Neck Tension and Pressure Signs

rustyinbend

rustyinbend

GySgt USMC 1976-1992
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    • Dec 9, 2018
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    Bend, Oregon
    #1
    I've been struggling in my 6.5-CM loads with pressure signs (heavy bolt, extractor swipe marks, etc.) in loads that absolutely should "not" be delivering them. I'm talking about velocities that are hundreds of fps slower (2400-ish) than factory ammo (2700-ish) ... that by the way ... shoots with no pressure signs whatsoever. I was at a match this weekend whining about this, and an experienced shooter (that won the match) told me to loosen my neck tension and see if that helped. I'd never thought about that, but it does make sense that if the bullet is too tight in the case mouth, it could build up internal pressure that causes pressure signs but doesn't translate into higher projectile velocity. I size necks with a bushing tie, and then expand prior to seating with a L.E.Wilson Mandrel Die ... but that die is specifically and only equipped with a generic 6.5 mandrel. Assuming it's money well-spent, I've ordered the 21st Century die and mandrel kit to give me the ability to precision size my case mouths (the kit has a range of mandrel sizes over and under the standard) ... and see if I can solve this problem.

    Anybody here agree ... that excess neck tension can cause pressure signs with low velocities? Just curious what the heavy-hitters here think about that. And ... am I on the right track with the 21st Century mandrel kit? Thanks! This whole journey is about continuous improvement ... at least that's what I keep telling myself.
     
    #2
    No, not agreed. If your neck tension is too tight, your bullet will plastically expand it (as opposed to elastic), and you’ll have roughly the same neck tension you would’ve had plus likely a deformed jacket.

    This sounds possibly like a carbon ring? Having very little data, of course. Do you have a borescope?
     
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    #3
    KnowNothing256 said:
    No, not agreed. If your neck tension is too tight, your bullet will plastically expand it (as opposed to elastic), and you’ll have roughly the same neck tension you would’ve had plus likely a deformed jacket.

    This sounds possibly like a carbon ring? Having very little data, of course. Do you have a borescope?
    Click to expand...
    I do have a borescope, and have checked that ... checks good ... I'm meticulous about keeping my rifles clean. Regarding your theory ... if it's "not" neck tension, then why does factory ammo shoot at consistently higher velocities with no pressure signs, when my handloads show big pressure signs at velocities that are hundreds of fps slower. That's what I don't get ... and why the neck tension idea made sense to me.
     
    #4
    rustyinbend said:
    I've been struggling in my 6.5-CM loads with pressure signs (heavy bolt, extractor swipe marks, etc.) in loads that absolutely should "not" be delivering them. I'm talking about velocities that are hundreds of fps slower (2400-ish) than factory ammo (2700-ish) ... that by the way ... shoots with no pressure signs whatsoever. I was at a match this weekend whining about this, and an experienced shooter (that won the match) told me to loosen my neck tension and see if that helped. I'd never thought about that, but it does make sense that if the bullet is too tight in the case mouth, it could build up internal pressure that causes pressure signs but doesn't translate into higher projectile velocity. I size necks with a bushing tie, and then expand prior to seating with a L.E.Wilson Mandrel Die ... but that die is specifically and only equipped with a generic 6.5 mandrel. Assuming it's money well-spent, I've ordered the 21st Century die and mandrel kit to give me the ability to precision size my case mouths (the kit has a range of mandrel sizes over and under the standard) ... and see if I can solve this problem.

    Anybody here agree ... that excess neck tension can cause pressure signs with low velocities? Just curious what the heavy-hitters here think about that. And ... am I on the right track with the 21st Century mandrel kit? Thanks! This whole journey is about continuous improvement ... at least that's what I keep telling myself.
    Click to expand...

    If your neck/shoulder are too soft they will not spring back properly and you will get heavy bolt lift. S&B 6.5 brass is like that. But you can turn any brass to shit by over annealing.
     
    #5
    918v said:
    If your neck/shoulder are too soft they will not spring back properly and you will get heavy bolt lift. S&B 6.5 brass is like that. But you can turn any brass to shit by over annealing.
    Click to expand...
    Totally get that. This is happening to my handloads on both virgin (Peterson) never-fired brass, as well as 1x-3x fired brass that's annealed (gently) in each sequence with my AMP. The fact that it's happening on virgin and fired brass makes me feel like over-annealing probably isn't a root cause, but it's definitely worth consideration. Thanks!
     
