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Thread: Velocity variation

  1. #1
    Member AKdutch's Avatar
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    Default Velocity variation

    I've loaded up a bunch of 300 SAUM rounds using 180gr TSX bullets and 65 grains of R-22. My groups at 100 yards are sub moa and around 2" at 200. I decided on this load for hunting and brought out the chono today to check vel. This is the first time I have chrono'd any rifle rounds. My first four rounds came out to be 2916, 2854, 2945, and 2840. I measured the powder for each case exactly to 65 grains and each bullet was seated to within a couple thousandths (the best my reloader would do).

    My question is, is this a normal spread for this round. I thought they should be alot closer. And if not, what could be causing this. Thanks, John

    Oh, and I might add that I have a pretty cheap chrono. Maybe this could be my problem.
    Last edited by AKdutch; 09-04-2007 at 01:21. Reason: more info

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    I would never expect velocity variations of 105 fps from that load nor would I expect those variations to give such good results. In a case and cartridge as the 300 SAUM, I will say velocity variations of up to 40 fps would be acceptable but 25-30 are attainable and preferred. One of the biggest causes of velocity variation is the primer. Some of them just cause erratic velocity. This case size and its normal powder charge are right on the edge of what works best, standard or magnum primers. The thing to do shoot again with the same load then try the other primer. Generally I will choose the magnum primer for 65 grains of RL-22 but if I got these variations more than once I would change. The wrong primer be it standard or mag, CCI or FED, can give wide velocity variations. Your group(s) may also be somewhat of a fluke, I don't know how many you fired, but if just one group, I'd definately do it again. A variation of 100 fps would cause vertical stringing. I have seen the primer faults traced to light firing pin fall, also. I've seen even new brass give these results when the flash hole was uneven and partially clogged with the punch residue. A flash hole reamer will help this.

    Another cause of velocity variations is the brass case, usually after several firings and resizings, the uneven neck tension, due to variations in neck thickness or concintricity and elasticity of the brass.

    As you say though, the chrony is always suspect and must be eliminated. Another excuse to do more shooting. I've seen expensive chronographs give bad readings then go back the next day and get correct readings all day long. I think it has to do with the sunlight. Chronys work best on calm, overcast days.
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    Member AKdutch's Avatar
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    Default primers

    Now I'm not real sure which it is. I am using Federal match magnum rifle primers. But that could also be the reason all my groups are almost straight up and down. The last group at 200 yards was 2.5", but they varied up to 3.7".

    One thing I did notice was the primers have a purple coating that covers the top of the primer on about 50% of the primers. The rest you can see the yellow primer substance below the purple, sealant I believe. It does look like pretty shoddy workmanship, but I figured it was normal as they are match primers. Thanks for the response. Yesterday was really bright sunny, so I might have to go shoot some more, dang it. I sure am impressed with my rifle (Rem model 7) as it is box stock except for a trigger job.

    One other question Murphy. My rifle seems to shoot larger groups after I clean it real good and then shoot smaller and smaller groups the more I shoot it. ?? Thanks for the response, John

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    Default

    Just curious, are you using an electronic scale for weighing the powder charges?
    I have a Pact scale and it tends to drift so I recheck it every 4-5 rounds and rezero it.
    Tennessee

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    Default scale

    I have a electronic Pact scale as well. I've never had a problem before, but it's something I will check. Have to wait until the work week is over before I go shootin again. Thanks

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    Default Shooting

    Plenty good to hunt with. Now get out there John.

