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Thread: Converting FPS to Chamber Pressure

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    Member North Polar's Avatar
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    Default Converting FPS to Chamber Pressure

    Anyone know how to do that? I figure I am below the max pressure by a good deal, but am hoping to figure out what pressure I am at. Also how can you compute foot/lbs? More out of idle curiosity then anything else on that one.

    Currently my loads are:

    Base Test: PMC Bronze 55gr Ball Ammo @ 2711 fps Average

    (All loads are in a 5.56 chambered rifle but loaded rounds are on .223 data)

    62 Gr SS109 @ 2702 fps Average

    55 Gr Nosler Partition @ 2311 fps Average (the book said 23gr was the most accurate. Going to dial that load up bit by bit)

    Hornaday Match JHP 52 Gr @ 2457 fps Average

    Hornaday Vmax 55gr @ 2583 Average

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    Member shphtr's Avatar
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    The reloading program "Load from a Disk" gives approx calculated pressures for different muzzle velocities which will be different for different powders of different burn rates....but to answer your question, yes, there are formulas to approximate those values but no, I do not know them - I do use the above program.

    http://www.loadammo.com/

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    Default Energy is easy Pressure is trickier

    Quote Originally Posted by North Polar View Post
    Anyone know how to do that? I figure I am below the max pressure by a good deal, but am hoping to figure out what pressure I am at. Also how can you compute foot/lbs? More out of idle curiosity then anything else on that one.

    Currently my loads are:

    Base Test: PMC Bronze 55gr Ball Ammo @ 2711 fps Average

    (All loads are in a 5.56 chambered rifle but loaded rounds are on .223 data)

    62 Gr SS109 @ 2702 fps Average

    55 Gr Nosler Partition @ 2311 fps Average (the book said 23gr was the most accurate. Going to dial that load up bit by bit)

    Hornaday Match JHP 52 Gr @ 2457 fps Average

    Hornaday Vmax 55gr @ 2583 Average
    Energy is easy Pressure is trickier

    V^2*m/7000*64.4

    V=velocity in feet per second
    m=mass in grains
    7000 is grains per pound
    64.34 is twice the acceleration of gravity (required because the formula needs to convert pounds (force) to slugs (mass)

    velocity squared times mass in pounds times 64.4 yields ft-lbs of energy.

    also check
    http://www.cruffler.com/BallisticCal...lculator.shtml
    or
    cruffler.com/BallisticCalculators/BallisticCalculator.shtml

    or just google the phrase "bullet energy calculation"

    As far as I know, you can only approximate the pressure (without installing measuring equipment into your firearm) The speed of the burn of your powder, the friction inside the barrel (of the bullet and of the propellant gasses) and every other thing.

    In the '60's there was a slide rule developed for estimating pressures based on velocity. Google the phrase "Hutton and Powley 'Pressure Estimation by Chronograph' " and similar phrases. Computer programs have made the slide rule obsolete, but not irrelevant.

    Remember, only believe half of what you see and one quarter of what you hear. That goes double for what you get from the internet. Even this Maybe especially this post.

    Do your own independent, confirming research when ANYONE gives you new facts on the web.

    Also remember, even the idiotic stuff might have a kernel of truth buried in there somewhere.

    Lost Sheep

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    Member North Polar's Avatar
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    Quote Originally Posted by Lost Sheep View Post
    Energy is easy Pressure is trickier

    V^2*m/7000*64.4

    V=velocity in feet per second
    m=mass in grains
    7000 is grains per pound
    64.34 is twice the acceleration of gravity (required because the formula needs to convert pounds (force) to slugs (mass)

    velocity squared times mass in pounds times 64.4 yields ft-lbs of energy.

    also check
    http://www.cruffler.com/BallisticCal...lculator.shtml
    or
    cruffler.com/BallisticCalculators/BallisticCalculator.shtml

    or just google the phrase "bullet energy calculation"

    As far as I know, you can only approximate the pressure (without installing measuring equipment into your firearm) The speed of the burn of your powder, the friction inside the barrel (of the bullet and of the propellant gasses) and every other thing.

    In the '60's there was a slide rule developed for estimating pressures based on velocity. Google the phrase "Hutton and Powley 'Pressure Estimation by Chronograph' " and similar phrases. Computer programs have made the slide rule obsolete, but not irrelevant.

    Remember, only believe half of what you see and one quarter of what you hear. That goes double for what you get from the internet. Even this Maybe especially this post.

