Pressure signs/chronographing

Discussion in 'Reloading' started by Big_Tex22, Jan 8, 2007.

  1. Big_Tex22

    Big_Tex22 Active Member

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    Just got my first chronograph, graduating into a little more in depth reloading I guess, or a least gradually able to afford a few more tools!
    My question is, while working up loads, are there indicators in velocity progression that indicate dangerous pressures?
    I mean if your working up say .5 gr increments in your charges, what do you watch in your velocities and extreme spread to dictate what the pressure is doing?
    I know a few of the simple indicators, primers flowing around the firing pin, split necks, difficult bolt opening, ejector pin marks on the case head, (are their any others?).
    But from what I read on here it sounds loike ya'll can observe pressure with what the velocity does on your load work ups?? Just curious!
     
  2. CatShooter

    CatShooter Well-Known Member

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    [ QUOTE ]
    ... I know a few of the simple indicators, primers flowing around the firing pin, split necks, difficult bolt opening, ejector pin marks on the case head, (are their any others?).

    [/ QUOTE ]

    Split necks are not a sign of pressure, but the rest... "primers flowing around the firing pin, difficult bolt opening, ejector pin marks on the case head,"... are all you need to look for.

    .
     

  3. BountyHunter

    BountyHunter Writers Guild

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    Measure case head expansion.

    Watch for huge MV jumps. if you do a ladder test you will find that your accuracy nodes coincide with MV nodes. ie normally the same say 3-4 bullets in a ladder that group together will also have very close MV while the one after will jump out of the group and in MV.

    BH
     
  4. .280Rem

    .280Rem Well-Known Member

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    The simple answer is that velocity is the single greatest indicator of pressure that we have without using pressure testing equipment.

    Here's the best way I know to describe it. The manufactures work up their data to SAAMI pressure specs, and in fact, usually stay about 2-3K psi under SAAMI max. I'll tell you how I work up my loads. I'll use the 30-06 as an example. I look up data from several sources. I like Nosler data, and Alliant powder seems to be good for me too. Alliant says 60 grs R-22 and a 165 Nosler Partiton get 2755 in a 24" bbl at 51K psi. Nosler says that same powder charge gets 2850. It doesn't give a pressure, but they're loading right up to 60K psi, and they say max is 63grs at 3002fps. (Some would say this is hot) So, at the same powder charge with the same bullet, you got a 100fps difference. Well, looking at most data you'll find that most max velocity given for most 165 bullets will top out around 2850-2950. So, with R-22, you start working up from say 57 grains, and when you hit that velocity range (assuming a like legth bbl), you've found your max load. Slower powders give better velocity with less pressure. If you went with H4350, then you might top out on the low end of that velocity range. In my experience, you can't get enough R-22 in the 30-06 with 165s to do any damage or even get pressure signs. In my 30-06 with a 23.6" bbl 60grs of R-22 and a 165 Hornady gets 2875. I shoot as my hunting load 57.5 grs of H4350 and 165 Accubonds at 2850. Some say 58.0 grs is max at that velocity. I'm sure I could go another half or even up to 58.5, but I'm where I want to be. The faster the burn rate of the powder, the quicker your pressure will rise, and you'll get higher pressures at lower velocities. Study all the data and you'll see the trends. To me the "max load listed" is the max velocity with the given powder. If I reach the max powder charge listed but am 200fps slower than the velocity listed...I'll add more powder. If I hit max velocity 1 or 2 grains short of the max powder charge, then I quit there. Velocity = pressure, but you have to factor in powder burn rate. 2850 with a 165 will give vastly different pressures with R-15, R-19 and R-22. I'd speculate you might not be able to safely reach 2850 with R-15. With R-22, you'd be at a VERY low pressure, some 9K psi short of SAAMI max according to Alliant.

    These two links show the burn rate vs. velocity vs. pressure, and also show how different bullets, because of their bearing surface, can affect pressure too.

    Learn to really read the data and look beyond what the "max listed powder charge" is and apply common sense, and you'll be fine. That new chrony is a valuable tool in reloading.

    Reloading Data

    Reloading Data
     
    Last edited: May 5, 2008
  5. jwp475

    jwp475 Well-Known Member

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    Velocity is a great indicator of pressure
     
  6. muleyman

    muleyman Well-Known Member

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    I'm not sure what to look for in velocity. When I was making up loads for my 338 win, I looked in several older printed manuals as well as some newer ones. The old books show 75 grains of IMR 4350 with a 200 grain bullet. The newer show anywhere from 71.5 to 73. I loaded up 73 and 74 grains with a 185 grain Barnes TSX. When shooting the 74 grainers I had several shots in and around the 3050-3100 mark. Then out of the blue one at 3260. No signs at all of pressures and the S.D. were very sporatic. On the 73 grains the S.D were a lot closer however just the opposite happened. Had all at or around 3120-3150 but then had one at 3000 something. Is this a sign of being overloaded, according to the Barnes manual, I'm right there on velocity. Also they printed fairly well, a completley factory gun shooting at around 3/4"-1" at 100 yards.
     
  7. .280Rem

    .280Rem Well-Known Member

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    You're gonna have some that deviate. It's one reason SAAMI set things the way they did. SAAMI didn't set the MAP on the ragged edge.

    Also, do you weigh each charge or just set it and throw them. Do you let your bbl cool down? A hot bbl can cause serious jumps in pressure.
     
