Balance of Jump

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  • Balance of Jump

    After using the reloading data provided by AA and other sources to get a handle on the basics of safe reloading I realized that to get a better understanding of why the rounds perform great or poorly with different powder loads vs the harmonics of my 16" AA bbl, I need to understand a little more about the 'Jump" to the lands. After recommendations from several members I now have the Hornady AOL Gauge with their 'Grendel' case as well as a Comparator to determine the cartrige length to the oglive.
    For my Gren this is what I have measured for the different bullets I have shot:
    ---------Chamber-------/------Loaded
    Bullet-- COAL--Comp----/COAL--Comp----JUMP
    120PH-----2.224- 1.744-/ 2.224- 1.744--- .000
    120NBT---2.243- 1.715-/ 2.250- 1.715--- .000
    120TTSX--2.283- 1.748-/ 2.260- 1.721--- .027
    123AMAX-2.265- 1.693-/ 2.260- 1.682--- .011
    123SMK---2.295- 1.797-/ 2.260- 1.762--- .035
    125 NP----2.250- 1.725-/ 2.200- 1.675--- .050
    130SCIR--2.295- 1.674-/ 2.260- 1.639--- .035

    You can see that the Jump varies form .000 to .050. Am I looking for the best Jump for each bullet or a general jump for my particular barrel? Or is it a matter of balancing the powder weight and Jump for each bullet? I haved some great sub MOA groups from the SPH and SMK's and should be doing better with the AMAX and NBT's.
    Last edited by Guest; 06-08-2011, 03:41 PM. Reason: Clarify

  • #2
    Originally posted by steel89 View Post
    Am I looking for the best Jump for each bullet or a general jump for my particular barrel?
    The former. Per-bullet, not per-barrel.

    Originally posted by steel89 View Post
    Or is it a matter of balancing the powder weight and Jump for each bullet?
    Yes, it's both. My personal question was which first, powder volume or OAL? The "optimal charge weight" concept indicates powder first, then tweak seating depth. I haven't done this yet, but I like the way it sounds.

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    • #3
      I've found that I cannot count on OAL and "jump" distance too much in an auto. Without going overboard on neck tension, it is extremely difficult to keep a bullet from creeping forward when the carrier/bolt slams it into the chamber. I've measured and remeasured after chambering then extracting and the amount of creep is never constant. There are too many variables: neck tension consistency, brass thickness differences at the neck, small amounts of lube left in case mouths, etc. Find the best OAL within .005-.010 and don't obscess too much more. IMHO bullet runout has more affect on auto accuracy, and there's no fix for this unless you hand chamber each round.

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      • #4
        So I'm getting ready to shoot a new round of AMAX w/8208. My last set of the AMAX were dissapointing. I did have a 25mph xwind over 100yds, so what did come out of it all was good Chrony data from 27.5g-28.5g. I would like to shoot sets using the same powder groups at different COALs. 2.260 will be my starting point (mag lth). How much should I back them down for the next sets? 2.255, 2.250 etc.
        I see your point on neck tension varying due to lubricants etc and the other aspects....good points. Some guys just write things off as 'my gun just shoots those better', but I want to get beyond that and see if I can get those marginal bullets to shoot great in my gun.

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        • #5
          What velocities are you getting with those loads? All of my best loads with the 123 Amax were loaded .015 off rifling and were running 2580 fps in a 24" barrel. I completely gave up on higher velocities with this bullet. I can run the 123 Sierra at 2660 fps with better groups. I would normally back them down in .010 increments until I see groups improve then move back up in very small increments to find the sweet spot. I'm gonna assume you've taken care of all the mechanical issues of your rifle (trigger, crown, optics, free float handguard etc.) and you are getting good groups with another load?? Another thing I've done that helped was to add an extra power buffer spring to increase lock time and take advantage of all the powder burn I can. At 16" you need all the lock time you can get without short stroking. It's a fine balance.

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          • #6
            My velocities ran between 2408fps @27.5g to 2487fps @28.5g. I have shot .75 to .54" groups recently with other bullets, but I can't seem to settle the 120NBT or the AMAX into the sub MOA groups I expect. My Gren has a free float JP handguard, PWS muzzle break (still want to try warped's break) and although I have the stock AA trigger, I polished it so it is more like a smooth two stage pull than the 'stock' gravel drag pull. AA Grendel edinine buffer, Nikon 3-9x scope. I am confident the rifle is shooting really great. I have to work on my techniques but my bench work is not too bad at 100yds. The winds are settling down so I want to make the next range day count. I am loading at 85-87F and shooting during higher temps. Humidity around 80-90%.

