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Pressure INCREASE due to added barrel length?

Discussion in '10mm Reloading Forum' started by Any Cal., Dec 28, 2011.

  1. Any Cal.

    Any Cal.

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    Oct 27, 2008
    Starting this thread fresh to keep from cluttering 21 Carriers thread on the subject. Please, post up thoughts or experience you have on the subject. I am interested, and have an opinion, but will wait a bit for others to chime in first.
     
  2. Yondering

    Yondering

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    I've also read on this forum that longer barrels cause higher pressure. Personally, I don't believe that; it doesn't make sense at all, if we're talking about Glock-length barrels. Pressure will exist in the barrel longer, of course, but peak pressure occurs long before the bullet exits, especially in a large bore cartridge with a relatively small charge of fast burning powder. Pressure is already dropping off by the time the bullet exits a G29 barrel; it's hard to picture why it would increase again if the barrel was longer.

    (If we were talking about a very short barrel, then pressure might increase with length, up to a couple inches.)

    I'll comment more later, but read this article: http://www.dtic.mil/ndia/2010armament/WednesdayCumberlandPhilipDater.pdf

    Edit: OK, back now, so look at page 14 on the link above, where they show bore pressure at different barrel lengths. At 5", the pressure was already down to about 25,000 psi, from a cartridge that measured 55,000 psi peak pressure. This is from a round with a smaller bore and larger volume of powder than the 10mm; peak pressure happens in the 10mm sooner than it does in a 223/5.56.
     

    Last edited: Dec 28, 2011

  3. TDC20

    TDC20

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    That was a really interesting article, Yondering. Thanks for posting that.

    I also have to agree with Yondering. I don't think the barrel length will affect the peak pressure, which will occur long before a bullet exits a 4.6" barrel. Take a look at some pressure curves that XmmAUTO posted here:
    http://glocktalk.com/forums/showthread.php?t=1372717

    I created a spreadsheet that applies a theoretically linear pressure/acceleration (incorrect, I know, but it proves a point). This shows a 180gr. bullet leaving the barrel at 0.592ms for a 4.6" barrel and at 0.676ms for a 6" barrel. The 0.592ms corresponds fairly close to what XmmAUTO's ballistics program calculated for his 4.6" test barrel. The theoretical velocity for the 4.6" barrel on my spreadsheet was set to 1296fps by setting a certain pressure profile. This same pressure profile was then applied to a 6" barrel and yielded a theoretical velocity of 1480fps. Now, comparing ACTUAL measured velocities for ACTUAL 180gr ammo loaded to 1296fps from my 4.6" barrel, that actually yields 1400fps from my 6" barrel. What I believe this demonstrates is that the pressure is dropping rapidly as the bullet is traveling that last 1.4" of barrel. This isn't proving anything, because there are a lot of variables, but it does add support to the argument that barrel lengths are not a factor in peak pressure. Again, the plots from XmmAUTO's pressure measurements would also support this.

    I will, however, end by saying this...all barrels are not created equally. There will be differences in chambering, bore dimensions, depth of rifling, surface roughness, etc. that can and will make differences in pressures when firing the same ammo in each. It's not an apples-to-apples comparison using a 4.6" Glock barrel vs. a 6" LWD barrel, but I don't want to confuse that with the primary issue here, which is strictly how barrel length affects pressure, all other factors being equal.
     
  4. Any Cal.

    Any Cal.

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    Oct 27, 2008
    I will say that I am of the opinion that barring some extreme circumstance, barrel length will not affect peak pressure in the least, unless it is a matter of the barrel being too SHORT for the powder to develop peak pressure. I will post some graphs later when I have time, so anyone that wants to can say otherwise.

    21 carrier mentioned total average pressure, which could be related, but I will wait for him to chime in as well.
     
