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How does seating depth/COAL affect pressure

Discussion in 'Reloading' started by country85, Aug 11, 2012.


  1. country85

    country85
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    I know that when I used to load for my .223 bolt action I used to load longer than "recommended" overall length. I've been loading Lee 310 grain flat points with unique, haven't got a chance to fire any yet but am curious on how seating depth will affect pressure, The shallower crimp groove(1.720) in these just kisses the rifling in my H&R so I'd really like to use it but am just unsure how it will affect things in a straight wall case, any help would be greatly appreciated!!

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    #1 country85, Aug 11, 2012
    Last edited: Aug 11, 2012
  2. WeeWilly

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    Deeper will increase pressure, less deep will decrease pressure. How much depends on the case volume, the powder used, etc. etc.
     

  3. dkf

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    #3 dkf, Aug 11, 2012
    Last edited: Aug 11, 2012
  4. cowboy85306

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    Here's the heck of it- loading bullet to touch origin of the rifling will increase pressure. Use established techniques in specific applications to reduce risk to the rifle and the shooter. Do A LOT of research as to what has worked for others, follow their recipe EXACTLY until you become expert. No need to risk a lot reinventing the wheel. Go to additional forums and see what the Single Shot rifle and TC Contender/ Encore guys do. Just my 0.02
     
    #4 cowboy85306, Aug 12, 2012
    Last edited: Aug 12, 2012
  5. country85

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    I agree with you about seating in the rifling slightly increases pressure, but seating the bullet shallower decreases pressure, so they will pretty much cancel the difference in pressure out, at least this is my experience with loading necked cases, I should have worded my question differently, I was wondering if there was any difference in loading a straight necked case and a necked case longer than normal
     
  6. fredj338

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    Read my little blog on OAL. Seating deeper in handguns always increases pressures. The opp for rifles. Seating longer for rifles increases pressures as your bullet is closer to the lands. Consider the fastest rifle powder is slower than the slowest pistol powder, deeper seating in a rifle doesn't do much to pressures, compressing rifle powders is pretty common. SO for slow magnum powders, seating deeper in handguns will increase pressures, but not as much as faster powders.
     
    #6 fredj338, Aug 13, 2012
    Last edited: Aug 13, 2012
  7. country85

    country85
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    AHHHHHH this is getting really confusing!!! lol Thanks for all the advice and reading your blog helped alittle bit. BUT I understand how wedging the bullet in the rifling can increase pressure, and I understand that in rifles compressed loads are common, but my known safe load of H110 and a 240 grain xtp is a compressed load in the 44. I guess thats what I get for getting a rifle in a pistol caliber
     
  8. WeeWilly

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    H110? If you are loading H110/W296, longer might give you some issues. This powder likes pressure and doesn't like a lack of pressure. Compressed or no space is better than free space with H110.
     
  9. fredj338

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    H110/W296 is actually a slow pistol powder that has some use in small volume rifle rounds like 30 carbine & 22Hornet. So yes, it takes compression well, many heavy bullet loads in the magnums will be compressed w/ H110.
    So don;t get over whelmed w/ OAL. It's pretty easy to remember. Shorter in handguns raises pressures, longer in rifle raises pressures. It's about that simple.:supergrin:
     
  10. country85

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    Lol thanks ya'll!!! Well I finally got to the range for a few today and got to fire some test shots. I decided to start with a powder that's a little more forgiving on pressure so I load some 310'ers up with 6.8 grains of unique. I didn't have a crono but I can tell you that the one's seated longer, are quieter and seem to have less recoil. Also have a nice wad of fire out the end of the barrel, which if I'm right means I"m not getting complete powder burn. The ones seated shorter were louder and had more of a pop in the recoil. Once I get my gas checks I'm going to load some up with H110.
     
  11. fredj338

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    I am not sure you understand the relationship of powder burn rates to pressures. Unique is certainly more forgiving than TG presure wise, but H110 is about as slow as it goes. The only issue w/ H110/W296 is they are NOT powders you down load. Starting load & max are usually less than 3% diff. Unique under a heavy bullet like that is an over pressure event waiting to happen. keep in mind the heavier the bullet the slower the powder in order to get higher vel w/ less pressures. Also, anytime the bullet is touching the rifling, pressures are going up. Without a small bullet jump, you hit max pressure on ignition. So a max load that is not touching lands will have lower pressures than the same max load "kissing" the lands.
     
    #11 fredj338, Aug 14, 2012
    Last edited: Aug 14, 2012
  12. country85

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    I should have been more clear. What I ment was a load that was more forgiving. 6.8 grains of unique is going to produce less pressure than a full charge of 22 or 23 grains of H110. I also seated alittle deeper than I orginally had begin with. I'm now no where near touching the rifling. I'm going to load some with H 110 and NO gas check, just for S&G's, so far my home made lube is doing great, better than store bought, with zero leading.
     
  13. fredj338

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    That load yes, but you do understand that just because the round produces less vel, recoil & blast does NOT mean it isn't running high pressures. You'll run well over pressure w/ Unique & heavy bullets long before you get anywhere near the vel/recoil of an H110 load.:shocked:
     
    #13 fredj338, Aug 14, 2012
    Last edited: Aug 14, 2012
  14. PEC-Memphis

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    <<<< There's the 3.2gr TG/147 gr FMJ/Mean SAAMI OAL Right There.

    I've done quite a bit if testing with titegroup in 147 gr. FMJ 9mm IDPA loads. I made 3.1, 3.2 and 3.3 gr loads with min/mean/max SAAMI OAL. I made 5 rounds under each variation - then made 10 rounds of each min/mean/max OAL for 3.2 gr. I chrono'd each loading. I didn't see statistically significant velocity changes between OALs for a given power loading.

    I know this goes against conventional wisdom - your mileage may vary.
     
    #14 PEC-Memphis, Aug 14, 2012
    Last edited: Aug 14, 2012
  15. unclebob

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    With TightGroup and 147 grain bullet the load is 3.3. 3,3 is max and you do not reduce the load below that.
     
  16. fredj338

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    Actual OAL dims would be useful. I don't see a lot of diff until OAL reduces by 0.030". changes by that small of an amount WILL SHOW vel/pressure changes. With some powders more than others. At 0.060" change, things happen rapidly.
    The faster the powder the more significant OAL changes will be, particularly in handgun rounds, particularly in small volume rounds.:dunno:
     
    #16 fredj338, Aug 14, 2012
    Last edited: Aug 14, 2012
  17. PEC-Memphis

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    I've only loaded about 7,000 147's @ 3.2 gr TG making about 130 PF without a single problem.
     
  18. unclebob

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    And the books say 3.3 max and DNR, Do not reduce. Yes I and at least 5 other people that I know have loaded Tightgroup and 147gr. bullets below 3.3 for many hundreds of thousand rounds without any problems. Look in Speer Manual # 13 Manual # 14 does not even list Tightgroup with the 147 gr. bullet any more.
     
    #18 unclebob, Aug 15, 2012
    Last edited: Aug 15, 2012
  19. PEC-Memphis

    PEC-Memphis
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    Well, that should tell you something.
     
  20. unclebob

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    No it just mean we have been lucky. Our loads now with the 147gr bullet is 3.3.
     
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