    #6
    depending on your needs I have been shooting slower rounds to stretch out my powder on shorter ranges where I don't need as fast a bullet like the 600 yard range I really don't need 2800+ fps its nice to have a few work ups that can go faster but I don't have access to powder to make it feasible full time yet for 1k and further Id defiantly use a faster round other wise 2680's to 2746 suit me well enough at 600 yards and under , I am sure when I get my new barrel and have to rework my loads again it could all change as my barrel now has 3800 rounds through it and showing signs it's end is a coming . Good luck to you with your competitions you can do it . I am personally more interested in getting better with my wind calls to which I at the moment stink at for me its a blind guess that I too frequently get wrong more than correct .
     
    #7
    I have heard of light charges exhibiting pressure signs just like fatter charges at the top of the load chart. As far as neck tension, the most you will see a neck shrink back after pulling a bullet is about .001-.002". Any neck tension tighter than .002" interference fit with the bullet will just deform the neck and shoulder of the case during seating.

    What powder and bullet are you running that is giving you pressure signs at slow speeds?
     
    #8
    gigamortis said:
    I have heard of light charges exhibiting pressure signs just like fatter charges at the top of the load chart. As far as neck tension, the most you will see a neck shrink back after pulling a bullet is about .001-.002". Any neck tension tighter than .002" interference fit with the bullet will just deform the neck and shoulder of the case during seating.

    What powder and bullet are you running that is giving you pressure signs at slow speeds?
    Click to expand...
    I'm seeing it with Winchester StaBall and Accurate 4350. Those are the two powders I have that are geared for my 6.5 rifles. I have a lot of Accurate MagPro, but that's not a good fit for this caliber (IMHO).
     
    #9
    rustyinbend said:
    I've been struggling in my 6.5-CM loads with pressure signs (heavy bolt, extractor swipe marks, etc.) in loads that absolutely should "not" be delivering them. I'm talking about velocities that are hundreds of fps slower (2400-ish) than factory ammo (2700-ish) ... that by the way ... shoots with no pressure signs whatsoever. I was at a match this weekend whining about this, and an experienced shooter (that won the match) told me to loosen my neck tension and see if that helped. I'd never thought about that, but it does make sense that if the bullet is too tight in the case mouth, it could build up internal pressure that causes pressure signs but doesn't translate into higher projectile velocity. I size necks with a bushing tie, and then expand prior to seating with a L.E.Wilson Mandrel Die ... but that die is specifically and only equipped with a generic 6.5 mandrel. Assuming it's money well-spent, I've ordered the 21st Century die and mandrel kit to give me the ability to precision size my case mouths (the kit has a range of mandrel sizes over and under the standard) ... and see if I can solve this problem.

    Anybody here agree ... that excess neck tension can cause pressure signs with low velocities? Just curious what the heavy-hitters here think about that. And ... am I on the right track with the 21st Century mandrel kit? Thanks! This whole journey is about continuous improvement ... at least that's what I keep telling myself.
    Click to expand...
    Ever measure the diameter on your Wilson mandrel? Or measure the inside diameter of some necks post expansion?
     
    #10
    6.5SH said:
    Ever measure the diameter on your Wilson mandrel? Or measure the inside diameter of some necks post expansion?
    Click to expand...
    That's an excellent suggestion ... "No" I haven't measured the mandrel and corelated it to a fired neck. Sometimes, the simplest suggestions are the hardest to see when you're too close to a problem. Thanks!
     
    #11
    Supposedly lower charge weights will show pressure signs as they won’t aren’t powerful enough to expand the brass enough to grab the chamber walls to keep the pressure off the case head. Up your powder charge to standard factory FPS and see what happens.
     
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    #12
    6.5SH said:
    Ever measure the diameter on your Wilson mandrel? Or measure the inside diameter of some necks post expansion?
    Click to expand...
    OK ... here's what I get:

    Fired brass Outside Diameter: .2960
    Neck Brass Width: .026 (.013 x 2)
    Interior Diameter: .2700 (.2960 - .026)
    Mandrel Diameter: .2650

    So ... what does this tell me?

    BTW ... I know I didn't screw up the measurements because the fired brass (.2700) fits nicely and without force on the mandrel (.2650).
     
    #17
    I tend to disagree about neck tension being related to this. The mandrel kit can’t hurt but your bullet is going to make the neck be whatever size it is and “tension” is limited by the spring back of the brass...if you sized .010” under bullet diameter, your bullet acts like an expander mandrel when it seats...if you then pulled the bullet, you’d have a new neck diameter, minus whatever amount of spring back your brass gives. Hope that makes sense. You said you are meticulous about cleaning. Are you getting your chamber clean and dry? No residual oil in your chamber? How about sizing lube on your brass? What’s your trim length? How far off the lands are you seating?
     