    Do your own independent, confirming research when ANYONE gives you new facts on the web.

    Also remember, even the idiotic stuff might have a kernel of truth buried in there somewhere.

    Lost Sheep
    Thanks for the info I'll have to do a wee bit of number crunching on my next day off. I was figuring it was going to be ballpark pressure anyhow.

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    Default

    Thanks for the info, I'll check it out.

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    Moderator Paul H's Avatar
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    It is impossible to convert velocity to pressure. If you aren't measuring the pressure, there is no way that you know you are below max pressure by a good deal if you can't measure it. Thats akin to say you are far away from the edge of a cliff if you're walking in a fog. If you can't see the edge, you don't know how close or far away you are. You may have a map of the area and an idea of where the edge is.

    Using published reloading data, and exactly the same componets, measuring your velocity and correlating for differences in barrel length it is possible to guestimate the pressure you are opperating at, but you aren't measuring it, and are just making a guess.

    Differences in chamber and barrel dimensions have an effect on pressure, you can take ammunition and run it through two different guns that are nominally chambered for the same round and see signifigant diffeneces in pressure. This is paticularly true with the .223 / 5.56 Nato as there are many different chamber designs, and barrel twists from 1-14 to 1-7. Also the guns are built on different actions, bolt actions, break open single shots, gas opperated autos, etc.

  7. #7

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    Quote Originally Posted by Paul H View Post
    It is impossible to convert velocity to pressure. If you aren't measuring the pressure, there is no way that you know you are below max pressure by a good deal if you can't measure it. Thats akin to say you are far away from the edge of a cliff if you're walking in a fog. If you can't see the edge, you don't know how close or far away you are. You may have a map of the area and an idea of where the edge is.

    Using published reloading data, and exactly the same componets, measuring your velocity and correlating for differences in barrel length it is possible to guestimate the pressure you are opperating at, but you aren't measuring it, and are just making a guess.

    Differences in chamber and barrel dimensions have an effect on pressure, you can take ammunition and run it through two different guns that are nominally chambered for the same round and see signifigant diffeneces in pressure. This is paticularly true with the .223 / 5.56 Nato as there are many different chamber designs, and barrel twists from 1-14 to 1-7. Also the guns are built on different actions, bolt actions, break open single shots, gas opperated autos, etc.
    I could not have said this any better.

    THERE IS NO WAY TO KNOW YOUR PRESSURE WITHOUT PRESSURE MEASURING EQUIPMENT.

    There are a lot of techniques, including MV, to guesstimate pressure, but they are all guesstimates! I personally think that MV is the least realible way because of the all the variables involved which Paul named just a few.

    My philosophy....

    When my brass is showing signs of excessive stress such as extractor marks, cratered primers, loose primer pockets, etc... and/or my bolt starts getting stiff, and/or my muzzle splits and peals back , then I figure I am exceeding the max pressure for my PARTICULAR RIFLE and BRASS and it's time to back off. when I back off and I'm shooting this particular load with easy bolt, no cratered primers, firm primer pockets (or at least not excessively loose), then I figure "all is well".

    JMNSHO

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    MR:

    My philosophy is that,,,

    If you back off only because of pressure signs, you could be WAY over Max by that time.

    I think it's safer to stop when you reach the top velocity of your data, assuming no pressure signs to that point.

    I think using MV in conjuction with loading data, is the MOST reliable way to estimate pressure. I'm sure it is for me, anyway.

    Smitty of the North
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    I kind of figured that there was no way to do it with precision. Mostly I was wanting to double check that my loads were still in spec.

    I am loading under max pressure, but one of the loads seems like it is compressed when the books says it shouldnt be.

    So far no signs of over pressure, just trying to be safe with it.

    I'll hopefully be taking some of the rounds to the range tomorrow and do an accuracy test and see how they do.

    I dialed back the max load by 10% for my first loads and then added 1 grain from there. Planning on staying there unless something happens tomorrow.

    Current load that I am using:
    .223 in a 5.56 chamber

    62gr SS109 bullet loaded with 25.5gr of varget powder and a cci 400 primer.

    I tested with 24.5gr of powder and it was only 300 fps slower then factory .223 so i figured 1 more grain ought to get roughly the same velocity.

    Max is 26.4gr with that load, so i am still .9 gr away from that.

    Anyhow, target pics to follow tomorrow, along with some brass pics.