  5. Gunnut80

    Gunnut80 Raoul_Duke

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    I found the explanation mudrush gave: http://www.glocktalk.com/forums/showpost.php?p=17684398&postcount=33

    He could be wrong but given his pressure testing equipment and the way he explained it I'm inclined to believe him.

    Of course, as I said, he could be wrong.

    And I wasn't meaning to be rude in 21Carrier's thread, Any Cal. No offense intended. I'm much better at talking than typing or writing!
     
  6. Yondering

    Yondering

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    Dec 3, 2011
    I'm very skeptical of mudrush's explanation. It sounds right at face value, until you look at the details closer.

    -He says with Silvertip ammo, the pressure tops out at about 3.5 inches, but his slower powders build pressure up to 6". Problem is, the powders that work in a 10mm just aren't that slow. See the article linked above; the 5.56 uses much slower powders than anything that would work in the 10mm, but the pressure peak happens long before the bullet hits the 5" mark.

    -He's saying the pressure is still at 37,000 psi when the bullet leaves a 4.6" barrel with his loads. If this were true, these would sound louder than most rifle rounds. The sound of the shot is related to the pressure released at the muzzle. 37,000 psi would be incredibly loud. The fact that the 10mm does not produce the sound levels that most high powered rifle rounds do, indicates that the pressure is lower when the bullet exits the barrel.

    I'd like to see some actual pressure measurements with different barrel lengths in the 10mm. I suspect that mudrush made a calculation error in figuring out bullet distance traveled vs time.
     
    Last edited: Dec 28, 2011
  7. Any Cal.

    Any Cal.

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    Alright, I've got a couple minutes. here are my thoughts, with some of the info that I base them on.

    Take a look here at the chart that a guy drew up on Quickload. I realize that it isn't an actual measurement, but it shows pressure against bullet travel in inches...

    [IMG=http://img406.imageshack.us/img406/1111/claysvs2400.gif][/IMG]


    You can see that even with lots of a relatively slow powder, peak pressure happens very early in the cycle.

    Now look here at Ballistics by the inch, and notice how the velocity jumps from 2" to 3", then less from 3-4, even less from 4-5, etc.

    http://www.ballisticsbytheinch.com/10mm.html

    If the pressure peaked at 5 or 6", then we would see drastic jumps in velocity of the same magnitude from 5-6", then a strong jump from 6-7", and so on, but we don't. All you see is a gradual decline in the rate of increase of velocity, as would be expected if the pressure peaked early in the barrel, like in the first inch or two.

    As far as anecdotal evidence, if there was a pressure peak at 6" of barrel in handguns, you would see everyone shooting 6"+ barrels, as that length would get them into the jumps in velocity where each inch would be worth 1-200 FPS in a 10mm. What we see happening though, is that there are only marginal increases going from 4-5", less from 5-6", etc., to the point that an 8" barrel is pointless because the return on the length is so minimal.

    If pressure peaks early in the barrel, then that leaves time of the bullet in the barrel to cause issues, and timing of the lockup to cause issues. If time in the barrel caused higher pressures, then why are the SAAMI specs for a cartridge the same, whether it is to be used in a pistol or rifle? If the rifle barrel works the brass too much due to the time in the barrel, then why aren't SAAMI specs dependent on barrel length? Shouldn't you be able to load .223 for an AR pistol much hotter, since the round would spend less time in the barrel? No, because the SAAMI spec is for Maximum pressure, not the time spent at pressure.

    What about the lockup, perhaps the barrel is unlocking when pressure is high? That I don't know enough to talk about. I can say that the pressure in the gun when it unlocks is far less than the peak pressure that the brass must endure at the beginning, and that when pressures are high, the brass is pushed against the chamber walls, keeping it from moving. That being the case, if the extractor can pull the case out of the barrel, that means the pressure has already dropped considerably, letting the case spring back close to its original size. I can not guarantee it, but I believe that the pressure will not drop that far until the bullet has exited the barrel.

