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> Powder temp and velocity
Powder temp and velocity
@ 09:43 am (GMT) |
Matt ReidI am currently running 2208 in my .308, and am wondering what the standard practice is to allow for temp induced velocity fluctuations.The app I have been using on my phone has a ´percentage vel. change per 15 degs of temp´. I have had this set to 1.5 %. Is there any way to establish this figure correctly without actually testing at different temps? I have heard of people putting ammo in the freezer and then running over the chrono to compare. Cheers, Matt |
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@ 03:41 pm (GMT) |
Nathan FosterRe: Powder temp and velocityHi Matt, 2208 (Varget), including all ADI/Hodgdon stick powders, are consistent between approximately -5 degrees C and 30 degrees.One thing I have noticed is that at the extremes of temperatures with ADI / Hodgdon stick powders, it is not uncommon to see changes in pressures that do not correlate to changes in velocities. For example, a reader from Canada recently reported that at -20C, he developed a highly accurate, very fast load for his magnum. But when temeratures came up to zero, the pressure was too high, flattening primers. Yet velocities remained for the most part, unchanged. Ultimately, the load had to be backed off. At the other extreme, I have seen velocities rise by 100fps due to multiple shots being fired on a hot day. However, had the ambient temperature been the only factor, as opposed to rapid fire, velocities would have remained the same. So its a difficult game and yes, the best method is to test your ammo for yourself. If this is not done, it is best to set the app at 0% With Winchester ball powders, I have seen velocities rise by 80-100fps between 0 and 30C, ambient temperatures being the only factor, not barrel heat. Ball powders are much more sensitive to temperature changes. Shotgun powders are the same. This is why it is imperative that military rifles are proof tested in controlled tests for extreme weather conditions. Below -20C, projectiles or wads can become lodged mid way down the bore, at extremely high temperatures, pressures can rise exponentially. If you shoot a suppressed rifle (light weight barrel), velocities can rise by 80-100fps during a rapid 3 shot string (30 seconds) due to vast amounts of heat trapped in the bore. This cannot be factored into an app because if you are having to take rapid follow up shots, there will be no time for scrolling through an app. If we are to summerize the above, we could say: A cool barrel is important as a means to control velocities. As the bore gets hot (above 40 degrees), the shooter may have to remove 1/4 minute and fire for correction. As long as ambient temperatures are not extreme, there is no need to calculate temp/velocity increases on your app when using stick powders. If ambient temperatures are extreme, it is best to test these factors to be certain of results. With ball powders, ambient temperatures can have a pronounced effect and should therefore be tested and a percentage inputted into your app. More to the story: I do find that ambient temperatures have a huge bearing on external ballistics (as opposed to the internal ballistics discussed above). Long range shooters need to take time to look at software etc to see the potential influences of environmental factors. Shooting in the middle of day can, especially up through long valley systems can see huge rises in the bullet. By the same token, as the air cools in the evening, combined with severe down drafts, bullet drop can be immense during winter. These environmental factors can catch the best of us out at any time, As opposed to say a 3" error caused by changes in internal ballistics, this type of error can be up to or over 1 yard or so at 1000 yards and beyond. With environmental factors, its a bit like the wind. You can use hardware such as a Kestral to measure the wind at your position but it means nothing in the hills, even 100 yards out, the wind can be completely different to where you are. The same can be said of temperatures. This is why it is so important to set up your long range shooting position early and then take time to observe what is going on out in front of you, its a quiet time, every other distraction needs to be turned off for a while, your rifle is set up, get yourself ready to shoot, then stop. Stop and look at the environment, using a pair of binos, study the wind on the grass and trees, study any airborne seeds, study the shadows, look for frosts on the ground in the low valleys. Look at the fore ground, the middle ground, then right out at your target area. Let it all paint a picture of the bullet path. Wind directions change so study the possibilities based on the shape and positions of the ridges. This time taken will serve you in good stead later on when decisions have to be made more rapidly. I advise LR shooters to always keep a set of printed drop charts as the primary ballistic info. A smart phone application can then be used as a form of assistance. Also, I can tell you that there is nothing more frustrating than watching someone trying to scroll through an app when the pressure is on, an animal suddenly appears at long range and decisions have to be made fast as the animal moves to and fro. Nothing worse than seeing the screen go blank as the phone shuts down on battery save, then the shooter has to start all over again, meanwhile his quarry is off over the ridge. To me, its important that guys have a printed drop chart ready for these situations, the phone for back up. This also helps save batteries for potential emergency use. If you are in your shooting position a couple of hours earlier than need be (as you should when LR hunting), try your best to use your drop charts, then take environmental factors into account and take a guess/estimation at the required correction, then see how this pans out on your app. The longer the prep, the better the final result, including speed of decision making if the pressure comes on. I think the 'Shooter' app is the best on the market. It is incredible how the phone can be used for measuring angles with this app. It is extremely useful, especially