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    Quote Originally Posted by Nitroman View Post
    Cool idea but wont work on my AR. At least i know someone makes that in case i start loading my other rifles. Thanks for the link

  12. #12

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    Quote Originally Posted by Smitty of the North View Post
    MR:

    My philosophy is that,,,

    If you back off only because of pressure signs, you could be WAY over Max by that time.

    I think it's safer to stop when you reach the top velocity of your data, assuming no pressure signs to that point.

    I think using MV in conjuction with loading data, is the MOST reliable way to estimate pressure. I'm sure it is for me, anyway.

    Smitty of the North
    Smitty,

    I respect your opinion on this, but... there are just too many variables associated with mesuring velocities with a chrony.... not to mention other variables in a cartride's velocity including barrel chamber, throat, freebore, rifling, bore diameter, etc. I would bet that if you took a factory box of ammo and fired 4 shots each out of 5 different rifles in the same chambering you would get 5 different velocities.

    Just last week I was doing some load development with 210 Bergers and as I was climbing the ladder I thought my velocites were a little low. The next day I went back to the range and changed the battery and the velocity jumped from about 2845 to about 2895 with the exact same load and rifle. Now mind you, this is with about a 14 yr old middle of the road chrony that is only 12" between sensors, which I think makes my point even more so. How accurate is the chrony equipment we use? How about the light conditions we are shooting in? I have had a lot of "quirky" readings with my chrony, but in good conditions with a good battery, I *think* it is fairly close (+ or - 30 fps)

    Your suggestion about the fact I may be way over max when I *notice* pressure signs id something that I can not completely refute. I will say this though... If I climb at 1/2 gr at a time, I find it hard to imagine that if at 68 gr of xxx powder I see no stress signs in my brass or sticky bolt, that I will have a receicer blow up in my face at 68 1/2 gr.

    Now in my very limited experience with handloading I have seen a few very peculiar results. The physics of handloading when considering all the variables... brass, bullets, powder, primers, headspace, seating depth.... well you git the point.... there are often suprises instore for us.

    Bottom line... my philosophy is... If my brass fairs well and my action functions within *normal* expectations... then *all is well*

    And it's even better when I break the trigger and the critter falls

    -MR

    PS...

    I'm a single old fart with no wife to worry about, so if an ambulance carries me or my corpse away from the range it aint no big deal.

    One of my philosophies of life is to... always live it "on the edge"

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    MR:
    Isn't velocity your BEST pressure sign?

    One time ago, I made some loads that gave me extremely high velocities. So extreme, in fact, that it couldn't rationally be attributed to an inaccuracy with my chronograph, or the use of it. (The velocity decreased as I decreased the load.)

    I knew, or at least I felt I knew, that the pressure had to be WAY above normal, yet there were NO pressure signs.

    (The minor problems I've experienced with my Lil Chrony, have been such, that there was no reading at all, but never a Low Reading.)

    But, all inaccuracies aside, I'm simply suggesting you don't continue to increase your powder charge, when your velocity is higher than the velocities from your loading data, just because you aren't experiencing pressure signs, OTHER THAN THE VELOCITY.

    There is loading data out there, that is based largely on pressure signs. Where it is still found in the later manuals, it is so stated. Even, when the data compares favorably with that from other sources.

    Not to belabor the point, but.....

    I convinced that pressure signs are the least reliable way to determine safe pressure, and for the reason given. By the time you have pressure signs, (those you mentioned) you're probably WAY over max.

    I'm also convinced that there are worse things than, "if an ambulance carries me or my corpse away from the range".

    May you always have a trigger finger, and at least one eye to sight with.

    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
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    Well working up loads for my 06 with 180 grainers I got to some pressure sings with 57 grains of powder, the manual said that 58 was max. The hell of it was they where only on some cases. Some showed flattened primers, some showed an ejector mark on the case, some showed both, and some showed none all on all the bolt opened easy. I backed off to 56 grains and the groups shrank to about an inch. I looked at the velocity the manual gave for min an max loads and only found a 100 fps difference. So I figure I am gaining brass life and staying plently safe and only lossing 25 or 50 fps, so that is a good trade.

    One thing that does bother me is that after the manuals warn that many things can effect pressure they work up loads with only one rifle. And then list a min and max from that. This seems a little hipocritical to me. If so many things can have an effect should they not test these loads in many guns and make sure that the rifle in question is not a fluk before puplishing? I would think that they run the risk of giving over conserative max loads or high start loads by only using one rifle.