    _Edit_ Looking at Yondering's PDF, the chart showing bore pressure vs. barrel length shows that at @ 5", that round had already developed peak pressure and was heading back down, and that was for a 5.56.

    Sorry for the book, but I figure if I explain myself, you can show where one part or another is in error, if it is.

    Gunnut-don't sweat it.
     
    Last edited: Dec 28, 2011
  8. Yondering

    Yondering

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    Last edited: Dec 29, 2011
  9. glock20c10mm

    glock20c10mm

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    :agree: Not much else to add to that!

    In the 6" barrel realm my theory is that the above thought is dead on.

    For all the powders that work from decent to super-good in 10mm Auto along with whatever combination of primer, COL, throat lead before rifling, and bullet, clearly we're all of the understanding that max pressure will be built up and just as quickly be dropping back down well before the bullet makes it beyond ~2" (after subtracting chamber length) of barrel length.

    And again recalling some of Mudrush's posts, he more than a couple times also talked about heavier weight springs just for this reason in 4.6" barrels alone. Why the situation wouldn't be enhanced further with a 6" barrel is beyond me.

    By the same token this is also just my THEORY at this point. I'ld suggest it's a pretty strong theory at this point though. And it sounds like some agree already. Anybody disagree???
     
  10. Any Cal.

    Any Cal.

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    Oct 27, 2008
    ^^^ The problem is that I'm not sure about that statement. A good 'smith could say for sure, but if the lockup is held by the brass sticking to the chamber, then the action could not work until the pressure subsided (bullet left barrel). I don't know exactly how events are timed in that respect.

    In any case, while it could cause issues, it wouldn't be an over pressure situation, it would be things like strange marks on the cases, or glock smiles that occured at a lower pressure, since the opening of the action would put the thinner portion of the brass out over the feedramp, rather than leaving the web over it. You would probably be able to see that if it was happening though, since the smiles would be higher up on the brass.

    -Edit- After looking a few things up, I am pretty sure the barrel will stay locked up until the bullet exits. Consider this. At the start, barrel and slide are locked together, and the firing pin has forward momentum. At firing, the 135-220g bullet is being pushed forward by gas. The gas is being backed up by 20+ oz of mass, or easily 40x the weight of the bullet. As the bullet is traveling forward, it is also pushing against the barrel due to friction, which along with the recoil spring is helping the 20+oz of the slide stay in position. While the bullet is in the barrel, there is little reason for rearward motion of the pistol.

    Once pressures have come up, the case is pushing against the chamber, which locks the slide and barrel together via the extractor/extractor groove. I don't think that the barrel and slide can unlock when pressures are high. If there was a need, it would be more likely for the extractor to rip a chunk out of the case head then to pull out a couple square inches of brass from the chamber, held in place by many thousands of PSI. It seems more reasonable to assume that the slide doesn't start it's rearward travel until the bullet has left the barrel, at which point there is no friction pulling the barrel forward, there has been time to overcome the mass of the slide, and the slide and barrel can unlock because pressure has dropped and the case is no longer sticking to the chamber.

    I don't know if it is related, but it is said that you can compensate a pistol to the point that there is no longer enough recoil left to operate the gun. If that is the case, it means that the recoil is coming after the bullet leaves the barrel/comp, not while the bullet is still in the barrel.
     
    Last edited: Dec 30, 2011
  11. glock20c10mm

    glock20c10mm

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    Well said!

    There are still some basic facts involved. Namely that within ~2" of barrel I believe we know for a fact that max PSI has dissipated some and is continuing to dissipate really fast.

    In my mind, one question remains. Does an additional 1.42" of true barrel length (4.6" VS 6.02") throw a wrench into what you said? I DON'T know. Not suggesting I do. I only continue to parrot it without know that same as you mentioned above; "I don't know exactly how events are timed in that respect." But what else is there?