with regard to large changes in environment, especially temperatures and altitude. I have managed to get by all of my years without this type of software, usually jotting potential environmental changes/ effects in my note book but was recently gifted an Android phone for work done on a rifle. With Shooter, I now have my back up for environmental changes. For you old buggars, smart phones such as the Apple ipjhone or my Google Android shouldn't really be called phones as phone calls are but one small function. The smart phones are mini computers that are relatively easy to use after a bit of practice and help from friends, the kids and the grand kids if need be. Its a bit of a return to the brick, the new phones have big screens for people like me who find LCD screens frustrating to use, tapping out each letter one at a time while saying the letters out loud so that everyone around has to go through the spelling of a word, as though walking through mud. The smart phone automatically synchronizes itself with suitable online shopping under the market tab where the shooter program (application) can be easily found and purchased for a few bucks. Shooter is a highly complex program, no ballistic stone is left unturned- unlike Angry Birds which is free in the market place and relies solely on Kentucky elevation when slingshotting the chickens to their target. |
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@ 05:03 pm (GMT) |
Matt ReidRe: Powder temp and velocityHi Nathan,Thanks heaps for the detailed reply. I was under the impression the 'extreme' range of powders were relatively temp insensitive so it is good to hear you confirm that. I am definitely keen to run with a printed drop chart in the field. I have just been using my phone in the interim as it's easy to tweak variables. I am running a Nokia phone, so unfortunately the android apps aren't available. The app I am using (Strelok) really only accounts for humidity and elevation. Can you clarify why you are seeing the large variations in drop as stated here: "Shooting in the middle of day can, especially up through long valley systems can see huge rises in the bullet. By the same token, as the air cools in the evening, combined with severe down drafts, bullet drop can be immense during winter. " Is this a function of topography or humidity if it is not related to ambient temp? Or is it solely related to up/down drafts? What would you recommend as far as info to put on a drop chart? |
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@ 07:36 pm (GMT) |
Nathan FosterRe: Powder temp and velocityHi Matt, for my work in the hills, temp effects the bullet in two ways:Increased or reduced air density. Warm air offers less resistance and a flatter trajectory, cool air offers more resistance, causing a steeper trajectory. Up and down drafts (catabatic winds). These follow the lay of the land, going uphill as the sun rises and down hill as the sun sets, having an effect on both the bullet and airborne scent. I find that as the sun sets in the evening, if shooting from way up high on a ridge top, down to the bottom of a valley system, the down drafts as the sun sets in winter can be huge and combined with low ambient temperatures if shooting into shadowed valleys, bullet drop can be severe. By the same token, in the heat of summer, if you watch thistle seeds for example, you can often see the seeds rise. Some will even become caught in what looks like a mini tornado, a lone seed gently spiraling upwards while other seeds pass by, following the prevailing wind, seemingly unaffected. But the bullet path will indeed be effected. In the past, LR shooters had to take a punt, see where the bullet landed, then jot it down in their note book. Today we are lucky to have software. I do find that the effects of temperature are more pronounced at low velocities, say below 1800fps or 1600fps depending on general experience. The further you push it, the more pronounced each variable. Within a given cartridge, there is a range at which the cartridge is generally easy to use as far as drop goes, then it will hit a range were for each 100 yards added, it might as well be an extra 400 yards, such is the difficulty- or challenge depending on how you want to look at it. Below is a two page drop chart I set up for a client yesterday. It looks a lot tidier on Microsoft Excel where it was generated, including borders which did not copy and paste to here. The two pager is to be put in a clear file back to back, then the top taped to keep it water proof. One copy in his day pack, one copy on his person. The Mil column is to be used with the mil dots on his Sightron scope, following the usual 3.6 MOA calibration. The chart below will look difficult to follow for the novice but it really is easy once you get your head around it and have had a bit of practice. I believe Microsoft Excel is the optimum tool for generating drop charts as it can perform conversions and re-calibrations (required for Bushnell, Leupold, Weaver brand scopes / turrets). Excel is not the easiest to learn but a boon once understood. Actually it looks like the drop chart won't paste properly here so will put the chart in the Knowledge base (bottom of the KB menu). http://www.ballisticstudies.com/Knowledge+Base/Sample+drop+chart+for+long+range.html |
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@ 10:54 pm (GMT) |
Matt ReidRe: Powder temp and velocityHi Nathan,Thanks for that. It seems a lot of it still comes down to experience despite the calcs! I found the free calc over at Berger quite handy for making up a basic drop chart. Bryan Litz seems pretty onto it, I think he is also the brains behind the Shooter app. Fortunately (or unfortunately?) my drop chart is significantly shorter than your sample, will stop at 600m until the skills are up to it.... now off to the laminator. |
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@ 07:37 pm (GMT) |
Matt ReidRe: Powder temp and velocityManaged to get out for a shoot last night, testing at 487m (532 yards). First 3 shot group is shown in pic, drop based of the drop chart. Nothing amazing but starting to feel a little more confident. Wind was a L to R, about 0.5 - 1 m/s. Allowed 2 clicks for it.