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            • #7
              Let me ask this on another tac: You want to try a hunting bullet that there is no prior published info on 'Grendel seating depths' and its a common weight (120gr). I will measure the chamber COAL using one of the bullets and get the oglive OAL using the comparator. Is it a general practice to set your first reloading depth with a standard jump, or is there a rule of thumb I can look to.

              Comment

              • bwaites
                Moderator
                • Mar 2011
                • 4445

                #8
                No rule of thumb. Most hunting bullets are somewhat blunter than the target bullets, so its important to do what you are doing. Recently I've read that starting .010 off the lands is a reasonable place, but there is no standard that I am aware of.

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                • #9
                  Great, Thanks.

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                  • #10
                    OK, I finally reloaded six groups of 123 AMAX using two different loads of 8208 XBR and one group of 120 NBT's using 8208 XBR.
                    Conditions at the range were better 87F and 10k Xwind. The results were unexpected - not because I noticed an accuracy difference with the AMAX as the JUMP changed, but the chrony picked up progressive velocity increases as the jump increased.
                    123 AMAX/28.1g XBR
                    2.260 / Vel-2406 fps / gp 1.475"
                    2.255 / Vel-2417 fps / gp 1.490"
                    2.250 / Vel-2423 fps / gp 1.606"

                    123 AMAX/28.5g XBR
                    2.260 / Vel-2469 fps / gp .612
                    2.255 / Vel-2488 fps / gp .740
                    2.250 / Vel-2506 fps / gp .528
                    120 NBT/28.5g XBR
                    2.235 / Vel 2538 fps / gp .924
                    I'm not using a great bench for shooting, so the results could be better, but it is giving me pretty consistant results.
                    Looks like I have improved my loads to where I consider them accurate enough to feel confident hunting with them. My only question now is will the NBT and AMAX be too explosive at closer ranges?

                    Comment

                    • bwaites
                      Moderator
                      • Mar 2011
                      • 4445

                      #11
                      Originally posted by steel89 View Post
                      OK, I finally reloaded six groups of 123 AMAX using two different loads of 8208 XBR and one group of 120 NBT's using 8208 XBR.
                      Conditions at the range were better 87F and 10k Xwind. The results were unexpected - not because I noticed an accuracy difference with the AMAX as the JUMP changed, but the chrony picked up progressive velocity increases as the jump increased.
                      123 AMAX/28.1g XBR
                      2.260 / Vel-2406 fps / gp 1.475"
                      2.255 / Vel-2417 fps / gp 1.490"
                      2.250 / Vel-2423 fps / gp 1.606"

                      123 AMAX/28.5g XBR
                      2.260 / Vel-2469 fps / gp .612
                      2.255 / Vel-2488 fps / gp .740
                      2.250 / Vel-2506 fps / gp .528
                      120 NBT/28.5g XBR
                      2.235 / Vel 2538 fps / gp .924
                      I'm not using a great bench for shooting, so the results could be better, but it is giving me pretty consistant results.
                      Looks like I have improved my loads to where I consider them accurate enough to feel confident hunting with them. My only question now is will the NBT and AMAX be too explosive at closer ranges?
                      I doubt that you will see them come apart badly at those velocities. At short range at 3000 FPS maybe, but at 24-2500 FPS, doubtful.

                      Interesting groups.

                      With lower powder loads your accuracy degenerated as velocity rose, indicating that they liked to be closer to the lands for accuracy. With the higher load, accuracy improved as the velocity rose, indicating that they like more velocity and more jump!

                      Very interesting!!

                      Comment


                      • #12
                        I appreciate the help working out how to measure my chamber by using a comparator and Hornady OAL gauge for each of the bullets I want to try. That takes a lot of the guess work out of developing the best COAL/Jump & load for accuracy. Good Stuff.

                        Comment

                        • PA_Allen
                          Warrior
                          • Mar 2011
                          • 333

                          #13
                          Steel89,
                          Nice set of data and thanks for reporting your results!
                          Best,
                          PA

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