    And now you bring up another question in my mind. That is; Who actually has checked the specific location where any 10mm brass case has blown out in what would have been the 6'oclock chamber position in a Glock, against bad smiles in hot hot loads that didn't blow in that same position?
     
  12. glock20c10mm

    glock20c10mm

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    I just want to say I think it's great we're hashing this out. TYVM Any Cal. for starting this thread! IMO too many GT members are starting to practically throw their OPINIONS around as loose fact at a minimum regarding this specific subject. Not cool when none of us know for sure. Not YET anyway.

    Here's to future knowledge - :cheers:
     
  13. XmmAUTO

    XmmAUTO 10mm Addict

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    Here's my perspective on Pressure vs Barrel length vs Lockup

    I'll start by saying that I'm no way a very educated person.

    Lets start at Lockup:

    It's my understanding that in a Browning designed short recoil system, the barrel and breech stay locked together until the bullet has left the barrel. Now looking at Newton's Third law every action has an equal and opposite reaction.This of course is how the slide mass is accelerated opposite the direction of the bullet. The question is when does the mass of the slide begin to accelerate? After googling " High speed camera 1911" there is footage that in my opinion shows that the bullet leaves the barrel before the mass of the slide begins to move. Think about it. Take a bullet and try to manually push it down a barrel from the chamber. It takes a lot of force to do. Why?
    DRAG and FRICTION. It is my understanding that the bullet is literally keeping the barrel from moving rearward through a function of drag caused by friction until it exits. The remaining pressure acting on the base of the bullet and cartridge case exert the rearward force necessary for cycling the action.
    Now in a straight blowback action the barrel remains fixed and the mass of the slide is directly acted upon by the force of pressure while the bullet is still in the bore as the two are not locked together. This is where we see movement of the case retarded by pressure.

    Of course this is just my hypothesis.

    Now on to pressure:

    Pressure along with other variables is what gets velocity. This is where "There is no replacement for DISPLACEMENT". Larger caliber guns use pressure more efficiently. It's simple, more area for pressure to act upon. Hence a 10mm will never be a 500 S&W Magnum. Now in a small caliber just as in a small engine to get more power we increase pressure which in turn drives the smaller mass faster creating more energy. Think small 4 cylinder turbocharged high compression engine vs naturally aspirated big block v8.
    The only drawback to increased pressure is being able to contain it without failure of the cartridge case, breach and chamber. This is why we are looking for FULLY supported chambers.

    Now barrel length:

    I would think that barrel length does play a role in pressure. In what way and how though, I don't know. I do know that peak pressure should be early in the cycle and taper off. How much pressure remains at bullet exit is dependent on a lot variables. When I get my test equipment up and running I'm sure we will learn more. I hope to also do testing in a 5", 6" and 10" barrel as well as a shorter G29 length barrel. Add up the cost and I'm looking at least another $1000 just in barrels. My project has been delayed and is taking time, please be patient.

    On a side note please keep in mind that the test results I posted were not calibrated in any way.
     
  14. 21Carrier

    21Carrier Until I Gota 29

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    On the pressure issue:

    I think longer barrels do increase pressure, but total average pressure, not peak. Whether more overall pressure can smile brass or not, who knows. I saw a previously safe load almost blow up a gun when barrel length was increased. To be fair (and as TDC pointed out), there were too many variables for us to really gain any useful knowledge from that situation (though I had previously taken it as proof, I have settled down and thought about it more). First, the longer barrel was conventionally rifled. Beyond that, the chamber is tighter, which should make any expansion in the case head more pronounced. Even a difference in finish or the coefficient of friction in the barrel could make a HUGE difference in pressure.

    At the end, I think a longer barrel increases total pressure, and I think any graph of time vs pressure should show that. The question is which smiles brass? Average pressure or peak pressure, or both?