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@ 07:39 pm (GMT) |
Matt ReidRe: Powder temp and velocityHope this is easier to see....
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@ 08:57 am (GMT) |
Nathan FosterRe: Powder temp and velocityThats a fair start. The height variables arfe what you need to focus on at this stage. Finding out 'why' is the most important aspect, why is it shooting roughly 2" high / .3 MOA high.So the variables you have to work with are: Is the scope true to its dials or does it need recalibrating Was the drop chart correct regarding environmentals and how they correlated to field conditions. Did I change my shooting technique. Such as the change from shooting off a bench verses prone on grass. Shooting over a sand bag versus a bipod. ES may be a part factor. The BC of the A-Max is not a variable, the G1 BC of .475 is correct and the G1 model is usable / practical in this instance. You did well with the wind drift. In the future, if you intend to hunt with the rifle, try also to learn to aim off rather than dial for wind so that you are practised with both methods. It won't be as neat and tidy/ small as the group you sent, but it will be useful for your own purposes. You are doing well, stick with it. When you get a chance, could you please post the rifle model exactly, the load and velocities plus typical group sizes at 100 yards. Would also like to hear if the rifle double groups at 100 yards. For example, if its a VTR, the high shots will most likely be a function of stress within the barrel steel that cannot be remedied. Thanks for sharing this. |
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@ 10:35 am (GMT) |
Matt ReidRe: Powder temp and velocityHi Nathan,Rifle is rem 700 spss in .308, 1 in 10 twist. Barrel has been trimmed to about 21.85 inch, and a muzzle forward suppressor is employed. Scope is Bushnell Tactical 10 x 40. I have a carbon fibre cheek piece I made at work for it fitted so cheek weld and eye alignment seems pretty good. Load is 45gn AR2208 in Norma brass, Fed 210 primers, coal 2.800, 168 A-Max. FL sizing at the moment. For the above grouping, I was shooting off my day bag as this is what I plan to use when hunting. Typical groups at 100m have been 0.4moa at best, probably averaging closer to 0.7moa. Dicovered recently, if barrel is cleaned prior to shooting, first shot will usually be 1 min high and 1 min left. I need to chrono this load more to confirm, last time I did the first four shots were 2714, 2701, 2697, 2705. In my calc I was running with 2707fps, although from my shooting results, 2735 seems to be a better fit. 
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@ 12:49 pm (GMT) |
Matt ReidRe: Powder temp and velocityAnother bit of a query... I prepped a load of brass in the weekend, after the shoot. Probably had 4-5 reloads through them. I noticed quite a few required trimming with my Lee gauge, as I hadn't trimmed them previously. Wondering whether the growing case necks had been causing an increase in pressure and pushing a few shots higher than others? Will definately be worth getting back behind the chrono to compare. |
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@ 03:50 pm (GMT) |
Nathan FosterRe: Powder temp and velocityHi Matt, yes, long cases will definitely increase pressures and velocity.If you intend to do a lot of shooting with the .308, you can trim a bit shorter than usual and it won't effect accuracy. To do this with your Lee pilot, simply sand a touch off the tip of the pilot, be careful though, the material comes away easily. Max case length is 51.2mm, I trim to 50.9mm. Thanks for supplying your reloading and rifle specs. For other readers, please be advised that 45gr of 2208 (Varget) does not usually yield over 2700fps. MV for this load is usually around 2600fps. Approximately 47 grains is normally required to obtain 2670fps. Many .308's, regardless of barrel length, will not produce acceptable accuracy above 2700fps (168gr bullets). Occasionally there is an exception to the rule, velocities can be as high as 2800fps. Generally, the sweet spot for 168gr bullets is 2670fps, regardless of barrel length. Using 2206H or H4895, the sweet spot loads occur between 43.5 and 44.5 grains. 2208/Varget is best suited to long barreled rifles as it is slower burning than 2206H but typically, high charges must be used to obtain the extra velocity and increase in performance that is being sought. Quite often, a longer (than 20") barrel makes little difference in the .308 so lets say the user has a 26" barreled rifle. My mindset is to find that 2670fps sweet spot with 2206H. If any extra velocity can be obtained via the long barrel or via slower burning 2208, all the better, but this is not my expectation. My expectation of the long barrel is that it will reduce recoil for extreme accuracy. Matt achieves the same with a forwards mounted suppressor. Sorry to depart from the root topic Mark but these are questions readers have. OK, Im am hoping your chrony is on the money, it most likely is but I have concerns I want to go over with you and for the sake of others. The Bushnell scopes (including tactical series) are not usually true to MOA in the turrets. A friend has just emailed me his calibration (Bushnell Tactical) which correlates my own findings with the Bushnell brand. 