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Can someone please explain why

Discussion in 'Handloading and Reloading' started by papajoe222, Sep 30, 2019.

  1. papajoe222

    papajoe222 Member

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    I’m fairly new to reloading and for the life of me don’t understand why a load for a lighter bullet requires more powder than used for one 10-15gr heavier. I noticed this when I first started loading .223Rem and just figured it had something to do with the different designs rather than the difference in weight. Then I began experimenting with different weight bullets for my .308Win and noticed that even for the same HPBT bullets, starting loads were heavier for the 150gr and 155gr than for the 168gr I’d been loading. My way of thinking would be just the opposite, like putting a 350hp engine in a full size car, but needing a 400hp for a compact.
     
  2. gotboostvr

    gotboostvr Member

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    More powder=more pressure
    More bullet weight=more pressure

    The brass can only withstand so much pressure till it gives up the ghost.
     
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  3. Milt1

    Milt1 Member

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    Basically, gotboostvr has answered your question!
     
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  4. Skeptic13

    Skeptic13 Member

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    If you take two powder charges that weigh the same and place them in different size spaces the one in the smaller space will create more pressure than the one in the larger space. Since heavier bullets are longer but seated to the same overall length as lighter ones the heavier bullet creates a smaller space inside the case and creates more pressure. You see the same issue in .308 vs 30-06 where it takes much more powder to create the same amount of pressure in the 30-06 even though you are using the same bullet. The 30-06 has more space.
     
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  5. twistytorn

    twistytorn Member

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    Think of the lighter bullet as allowing you the opportunity to add more powder before reaching the pressure limitations of the rifle. The same pressure acting on lighter bullet = more velocity.
     
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  6. Swampman

    Swampman Old Fart

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    Reduced case volume is important, but as gotboostvr said, bullet weight is likely to be the main contributing factor.
     
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  7. MEHavey

    MEHavey Member

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    Modern powders are progressive burners --> the more pressure, the faster the burn rate increases --> the faster pressure rises (spiraling upwards)

    Light bullets don't produce the resistance needed to raise that progressive-pressure as quickly as heavy bullets do.
     
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  8. Demi-human

    Demi-human Member

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    Almost.
    The heavier one requires less, else it would be too much pressure.
     
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  9. papajoe222

    papajoe222 Member

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    So......................a heavier bullet doesn't begin its forward move until so much pressure is built up behind it.. Because of its weight and resistance to friction, pressure is able to build up behind it using less powder than a lighter bullet that begins its movement easier, and so needs more powder to burn behind it to achieve sufficient pressure before it does.
    Makes sense. And I wondered why I had such a hard time in physics class.
     
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  10. Bfh_auto

    Bfh_auto Member

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    You are changing multiple variables. The increased powder charge also increases velocity.
    If you run two bullets of different weights with the same powder charge, the velocity doesn't change very much but the pressure is different.
    Look at data for a 223 with imr4064. A starting charge of 23 gr gives nearly identical velocity with a 55 and 60 gr. But the 60gr has a higher pressure.
    To make the pressure equal, you must use more powder which gives you more velocity.
    Clear as mud?
     
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  11. LiveLife

    LiveLife Member

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    It's Newton's second law of motion - https://en.wikipedia.org/wiki/Newton's_laws_of_motion

    Force = Mass x Acceleration

    So if you decrease weight (Mass), then you need to increase powder charge (Acceleration) to produce same chamber pressure (Force).
     
  12. Toprudder

    Toprudder Member

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    I remember doing a load workup with two different weight bullets from the same manufacturer. The charge ranges happened to overlap. I noticed that, for the same charge, the heavier bullet was actually faster - more pressure. I will have to dig around and see if I can find the data.
     
  13. Toprudder

    Toprudder Member

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    From my 45acp load data:

    Xtreme bullets, 200 RN vs 230 RN

    Nitro 100NF powder
    200 - 4.4gn, 704 fps
    230 - 4.3gn, 721 fps

    RedDot powder
    200 - 4.7gn, 731 fps
    230 - 4.7gn, 758 fps

    Trailboss powder
    200 - 4.5gn, 654 fps
    230 - 4.5gn, 657 fps
     
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  14. murf

    murf Member

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    and this is called "inertia", the resistance to motion. the more weight, the more resistance to motion (speeding up, or slowing down).

    murf
     
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  15. Metal God

    Metal God Member

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    ^^^^ that ^^^^

    I'll add this is all about burn rate , barrel length , bullet weight and space behind the bullet as it travels down the bore .

    In some ways it's simple physics . Lets start with the idea of the powder/gases expanding at a certain rate . Those gases are going to expand at a specific rate no mater what once ignited and continue to expand at that rate filling any space available until there is no longer any powder to burn . Think of it as filling a air tank with compressed air at a specific rate . A little one litter tank will fill much faster then a one gallon tank .

    OK now place and obstruction in the way like a bullet preventing those gases to expand , what can happen ? Well if the bullet is the right diameter for the bore the pressure being generated by the expanding gases will push the obstruction ( bullet ) out of the way and down the bore at a sufficient enough speed to allow the expanding gases to fill the ever growing space in the bore behind the bullet . This is when burn rate and bore diameter become very important . As the bullet travels down the bore it's leaving more and more space behind it for the gas to fill . This is a good thing because the gas needs a space to expand into .

    However at 2" down the bore a 9mm bullet will have less space behind it to fill then a 45 will do to the diameter of the bullets respectfully . The weight of the bullet is now also becoming important as well . The lighter the bullet in a given cartridge the quicker it can be pushed down the bore at a given pressure . Resulting in the space behind the bullet to increase faster as well allowing more space for the expanding gases to fill . Keeping in mind powders have a specific burn rate which is how fast the gases will expand . Meaning the further the bullet goes down the bore the more space there is for the gases to fill .