    On unlocking:

    I think the gun starts to move the moment the bullet does. Remember, that does NOT mean that UNLOCKING begins at that moment. A Browning type action allows for some rearward movement before the barrel and slide unlock. Why do I think the barrel/slide start to move immediately? Well, what's causing them to move in the first place? They are moving rearward because the bullet got fired forward, right? So why would the slide and barrel not start moving at the same time? If they don't start to move until the bullet is gone, that doesn't make sense. How can the bullet move them if it's in the air 20 feet away? It can't. If the bullet is what's doing the moving, I'm pretty sure it has to happen while it's in the barrel. It just happens so fast, and the slide is moving so slowly in comparison, it seems like it's not moving.

    I think I understand this right, but correct me if I'm wrong: We have two things going two opposite directions, and both should have the same momentum. Object one is the slide/barrel assembly (called S from now on), and object two is the bullet (called B from now on). Momentum is described by the equation p=mv, where P=momentum, m=mass, and v=velocity. Since momentum is always conserved, we know that both S and B must have equal momentum (S=B). However, because B's mass is so low in comparison to that of S, it creates a similar difference in velocity between the two. So if we throw some numbers in there, we get this:

    m1=180gr bullet mass = 11.66g
    m2=G20SF slide/barrel mass = 625g (guess based on 780g unloaded mass for whole gun)
    v1=180gr bullet velocity = 1300fps
    v2=G20SF slide/barrel velocity=?

    (m1)(v1)=(m2)(v2)
    (11.66)(1300)=(625)(v2) Solve for v2 and get 24.25fps as the velocity for the slide/barrel combination.

    So that means the bullet is going 54 times as fast as the slide. What all this means to me is that even with slow motion video, it likely just LOOKS like the slide is not moving while the bullet's in the barrel. In the time that the bullet moves the roughly 3.5" down the barrel (discounting chamber length), the slide only moves .06". That's not enough for us to see in a video, so it LOOKS like it's not moving until the bullet is gone. Again, correct me if I'm wrong with my math above. It's been a while since I took Physics.

    I am pretty sure that the slide/barrel HAS to move immediately. The slide NOT moving until the bullet leaves the barrel would seem to constitute a suspension of the laws of physics. If I understand the Browning short-recoil locking mechanism right, it does not mean that the slide/barrel is locked to the FRAME, but rather that they are locked TO EACH OTHER. They are able to freely slide along the frame as soon as the bullet moves, but they remain locked to each other, until the bullet leaves the barrel, and pressure lowers. You can see this by pushing on the muzzle of your Glock. The slide and barrel will move back about 1/8", while remaining locked together. After all, the brass being pressurized and all holds the slide and barrel together. That does nothing to hold the slide/barrel assembly in place on the frame. While in battery, the only thing that holds the slide forward is the recoil spring. You could weld the slide and barrel together, and you'd still be able to move the slide/barrel back that 1/8" or so until the barrel's lug hits the locking block.

    Also, Mudrush was clear that he recommended stronger springs ONLY to make sure the slide was fully pushed into battery. If I remember correctly, he was not big on believing that stronger springs helped slow unlocking. He said it was just insurance to make sure the gun didn't fire while slightly out of battery.
     
    Last edited: Dec 30, 2011
  15. nickE10mm

    nickE10mm F.S.F.O.S.

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    I believe your math is correct. Well said.

    As for what Mudrush thought .... I kinda remember being in a thread talking about this very thing with him and he menitioned that it DID slow unlocking.... which I disagreed with. I did mention that a benefit of stronger springs would be to make sure the rounds were fully seated in the chamber to prevent a case rupture. (Glocks have a tendency to fire a TINY LITTLE BIT more out of battery than other designs: the 1911, for example. :))

    Great thread and discussion. I've said it before and I'll say it again, I wanna see a test of someone running a hot load and getting brass smilies.... and then switching to a heavier spring setup and subsequently NOT getting smiles. That would help the "unlocking theory".... Until then, I'll stick with my premise that, while the heavier spring will help with other aspects of shooting hot loads, slide unlocking speed isn't one of them. The only thing that should slow this unlocking speed should be SLIDE MASS.
     