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    #19
    OREGUN said:
    I tend to disagree about neck tension being related to this. The mandrel kit can’t hurt but your bullet is going to make the neck be whatever size it is and “tension” is limited by the spring back of the brass...if you sized .010” under bullet diameter, your bullet acts like an expander mandrel when it seats...if you then pulled the bullet, you’d have a new neck diameter, minus whatever amount of spring back your brass gives. Hope that makes sense. You said you are meticulous about cleaning. Are you getting your chamber clean and dry? No residual oil in your chamber? How about sizing lube on your brass? What’s your trim length? How far off the lands are you seating?
    Click to expand...
    All good questions. Answers follow:
    • I regularly clean my chamber, but did not know residual oil was a problem. Will go for a dry (drier) chamber in the future.
    • I wet-tumble pre-seating to ensure all lube is removed.
    • I caliper all post-sized cases and trim to (below) the case length specification (1.92).
    • I seat .03 off the lands.
    Question back to you ... What is the best methodology or tool to clean a bolt action chamber and get it dry? I use a chamber brush to get the big stuff, and Swab-Its for the final clean and dry.
     
    #22
    If memory serves, aren't you shooting an MRAD? I have a 6.5C in the MRAD and could take some measurements of my brass for comparison. Relatively new to cf reloading and not sure if this would help you.

    This is the kind of stuff I learn from.
     
    #23
    rustyinbend said:
    All good questions. Answers follow:
    • I regularly clean my chamber, but did not know residual oil was a problem. Will go for a dry (drier) chamber in the future.
    • I wet-tumble pre-seating to ensure all lube is removed.
    • I caliper all post-sized cases and trim to (below) the case length specification (1.92).
    • I seat .03 off the lands.
    Question back to you ... What is the best methodology or tool to clean a bolt action chamber and get it dry? I use a chamber brush to get the big stuff, and Swab-Its for the final clean and dry.
    Click to expand...
    I use a couple of patches wrapped around a brush on a short cleaning rod (came with a pistol, .45 caliber on about a six inch handle). Slippery stuff in your chamber can cause your case to slip back on firing and smack the bolt face. Probably not your problem, unless you are “oiling” your chamber as a final step in your process.

    What bullet and charge weight?
     
    #25
    Cascade Hemi said:
    What are the load details?
    Click to expand...
    Example of a load showing pressure signs at hundreds of fps lower velocity than my factory cartridge:

    Rifle: Tikka T3x TAC A1 in 6.5-CM
    Brass: Peterson - Virgin (new)
    Primer: CCI LRP #200
    Powder: Winchester StaBall 6.5
    Charge: 41.2gr
    Bullet: Berger 140gr Hybrid Target
    Velocity: 2416

    Factory comparison is a Hornady 147gr ELD-M that gives me about 2660 fps in velocity with zero pressure signs.

    One note ... I still get pressure signs on fire-formed brass that's sized, annealed, etc. ... so it's not the new brass / old brass thing. That said, I haven't tried fired and sized Peterson brass ... that's probably my next investigative step. My once/twice-fired Hornady brass definitely shows the same pressure signs at low velocities.
     
    #29
    Cascade Hemi said:
    Can you post a picture of the pressure signs?
    Click to expand...
    Heavy bolt, ejector swipe, ejector print ... that's what I'm seeing.

    1622123224049.png
     
    #33
    Super long headspace can give you case thrust that can present like pressure issues but it’s odd to me that your velocities are so low at that charge weight. What kind of chronograph are you using and is there any chance it was glitching (low batteries) when you recorded the 2416?
     
    #36
    I’m kinda out of ideas. I’ve been sitting here thinking about what I would do if it was me. I would do two things. I would seat bullets at .010”. You might be closer to the lands than you think. Did you measure the distance to the lands using your bullet specifically (140 Berger) or is your .003” number based on some generic estimation? Every bullet shape is different.

    Hybrids tend to like jump. I can get pressure signs in my 6.5 by seating within .005” of the lands on loads that show absolutely no pressure signs at .015” jump. I start load development at .010” back from whatever I’ve measured as “touching”.

    So, 1) seat deeper and
    drop all the way down to the lowest recommended charge weight in your loading manual. Build back up from there.

