Make sure your ammunition has the legs...
- Thread starter Stalker62
- Start date
Crono to check your going to stay fast enough to keep supersonic and you load can get into single figures ! shoot some 1- 5-800 stuff besides the obvious , folks can get touchy about someone messing around a while not getting on the paper when the range is busy .
Interesting point - my TR rifle has a 30" barrel.
Actually, they moved from the 173gn FA-designed FMJBT bullet from 30-06 times which has excellent ballistic properties, but grouped poorly. Sierra's remit was to design a similar weight bullet with similar external ballistics properties but which matched the 168gn Sierra MK's superior ability to produce small dispersions.
The 168gn SMK was used in the US military M852 7.62, but primarily as a match round. It (in the form of the Federal GMM factory round) is very popular amongst US law enforcement agencies and was the 'gold standard' for the M14 /M1A period in US Hi-Power XTC competition which is shot up to 600 yards. However, the 168gn is a relatively short-distance model having been designed as a 300 metre bullet way back in the 1960s for UIT competition originally. It has an over-steep boat-tail angle that creates drag and instability once into trans-sonic flight which occurs around 700 yards in typical 7.62/308 use and 'standard ballistic conditions'. It is therefore not a 1,000 yard bullet at 308 Win velocities in normal conditions.
The 175 won't stay supersonic to 1,000 in the nominal US military sniper rifle / M118LR combination whose MV spec is 2,580 fps. With an average G7 BC of 0.243 its terminal 1K speed is a calculated 1,082 fps in 'standard conditions', some 40 fps below the speed of sound. 2,660 fps MV theoretically just achieves supersonic flight throughout, but that doesn't take account of MV spreads, so higher MVs are needed to ensure the slowest bullet in a string remains supersonic.
Ah, if only things were so simple. BC in the commonly used form is a combination of sectional density (SD) and the bullet's 'form factor' which is calculated against that of a 'reference projectile', the old G1 being very close in shape to that of a typical .22LR 40gn 'solid' round-nose / flat-base bullet; the now preferable G7 is based on a modern artillery shell design with a much longer pointed streamlined nose and a 7-degree angle boat-tail.
SD is a calculation based on the projectile's mass and diameter and is therefore fixed for any bullet of a given weight and calibre. ie the SD value of any 150gn 0.308" bullet is 0.226 for instance and that applies whether it has a highly streamlined (low-drag) shape, or whether it's a flat-nose design for the 30-30 in a tubular magazine and has the aerodynamics of the proverbial brick-built sh*thouse. Going heavier for two identical bullet shapes increases the BC simply because of the SD improvement. However, a heavier bullet cannot be driven at the same MV as its lighter counterpart in the same barrel, maximum pressure etc, so may offer no practical improvement on the range.
The drag bit of the equation is the 'form factor' which gives a projectile a numerical value comparing it to the G-whatever 'reference shape'. The 'reference projectile' or basemark is always valued at 1.000, and as this is a drag-based measurement, lower values are better being 'slippier' in the air. So, if bullet A is calculated as 0.900 form factor, it produces 10% less air-drag than the reference; if bullet B is 1.100, it produces 10% more drag (at any given speed as drag isn't constant across all speeds). A good 155 match bullet can easily have a much higher form factor than an average or poor 185gn design ('good', 'average', or 'poor' only in the context of in-flight drag). The 155gn 308 Berger Hybrid has a form factor value of 0.953 (almost 5% reduction on the G7 'reference') which when taken as a ratio with the SD for all 155s gives a very creditable G7 average BC of 0.245; the 185gn Lapua Scenar which although a good performing match bullet design is relatively blunt and has a high 'form factor' value of 1.13, creating some 13% more drag in flight than the G7 'reference' ........... but has a much higher SD of 0.279. Turn them into BCs and they end up with a nearly same value of 0.245 (155) and 0.246 (185). So, at the same speeds, they perform theoretically anyway, identically. But of course, you can't drive them at the same speeds - all other things being equal the 185 Scenar will have a significantly lower MV and therefore performs worse in both coping with wind and retained speeds.
Where you are correct, is that the heavier the bullet, the more leeway the designer has to improve the shape and reduce drag in flight (ie improve form factor). 0.95 is likely as low as anybody will get a 155gn 308 in a conventional lead core bullet, but the Berger 200gn 200-20X Hybrid from the same designer and period (Bryan Litz of Applied Ballistics LLC working as a consultant for Berger Bullets) has a form factor value 0.919, and his 215gn Hybrid design is even better at 0.898. (The very best of the lead-core 6.5mm bullets are better still at just below 0.89.)
The 308 175gn Sierra MK which is quite long in the tooth now dating from c. 1990, is relatively high-drag with a form factor value of 1.085, ie 8.5% more than the G7 'reference projectile's) 1.000. Even with its higher SD thanks to the additional 20gn weight over 155gn designs, its resulting G7 average BC ends up as 0.243, marginally lower than that of the 155gn Hybrid, and not much higher than that of Sierra's 2nd model 155gn MK (0.237). However, one can realistically expect to push those 155s out at 100 fps higher MVs and ballistically anyway they'll perform a little better across 1,000 yards.
