bolt action (Remington 5R), and what you described is what I was expecting to see. I was expecting to see the groups either open or close as I got closer to or further away from the right seating depth. I guess I just got lucky and the load that was .015" longer was the perfect load?

    As for how I got to that point, I used a dummy round to find where the groves started. The groves in this rifle were way too deep and my OAL length where it made contact was at approximately 2.910". I then backed off .050" from that as a safety factor. A cartridge with the bullet touching when it goes off will increase pressure. Bench rest guys want the bullet right at touching but they are developing their loads for that and backing off on the charge to compensate for the increased pressure. So I backed off .050" from touching and then started working my way to that point stopping before I get too close. My longest loads were still .020" from touching.

    In this case I'm way past the manual on OAL. I just didn't expect that much of a change from a .015" change in depth? I guess .015" really is a pretty big change in seating depth.

    FMJ, I was shooting off a bench with a bi-pod and bag at the rear of the stock. I personally think most of use are capable of shooting great groups but our equipment is the biggest limiting factor? But I've also been shooting a long time and shot competitively on the rifle team in high school. And this still could have been pure luck? I will definitely load some more rounds at this length to verify my results.

    I'm just trying to figure out how many rounds to load when testing a load. I don't want to load too many because at some point I'm wasting shots and time. In this instance I was trying 4 different loads with 5 rounds each. That's 20 rounds and could have been a lot more if I loaded more rounds for each load. At the same time I want enough shots to have confidence in my results. One 3 shot group could be misleading because you could get lucky and shot one good group, or one bad one. 10 shots per load would give pretty reliable results but that's a lot of shooting and time. In this case that would have been 40 rounds. So I'm looking for the bare minimum shots needed to have some confidence in the group. Right now five seems about right for me?

    As for cleaning, I clean every 20 rounds as a general rule which is in part why I loaded 20 rounds to try on that trip. After I verify that I have found a great load for this gun I'll shoot more rounds and figure out at what point my groups start opening up or POI changes. I'll then change my cleaning to match what I need for this gun.:)
     
  11. ryguy00

    ryguy00 New Member

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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.
     
  12. steve4102

    steve4102 New Member

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    Don't know about pressure "inside the case" but chamber pressure decreases with a decrease in OAL.

    Here is some actual pressure tested data.

    [​IMG]

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    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.
     
  13. JonM

    JonM Moderator

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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
     
  14. TLuker

    TLuker New Member

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    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. :)