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    Default I'm goin

    next Sat for 10 days. Drew an any bull tag for the Ferry trail. Should be a gimme. JM

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    Member Jim Finn's Avatar
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    Default

    When you say your are seating the bullets within a couple of thousands, do you mean within a couple to the lands or plus or minus a couple from some other distance from the lands? I am asking this as I have had the best luck with the TSX bullets folowing Barnes' advice to keep them 0.030" or more back from the lands. I have also noticed that there can be a couple of thousands variation in the bullets themselves.
    If you are indeed loading very closed to the lands and the bullets have a few thousands variation themselves, that cloud possiblly lead to some velocity variation.
    I have noticed that seating bullets close to the lands can lead to increased pressure signs and velocity.
    Just a thought.
    Like some of the other guys have written, you already have a rig and load that is more than ready to hunt with.
    Good luck.
    Jim

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    Default

    I too have a Pact electronic scale along with some vel variations. Your post caused me to compare my powder weights to my balance beamed scale. There was more variation in the electronic than the mechinical by .4 grains!!! I will be calling Pact and find out why. Not a happy camper. Thanks for the post; it caused me to do the comparison. J.

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    Default Crustyjohn

    Judging from the size of your groups it would seem probable that your velocities are varying as much as you measure. 100 ft/sec should give on the order of about 0.7 inches of drop at 200 yards with this bullet - your groups should be oval with the long axis on the vertical if there is this much velocity variation. Your chronograph should not be too close to the muzzle, most chrono manufacturers recommend 15 feet or so, I have had good results with my bargain basement shooting chrony at less distance (about 5 feet with cartridges in the .243 - .270 class). Not enough distance will cause propellant products and shock waves from the gas as it accelerates past the bullet just after muzzle exit to give erroneous readings. Regarding primers, in my experience your cartridge volume is borderline between requiring a magnum vs standard large rifle primer with a double base stick propellant. Since Federal 215 primers are about the most energetic available I would suggest you try something a little milder. A double base propellant with the high NG content of RL 22 shouldn't be all that hard to ignite. A CCI 200 or Federal 210 might give you better groups but they also might not.
    I agree strongly with Jim Finn on the issue of seating distance. You might try smoking or inking the bullets and chambering them in a dummy round to determine the length of the leade in your rifle so as to pin down the optimal seating depth with your TSX bullets. If you're not familiar with the technique of smoking bullets a visit to most any reloading website should solve the problem. Hope you find this helpful and good luck with your quest.

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    Default Thanks

    for all the responses. I was referring to the overall length for the cartridge measurements. The bad thing about this caliber is the length of the receiver? where the bullets are held. It will not allow me to seat the bullets within .030 of the lands. I used the smoke technique to measure my overall length and the bullets would not fit in the receiver. So I am closer to .050 out from the lands I think, I have it written down somewhere. I remember I was pretty disappointed with where I had to leave them. But if I havenít stated, this is my first time loading rifle rounds, except for 45/70.

    I also may have been shooting too close to the crono, as I was only about six feet away. This very well could be my problem. I will try it farther out tomorrow, and if not I will try my other primers. Thanks again, John

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    Default Chrono

    Went out today and tried backing away from the chrono a bit. I shot from about 20 feet back found the variation was less this time. The variation was still from 2790 to 2816fps, but I can live with that. Thanks for all the help and advice. So far I'm 2 for 2 on getting problems solved with your help. John

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    Default

    In the quest for accuracy, leave the crony at home. What do you care what instrumental velocity shows when you have a combo that is accurate.

    It is fine to know what the velocity is for a given load, so you can do drop calculations for the times you need to go beyond your sight-in, but the time to check that is only after you have a load that's working for you.

    The other time when a crony is handy, is when you switch loading components. That's when it is great to know what the change will do to effect a known good load.

    Such changes as hot or milder primers, powder and brass for case neck tension. The work is finding a load that works for you and the maintenance is keeping the load working for you.

    I'm a firm believer in getting enough components when you find a good load, to load enough ammo to wear the barrel out. This is why a guy needs kegs of powder and cases of primers. Once you have found what works, why would you go wondering around, trying to find that sweet spot again.

    This brings us to chasing the throat erosion factor to maintain accuracy and still being able to load from the magazine. In a word, forget-it!
    All of that is fine from the bench for Saturday at the range, but that is not how you hunt, taking what works for a bench rest competitor and what works for a hunter is not the same thing. Thank goodness for that!