4 clicks should give 1 MOA which is 1.047". But the Bushnell's usually calibrate at 4 clicks give .973". My Friends Tactical is yielding .938" per 4 clicks. Ok, so what readers have to be careful of, is not to assume that the scope is actually true to MOA, then start mucking around with environmental factors and BC's in their ballistic software, trying to make it all match. You see, with a bit of fiddling, you can sometimes make it all match up, only to be caught out again when you step it out another 100 yards or so. I hope that makes sense. If using the Bushnell brand (also Leupold and Weaver), scope calibration and an understanding of Microsoft Excel software becomes important. The same goes for your MV regarding potential problems. As I said, normally, 45 grains will yield about 2600fps. But if your chrony is wrong and is reading 2700fps, you can end up again in the trap of mucking around with the software to make it all verify, then get caught out down the track when stepping out an extra 100 yards. So there are a few pitfalls here. I am hoping that your load is spot on and true to its MV, it does happen from time to time, the occasional rifle will give an extra 100fps in comparison to its brethren. The accuracy level sounds pretty good. It tells us that the bore is a good one, if the bore was a dud, groups would be out around 3MOA or more, really its as simple as that. To get the last bit of accuracy: Make sure the trigger is down around 1.5lb Action bedded and stock stabilized if plastic Keep working on technique as always, sling tension etc. With the high shots, make sure that the rifle has not slipped back under recoil, resulting in shots 3 and 4 being fired with the barrel resting on the day bag- can catch anyone out. Also, make sure the butt stock sits high on the shoulder, if its low, the butt will slip down under recoil and fire high. Do your group testing over sand bags, then move to the day bag to obtain your final zero and to determine what your level of field accuracy will be. When aiming over the day bag, take note of how much wobble is occurring in the cross hairs. Lets say you have .13" wobble. If your sand bag groups are say .3", you can immediately determine that more than likely, your day bag group will be .413-.450" . I really like your day bag by the way, good to see your set up, looks very sound with the rear rest under the butt, good to see. A wide flying fouler is pretty normal. The only way around this is by using moly coated bullets- if you want your clean bore shot to be on the money out long. I tend to be like most guys in NZ, simply fire one off before setting out into the hills. Thanks again for sharing Matt, it will help many people. |
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@ 05:24 pm (GMT) |
Matt ReidRe: Powder temp and velocityThanks again Nathan. I forgot to mention that action has been fully bedded and fore end stabilised as per your instruction.When I initially chrono'd that load, 3 shots yielded 2635,2637 and 2634 respectively. I went with this load because of the low ES despite velocity being low. The more recent chrono gave the higher results, possibly due to my lack of trimming? I was a little confused and put it down to the chrono. I did run weigh the powder capacity of the Norma vs the Win brass and from memory it was a grain less. As I understand the max load in Win brass is 46gns. My thinking being that Norma brass may produce a slightly higher velocity for the same charge as Win brass would. |
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@ 03:34 pm (GMT) |
Nathan FosterRe: Powder temp and velocityHi Matt, yes, that all looks about right. 2600fps was was I was expecting while 2636 would seem about right right with a heavier walled brass.Time will iron out all of the smaller problems. You are on the right track. |
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@ 02:56 pm (GMT) |
Matt ReidRe: Powder temp and velocityJust to follow up on this a bit, last time I was out with the chrono, first shots were showing as high as 2850fps, despite no pressure signs. It was late in the day and sun was casting across chrono which I have heard can cause issues. Seems the chrono isn't quite on the money at times which probably wasn't helping things.I have also changed out the Bushnell 10x scope for a Vortex, primarily for the vari-power function so hope to post a review at some stage. Cheers, Matt |
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