    I would assume the simplest explanation which is essentially that you are overpressure, despite being down in the charge weight. I thought I’ve read elsewhere that Tikka barrels tend to be slow…that could be total bullshit too.

    keep us posted. I’m super curious.

    don’t blow your face off. 😁
     
    #37
    rustyinbend said:
    Virgin: 1.5775
    Fired: 1.5790
    Sized: 1.5760
    Click to expand...
    You should have more case growth than that on fired brass, you shouldn’t ever have to size smaller than virgin brass. Your slow loads are not expanding the brass to the full chamber size.

    clean the shit out of your chamber, make damn sure your lube is all off the brass, and shoot some loads at 2700fps.
     
    #38
    rustyinbend said:
    Virgin: 1.5775
    Fired: 1.5790
    Sized: 1.5760
    Click to expand...
    You should have more case growth than that on fired brass, you shouldn’t ever have to size smaller than virgin brass. Your slow loads are not expanding the brass to the full chamber size.

    clean the shit out of your chamber, make damn sure your lube is all off the brass, and shoot some loads at 2700

    stop worrying about seating depth and neck tension for right now, load to book data length for now and get your velocities up
     
    #40
    Do you have carbon blowback on the fired case below the neck? That’s usually a good indicator of poor brass-to-chamber sealing.

    My virgin Lapua grows from 1.577 to 1.5775 during the first firing. I think you are sizing a little more than you need to but I don’t see anything there to panic about.
     
    #43
    TxWelder35 said:
    You should have more case growth than that on fired brass, you shouldn’t ever have to size smaller than virgin brass. Your slow loads are not expanding the brass to the full chamber size.

    clean the shit out of your chamber, make damn sure your lube is all off the brass, and shoot some loads at 2700

    stop worrying about seating depth and neck tension for right now, load to book data length for now and get your velocities up
    Click to expand...
    Already tried that ... loaded to 44gr that gave me velocity of 2699 ... and the pressure signs were off-the charts. Could barely get the bolt open, and the case base was loaded with ejector swipes and prints. I'd think I don't want to try that again.
     
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    #44
    I'm going to do a major league chamber cleaning / drying ... size some once-fired brass ... decrease neck tension slightly ... and build a ladder test that already gave mid-range pressure signs and see if it happens again. Those are kind of my only options at this point. Agreed? Any other recommendations?
     
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    #45
    rustyinbend said:
    OK ... here's what I get:

    Fired brass Outside Diameter: .2960
    Neck Brass Width: .026 (.013 x 2)
    Interior Diameter: .2700 (.2960 - .026)
    Mandrel Diameter: .2650

    So ... what does this tell me?

    BTW ... I know I didn't screw up the measurements because the fired brass (.2700) fits nicely and without force on the mandrel (.2650).
    Click to expand...

    I found a Peterson case and sized it and the neck thickness is .001” thicker than Hornady so you have plenty of clearance in your chamber. Your .003” shoulder bump can’t be it neither.

    What if it’s a staball problem that only manifests itself in heavy brass?

    When I tried it in 171gr brass I got heavy bolt lift and ejector marks too. Out of curiosity I’ll try it in Federal brass and report back.
     
    #46
    rustyinbend said:
    I'm going to do a major league chamber cleaning / drying ... size some once-fired brass ... decrease neck tension slightly ... and build a ladder test that already gave mid-range pressure signs and see if it happens again. Those are kind of my only options at this point. Agreed? Any other recommendations?
    Click to expand...
    Try to only size down to a shoulder that’s only .002” smaller than “fired” size…anywhere in the .002-.004” smaller is fine.

    Double check the relationship between the seated ogive and the lands. A short-throated factory chamber would be a tad unusual but make sure you are jumping some.

    Chances are high that your neck tension was good in virgin brass so you can use that number to set neck tension in your sizing process. For example, my brass neck is .290 new and I send it over an expander mandrel that measures .263 and it seats really smoothly…so initially, I just try to get back there….a .289 or .290 bushing and then once over the expander mandrel.
     
    #48
    Thanks everyone ... lots of stuff to clean, size, measure, build, and test. Somewhere in all that, I'd expect to stumble across the solution, even if it's only by persistence and blind-ass luck.
     
    #49
    I can’t explain the slow speeds you’re experiencing but I had pressure with my Berger’s seated .010 off the lands. Seated them deeper to .045 off and the pressure went away and my SD dropped.
     
    #50
    tnc said:
    I can’t explain the slow speeds you’re experiencing but I had pressure with my Berger’s seated .010 off the lands. Seated them deeper to .045 off and the pressure went away and my SD dropped.
    Click to expand...
    I seat to about .025 ... but I'll build some tests backing off that far, and see what happens. Thanks.
     
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