    OK so now we have a powder/gas expanding at a specific rate and a bullet that is traveling down the bore at a rate fast enough to out run the expanding gases . If you now add weight to the bullet it not only doesn't move down the bore as fast because of basic physics you also have added more baring surface which resist it's ability to move freely down the bore as well do to more friction . OK with a heavier bullet you now have made it much harder to push the bullet down the bore resulting in the bullet moving much slower then a lighter one . But wait we still have those same expanding gases expanding at the same rate . However now you have an obstruction moving slower down the bore . So now the space behind the slower moving heavier bullet is expanding less resulting in less space for the expanding gas to fill , remember the gas is going to keep expanding at it's specific rate regardless of what's in front of it trying to slow it down . This is why you put less powder for a heavier bullet , because the heavier bullet can't get out of the way of the expanding gases fast enough like the lighter bullet can . This is also why if you want to push a heavy for cartridge bullet faster you use a slower for cartridge powder . The slower powder allows the bullet to get further down the bore before it hits it's peak pressure resulting in more space behind the bullet for the gases to expand into .

    I hope I just made sense right there . I've been writing and going back a rewriting for so long now I'm not sure what all I've said lol .
     
    Last edited: Oct 1, 2019
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  16. jmr40

    jmr40 Member

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    There is also case capacity to consider. In the same caliber heavier bullets are longer and take up space in the case. You couldn't fit the same amount of powder in a 308 case with 180 gr bullets that you could with a 150 gr bullet if you tried. In simple terms if you're shooting 150's with 47 gr of powder, 165's with 46 gr of powder and 180's with 45 gr of powder the pressure will be very nearly the same with all of them. And it is pressure that is the limiting factor.
     
  17. Metal God

    Metal God Member

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    Yes but really no that's not the reason . You can seat the bullet longer , use ball powders that don't fill the case as much , use drop tubes or even vibrate the case to force the powder to settle . There are any number of ways to get the extra room you need to seat the heavier bullet . IMO it's much more about the burn rate , bullet weight and available space behind the bullet . Example you can use a fast burning powder like Titegroup in a 45acp with heavy 200 and even 230gr bullets but it's not a good idea to use it in 9mm and heavy 147gr bullets . even though the 45 is likely moving slower then the 9mm . Why ? As I pointed out earlier when both bullets are half way down the bore . The 9mm has much less space to fill behind the bullet . So those fast expanding gases fill that space up to fast behind a 9mm bullet . How ever the 45 diameter bore has much more available space for those expanding gases to occupy . Keeping in mind that the bullets are moving forward creating more and more space for the gases behind them . The further down the bore each bullet goes the % of space being gained in the 45 is much more then the 9mm allowing the faster gases to continue to expand with out over pressure . While the 9mm would be way over pressured because the bullet can't get out of the way fast enough . That's where your pressure issues are not because your bullet is seated deep or your charge is compressed .
     
  18. FROGO207

    FROGO207 Member

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    Reading through this thread has reminded me that there are many talented people who contributed to the science of reloading over the years so we as reloaders can safely ply our craft without a degree in rocket science with success. Also that there are a bunch of members on here that are willing to share their knowledge freely for the good of the site. THR rocks.:thumbup:
     
  19. edwardware

    edwardware Member

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    It doesn't.

    For the same charge of powder, a lighter bullet will produce lower peak pressure than a heavier bullet, because it's lighter (easier to accelerate down the bore). Consequently you can use a larger charge behind a lighter bullet while still respecting the same peak pressure; you aren't required to, but you can.
     
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  20. lordpaxman

    lordpaxman Member

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    Poor Titegroup, everyone picks on it :). I appreciated most of your post, however, there's just a few competition shooters that would disagree with your statement. That and Hodgdon does supply 9mm TG data for 147 gr bullets. The wilder crowd will also load a 160 or 165gr 9mm with TG.
    Your description of the available volume over time is correct, but, the published load data for TG reflects a correspondingly smaller charge for 9mm vs .45. And of course, the mass of the bullet factors in to this as well.
     
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  21. Walkalong

    Walkalong Moderator Staff Member

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    True, but I would agree with @Metal God that Titegroup in 9MM with 147 Gr bullets is not nearly as user friendly as Titegroup in .45 ACP with 230 Gr bullets. A lot less wiggle room.
     
  22. Metal God

    Metal God Member

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    Lordpaxman ' I didn't say Titegroup can't be used in heavy bullet 9mm loads just not a good idea . Loading it down may be a good option but in 9mm and heavy bullets Titegroup can get a little squarely at the higher pressures .

    Was my overall point and sorry if it did not read that way .:)
     
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  23. lordpaxman

    lordpaxman Member

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    @Walkalong I'm not trying to start an argument here, but you can get in to dangerous territory with TG in either caliber, stay between the lines and things are ok. Step outside the lines and you're at risk. I run TG in both, but I don't feel more comfortable in one or the other.
     
  24. lordpaxman

    lordpaxman Member

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    @Metal God I completely get it, believe me!!! I understand TG's squirrely nature probably more than most, but I wouldn't promote the idea that it's somehow safer in .45 than 9mm. Uber fast powders have to be respected in any caliber.
     
  25. Texas10mm

    Texas10mm Member

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    My problem with TG in 9mm is the extreme sensitivity to seating depth.

    I cast and load three different heavies for the 9mm. One, the NOE Elco can be seated out quite a ways due to the design of the bullet. This is a plus because even in HP form it's a 151 gr bullet. The others have to be seated quite a bit deeper.

    IMHO if Titeboom is the answer, you phrased the question wrong.
     
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