  15. #15

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    [QUOTE]
    Quote Originally Posted by Smitty of the North View Post
    MR:
    Isn't velocity your BEST pressure sign?
    No... I do not think velocity is the best indicator of pressure for all the reasons I listed earlier and here they are again...

    1) How do you know you are getting an accurate velocity reading?

    2) Differnet rifles will shoot the same load at different velocities. I have chrony'd factory ammo at greater velocities than the manufacturers list. Should I throw away or return the ammo?

    3) Many factors affect pressure, including seating depth and proximity of bullet to the lands. Seating depth affects distance to the lands as well as case capacity which will both affect pressure.

    4) Chamber, throat and bore dimensions can affect pressure which can and often are different from rifle to rifle.

    5) Different rifles and brass combinations can handle pressure better than other combination due to quality of materials and workmanship. Lapua brass will handle pressure much better than Fed brass.

    And here are a couple of other considerations....

    Not all data matches. If you visit the Nosler and Hodgdon sites and check the data for 300 RUM/200 gr AB using IMR 7828, you will find a huge differenc in their data.

    Hodgdon max -82.0gr/2857 fps
    Nosler max - 87.5 gr/3102 fps

    Now that is a huge difference. The Hodgdom max is lower than the Nosler starting load. Which is right? Also, I believe manufacturers will publish conservative data, some more conservative than others.

    Also, you cant find data for every bullet/powder combination. When I started loading for the 300 WSM I could find no published data for H4350 which is one of the best performing powders in that cartridge (until RL17, which ia a new powder and also no publihed data for certain bullets I am using) I had to research on the net to get a "good idea" of where to start and what my max's might be. Also, published data is always based on SAAMI specs like COAL. If we are going to strictley go by what is published then forget about experimenting with seating depth, neck tension or other than listed primers and brass, etc.

    One more thing. How are we defining pressure and max pressure? Is it the SAAMI spec based on their testing with x,y and z equipment? Or is it the subjective safe operation and firing of your rifle and cartridges?

    So no, I dont think velocity is the best indicator of pressure. You can use it as a tool. The touchy feely and visual sigs might not be completely accurate indicators of pressure, but in my limited experience they seem to work fairly well.

    For me, once again, if all is well with my rifle and brass, life is good

    Regards,

    MR

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    The way I see it trying to tell chamber pressure from muzzle velocity is something like sticking your hand out the truck window to see how fast youíre driving. You get some idea, some feedback from it, but what if you have a 45mph tail wind and you are going 35mph? If you have no other feedback you will think you are going 10mph backwards wont you?

    Published load data has a built in safety margin to account for the variations in guns and keep us from getting injured. If we stay within the published data we donít need to know the exact pressure we are making because we have a safety built into the data. We need to know muzzle velocity to know what the bullet will do in flight and on impact. If we push beyond the tested/published data there is need to know the pressure we are making and it better not be a guess from muzzle velocity any more than guessing you have oil in your truck motor by looking at the temp gage.
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    AD, and MR:
    I'm not guessing as to pressure. I'm playing it safe with pressure, rather than trying to wring the last fps out of a cartridge. Of course, I'm trusting that if the velocity isn't much different than the data, the pressure is probably within limits.

    If I'm getting the expected velocity, I'm happy. If I get pressure signs first, I'd back off. And, I like the built in safety margin.

    I believe that velocity is the best (safest) indicator of SAFE pressure, because if your velocity is a LOT higher, the pressure is very likely higher too.

    Granted, there CAN BE huge differences in rifles, data and other variables, but I trust my chronographed velocities.

    Obviously, "the SAAMI spec" is based on more reliable factors than pressure signs.

    Factory loads with "greater velocities than the manufacturers list" could indicate higher pressure in a particular rifle, and they could be risky. I wouldn't throw it away though, not without saving the brass, anyway.

    I figger that pressure signs tell you, you have high pressure, but you can have high pressure WITHOUT the signs, and that's what I'm warning against.

    The practice of increasing your powder charge, or varying other things, until you see pressure signs, and then backing off a bit without regard to the velocity,,,, because it isn't to be trusted,,,,

    "Wow, I have this great load. I'm getting more velocity, than the factory loads, the loading data, and Smitty".

    is IMO, out of date, due to the risk.

    Maybe, if I had a lot more handloading experience, I would have more confidence in pressure signs, and I'd feel different, but then maybe not. I'm very stubborn.

    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
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