    Last edited: Dec 30, 2011
  16. Taterhead

    Taterhead Counting Beans

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    I remember in earlier postings that Mudrush claimed that heavier springs keep the slide and barrel locked together longer. Buffalo Bore claims this too. Later, Mudrush did seem to retract a bit from this theory, and later emphasized the desired positive return to battery that you mention.

    Pressures and loads were clearly a work in process for Mudrush. It was very interesting to follow as he endeavored to apply theory to his real world business where the stakes were substantial.
     
  17. Taterhead

    Taterhead Counting Beans

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    Carrier, you explained the physics pretty much as I understand it. It has also been a long time since my last physics class so I might be missing something. I agree with Nick that I am skeptical about recoil spring weight influencing unlocking of the barrel and slide until you get to a spring weight of some substantial magnitude.
     
  18. nickE10mm

    nickE10mm F.S.F.O.S.

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    Exactly.... because, at least it SEEMS, that the only way the spring would have any effect on unlocking is in its installed, (more of less) uncompressed state.... and even then, only due to the fact that it increases the friction of unlock due to the geometry of the barrel breech and lower lug .... Aside from that, the spring will only serve to DECELERATE the momentum already imparted on the rearward slide travel....

    It would take a LOT of spring to make any discernible change in unlock speed if you ask me.

    Of course, I have no idea and could be wrong..... Makes me wonder how much even JMB or Gaston even knew about this topic. Not exactly common knowledge.
     
    Last edited: Dec 30, 2011
  19. XmmAUTO

    XmmAUTO 10mm Addict

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    Ok, After doing more reading I'll say that my above post was slightly flawed and that the slide/barrel mass will start to accelerate at the same time as the bullet. However there is still and frictional forces in play with regards to bullet and barrel. How much so? Not quite sure. I do know that when the bullet exits the barrel, the slide/barrel mass has not moved any appreciable amount as to cause a dangerous amount of unsupported cartridge case.
    See pic https://picasaweb.google.com/lh/photo/s2Vkft-_NQSloHsB3ynCcNMTjNZETYmyPJy0liipFm0?feat=directlink

    Say we take a theoretical slide/barrel mass to bullet ratio of 40:1 for the bullet to travel 4.6 Inches the slide/barrel will travel .115"

    Also there is a primary and secondary recoil. One caused by the acceleration of the bullet and the other by acceleration of a small volume of high velocity gas at the moment of bullet exit.

    It is correct that a recoil spring will not cause the slide to remain locked for a longer amount of time (in any amount that would be noticed). A higher spring rate will decelerate the slide faster. A 20# recoil spring is rated at full compression. As the spring is slightly compressed when installed there is a slight amount of force applied by the spring, maybe 2#?

    Xmm
     
  20. Any Cal.

    Any Cal.

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    21Carrier, maybe the barrel and slide DO move .06" while the bullet is in the barrel. It still doesn't change anything. Whether they do or not, there is also a lot of energy available in the expanding gases behind the bullet as it leaves the barrel. As soon as the bullet leaves the barrel(say .01" out), there are thousands of PSI pushing rearward on the casehead, so even if they hadn't started moving, they would be at that point.

    Even if the barrel/slide are moving backwards, that doesn't mean the gun has unlocked. It can't until pressure comes down enough to let the brass shrink back down.

    As far as average pressure or peak pressure deforming brass, look up some specs for brass and see what they are. If there is a time component in the specs, then average pressure would be something to consider. However, I think it much more likely that 37,500PSI would have a larger effect than an average pressure of 10,000PSI, or even 20,000PSI. The average pressure isnt nearly as high as the peak. Look at the chart I posted, that is with a lot of a fairly slow powder. Most loads in a 10mm will have much less area under the curve, so a lower average pressure.