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    Quote Originally Posted by Big Al View Post
    In the quest for accuracy, leave the crony at home. What do you care what instrumental velocity shows when you have a combo that is accurate.

    It is fine to know what the velocity is for a given load, so you can do drop calculations for the times you need to go beyond your sight-in, but the time to check that is only after you have a load that's working for you.

    The other time when a crony is handy, is when you switch loading components. That's when it is great to know what the change will do to effect a known good load.

    Such changes as hot or milder primers, powder and brass for case neck tension. The work is finding a load that works for you and the maintenance is keeping the load working for you.

    I'm a firm believer in getting enough components when you find a good load, to load enough ammo to wear the barrel out. This is why a guy needs kegs of powder and cases of primers. Once you have found what works, why would you go wondering around, trying to find that sweet spot again.

    This brings us to chasing the throat erosion factor to maintain accuracy and still being able to load from the magazine. In a word, forget-it!
    All of that is fine from the bench for Saturday at the range, but that is not how you hunt, taking what works for a bench rest competitor and what works for a hunter is not the same thing. Thank goodness for that!

    AL,

    I'm not sure what you refer to here, the highest possible velocity or velocity variations, but all my best loads in well over 100 rifles had small velocity variations and low SD (standard deviation). I have never found the best load for any rifle with 100 fps velocity variation.

    In fact the selection of components, primers, brass and neck tension, powders will all adversely affect accuracy of the very best bullet, because of velocity variations.

    Now if one uses a chrony to chase the highest velocity for a hunting load, that rarely ever gives the best accuracy and of course other factors, such as high pressure, can make that a bad idea. But certainly a chronograph is not only a useful tool to find the mean velocity but also for the variation, and that is the biggest value of it. I have never found the best accuracy from any of the rifles I've chronographed, when the load had wide velocity variations.

    Also, there are very few inaccurate bullets out there. Just about any bullet of today has more than enough accuracy for any good shooter. It is the selection of the right comoponents and their combination that brings out this accuracy.

    And I think, if that's what you mean in your last sentence here, you're right about the most accurate not necessarilly the best for hunting. This is why I only develop hunting loads for hunting rifles with hunting bullets.

    The process should work from the bullet backwards, but most do it the other way around.
    Last edited by Murphy; 09-17-2007 at 11:34.
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  15. #15
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    The only reason I see for a crony is the same reason the arsenals use them for. They are loading to a velocity with different lots of powder.

    This is of course, is needed for the fellow that is using the combo to fight with, no sight changes need apply.

    I would not agree about SD being that important at least not between point blank and 300 yards. For long range of course that is the case, this is where low SD does come into play for the long range bench rest crowd. It does really show out at 1000 yards.

    Now that we have returned to the real world for the moment. Years ago many of us in the bench rest crowd, one and two hundred yards, came to the conclusion that our cronies only added a factor of confusion to an situation where accuracy had been achieved. SD means squat at these yardages as has been proven time and again.

    Some times theories don't translate to the paper target. This is a prime case. Shooters that worked on reducing SD had no better groups than the shooters that paid no attention to SD. A reading of PS magazine articles from 20 years ago addressed this subject among the top bench rest shooters of the day. This has not changed. Please remember these folks to a contestant, shoot dropped not weight charges at the matches.

    I use the same measures here in good old Alaska to throw charges as I used in the lesser 48 for bench rest matches. With the best of measures, a 3/10 of a grain change from center does not translates to any change in group size. At 600 and beyond this is not the case and SD does rear it's ugly head.

    This is where I find the trouble with the use of a crony as a tool for accuracy, cause this will not be a prediction for accuracy and only throws in another reason for confusion.

    Instrumental velocity from a group of shots, to know what I have to know for drop, well that is useful. Beyond that, I will not change anything when I've found a combination that produces the smallest groups.

    I find that the preparation of my brass and the loading with well made loading dies in a good press makes more of a difference on the target, second only to a well built rifle with a good barrel from one of our top makers.

    In this, I'm always willing to put my money. Rereading this, I guess I do just that.

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