    Last edited: May 8, 2008
  8. Catfish

    Catfish Well-Known Member

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  9. .280Rem

    .280Rem Well-Known Member

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  10. muleyman

    muleyman Well-Known Member

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    .280REM, I do weigh each and every powder charge. I dump close to my desired weight and then trickle in the rest. Also had just gotten a new digital scale and was scared to use it so I actually weighed it first with the digital and then again on the old standby RCBS 505. When shooting, I only shoot three shot groups and let the barrel cool. This is usually done while looking closer at the targets.
     
  11. boomtube

    boomtube Well-Known Member

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    Unfortunately, velocity can be a poor indicator of peak pressure and that's the kind that makes guns come from together, especially so if the powder is too fast for the load. Like, a very low velocity would come at a very dangerous pressure with a charge of Bullseye. Yeah, that's a pretty extreme example but I use it to illustrate the principle that we really can't be sure of the pressures by simply recording the velocity.

    In my experience, increases in velocity (in rifles) tends to flatten as charges exceed normal levels; pressure quickly goes way up for little or no velocity gain. That's what makes attempts to gain more velocity by simply cramming in more powder so dangerous. BUT - if you have attained the published velocity BEFORE reaching the book max for a normal powder, that IS a good sign you have reached the full standard pressure, so stop there!

    Best to stick with case observations and bolt lift to judge pressure, IMHO. But back off at least 5% if you see any of those signs, they are for sure indications of being far over max.
     
    Last edited: May 8, 2008
  12. glassman

    glassman Active Member

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    Well, I will have to go along with what boomtube is saying:
    "Unfortunately, velocity can be a poor indicator of peak pressure and that's the kind that makes guns come from together, especially so if the powder is too fast for the load. Like, a very low velocity would come at a very dangerous pressure with a charge of Bullseye. Yeah, that's a pretty extreme example but I use it to illustrate the principle that we really can't be sure of the pressures by simply recording the velocity."


    I have used a chonograph since the 80's and I have never relied on it to tell me anything about what the pressure was.
    Some barrels are faster than others and some are slower. So you can't really rely on the velocity as to what the chamber will take for pressure.
     
  13. .280Rem

    .280Rem Well-Known Member

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    Again, velocity is the best indicator of pressure in the absense of any real pressure testing equipment. That is not to say that, for example, in a 30-06 shooting a 165 at 2850 that the pressure will be the same or safe regardless of the powder used. You must be able to interpret your pressure from the velocity based on the components you used. This information is obtained via published data. It's why the max load for each given powder is at a different velocity. Its why in cases such as the 30-06, that the slower the burn rate the higher the velocity at a given pressure the manufacturer has set as it's ceiling. But, still, when using a particular powder, your pressure is indicated by the velocity, relative to that particular powder and bullet weight. At a given velocity, pressure will increase as the burn rate of the powder gets faster, until at some point, that velocity is no longer being acheived at a safe pressure. Conversly, at a given pressure, slower burning powders will give increased velocity than faster ones. Still, the velocity is the primary indicator of pressure, in the absence of actual pressure testing equipment. If you're working up 30-06 loads with R-15 or R-22, your velocity is going to tell you when you've hit max, but that max safe velocity is going to be lower with R-15, but never the less, it will be the primary indicator, at least until you start going over safe pressures.

    You are correct in stating pressure isn't linear. Also agree that if you hit max velocity prior to max powder charge, you've hit max. Conversly, if you haven't hit max velocity at a max listed powder charge then you haven't hit max. The max safe velocity will depend on the particular powder and bullet combination, but the velocity will tell the story.

    As to your last paragraph...the consensus among many gun writers, and others in the industry, most of which have spent significant time in labs with pressure testing equipment and guns, is that the "classic signs" are not realiable. Many of them can "appear" and give false information. Cratered primers aren't always an indicator of pressure, but can indicate a large firing pin hole. Factory rifles often still have burrs in places and can leave shiny marks on brass. Different manufacturers of primers have differing hardness. Heck, many a factory round has flattened a primer. However, when the classic signs do actually appear, they are quite often significantly over safe pressures. Most say @ 70-72K psi in the rounds that have a SAAMI max of 60-65k psi like the .270 or 30-06. Meaning you can be shooting loads over safe pressures, but without them showing any classic signs, but any change, like a very hot day, or shooting the barrel hot, can result in catastrophic pressures. SAAMI max pressures are set with these deviations in mind so that you can load to SAAMI max, and the expected "normal deviations" will not put you in to thermonuclear pressure territory. Further, most data, is set a little below SAAMI, so there are 2 "cushions" built in to most data.
     
    Last edited: May 9, 2008
  14. .280Rem

    .280Rem Well-Known Member

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    You really should use your chronograph to indicate pressures, because it does if you read it and your data correctly.

    The fast/slow barrel thing is true in a sense. But it's, in a way, been largely debunked as you seem to understand it. If you shoot the same load in 2 "identical" (ie two Remington M700s 30-06 with 24" bbls) and one gives higher velocity than the other with the same load, it's also giving higher pressure. There are no free lunches with velocity and pressure. A particular barrel may give higher velocities at the same powder charges you loaded for other rifles, but rest assured, the pressure is higher too. So, in reality, there is no fast/slow barrel, only different barrels. This is also the reason you hear many people proclaim that "X's data is optimistic, you can't get that velocity at that powder charge." In one sense they may be correct in that you can't get their velocity with their powder charge in your gun, but you can safely get near the same velocity at near the same pressure in the same legnth bbl, but you'll have to use more powder to do it. Most likely if they're producing data, they have test bbl's cut to very exacting SAAMI standards...tight tolerances. They're going to get higher velocity at lower powder charges...due to higher pressure at a given powder charge because of the tighter tolerances than your or my mass produced M70, M700, Sako, Browning or whatever.