Ballistics aren't everything though. As a rule, the longer the bullet design in a calibre/weight and the longer its nose section, the more finicky it becomes in its tuning. The 175gn SMK has a perfect tangent-ogive nose, is very easy to tune and very tolerant of chamber form, throat erosion and so on making it an easy bullet to shoot well, and importantly for the military, keep shooting over an extended barrel life. Likewise, the original 155gn Sierra MK (p/n 2155) is a superb design in this respect and is why the GB NRA specified it for the RUAG and now GGG 308 Win Match ammo it issues in major competitions, rather than the longer-nose later 155 MK (p/n 2156). Some bullets have been pushed too far in design terms it seems in the pursuit of low form-factor values / high BCs. There is a limited production design of 200gn 308 that has a measured form factor of 0.845, one of the lowest of any lead-core design in any calibre and a correspondingly impressive BC. I've yet to hear of anyone getting good results from it in long-range competition though!
Smellydog
Well-Known Member
Sorry - what have I done to deserve that!?Hmmm. From what I read on this andallmanya few of his posts there is more than the bullet at risk of becoming unstable!
Just saying….
Foxyboy43
Well-Known Member
Gosh no - not you sir, rather a certain chap who seems to have almost mastered the black art of reloading!
Gosh no - not you sir, rather a certain chap who seems to have almost mastered the black art of reloading!
Things have certainly changed since I last shot deer in the Highlands as 1000 yards was the max distance one walked to recover the FCS quad bike after pulling the trigger.
K
K
Yes - providing that bullet would be suited to the barrel and chamber dimensions in any twist.
It was common at one time for F/TR competitors to specify 10-twist barrels and a chamber throat that would allow use of both 155s and 210s, the 155.5gn and 210gn LRBT Bergers common pairings. Depending on the match conditions, they'd choose the bullet at the start of the day as the 155s held tighter elevations and gave marginally higher scores in 'easy' conditions; the 210s gave more recoil, a larger elevation spread, but bucked wind changes better. (Obviously, the shooter knew his zeros and scope elevation adjustments exactly for each bullet as they weren't the same.) This practice is done by some too with light bullets chosen for short-distance matches; heavy for 900/1,000.
The 155gn bullet was driven at ever so slightly (Ahem!) faster MVs than 2,640 fps too, anything up to 500 fps quicker.
That's assuming we're talking twist rate / stabilisation factors only. Litz gives the 155 AMax an average G7 value of 0.213. At 2,640 fps at the muzzle, the Berger calculator says it'll only be doing 1,023 fps at 1,000 - 200 fps below the sound barrier. The old 155 AMax also had a double-figure boat-tail angle (13-degrees) which risks the airflow delamination problem that kills the 168 SMK at tran-sonic speeds.
So, do I think this is a good 1,00 yard combination? Not just No!, but Hell, No! as someone in a movie said. Ballistics are funny though, so there's always an outside chance it'd work. It certainly won't be any good on sound operated e-targets (Shot-Marker and Silver Mountain) either at this distance given the absence of supersonic 'crack' to activate the target microphones. You'd also need an available scope elevation adjustment of c.45-MOA on a 100 yard zero.
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John Gryphon
Well-Known Member
Some comparo BC`s here.
308 Winchester Load Data
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Stalker62
Well-Known Member
Notwithstanding the drama I had actually getting onto paper...
We have had no issues with the electronic markers at 1,000 yards at Bisley.
Try applied ballistics for long range shooting by litz. It’s the first book in his series wich introduces a lot of the ideas of ballistics.Thank you; that makes sense.
I have just tried to order :-
Modern Advances in Long Range Shooting by Bryan Litz, but that is proving harder than a hard thing.
While you are on fire, have a look at this thread:-
Zero Targets
and see if you know the answer to another great ballistics mystery...
Stalker62
Well-Known Member
Well now.
That is now in my Amazon 'basket', and awaits authorisation from the Finance Manager and Account Holder...
Like the older Kongsberg system, the Intargo (??) Bisley e-targets use an internal membrane that allows hit position to be plotted, so doesn't require supersonic bullets.
There are pros and cons to this type, the main cons being greater initial cost and wear and tear on the membrane over time through multiple bullet holes in a small area. The main pros are no need for supersonic bullets and absence of a vulnerable external microphone at each corner of the target frame.
That's the book that covers all the basics and explains how it all works and hangs together. Definitely the one to buy and read to acquire an understanding of what is required in bullet performance, MV, external ballistics, bullet design factors and identifying optimum stability/rifling twist rates etc for shooting at moderate distances (ie c. 1K for cartridges like 308 and another few hundred yards for the real ballistic high-achievers. (ELR shooting and ballistics take you into much more advanced territory and practices.) He avoids the math wherever possible and writes in plain language.
The modern Advancements books are very different - collections of essays each looking at a single topic on subjects such as types and accuracy of chronographs. For example: how MVs change (increase) as a barrel 'runs in'; unconventional carbon fibre wrapped and concrete tube barrels; efficacy of various handloading techniques such as deburring flash-holes and annealing (you'd be unlikely to spend on an AMP after reading that one); whether VLD type bullets 'settle down' after two or three hundred yards as many claim (they don't!) ........ and so on. They are all practical exercises with results involving several cartridges and calibres and LOTS of shots and results are usually MV/ES/SD effects, exhaustively measured, tabulated, and with correlations identifies and marked, only a few with group size results. They also include a lot of statistical stuff, which I'd have to warn people can be quite depressing - like a single 5-round group on its own tells you nothing worthwhile because groups from one load like everything else vary widely across a considerable size range, so you might be looking at an exceptionally large or small example at one end of the range and need the average of lots of them with the SD of sizes calculated to be statistically valid. Some of his exercises and results are very controversial, in particular his findings on screwed-on barrel-tuners which caused outrage amongst tuner makers and users. But the point is that whilst relevant to long-range precision shooting, they aren't directly concerned with the basics of external ballistics, bullet stabilisation, bullet features and design, and similar matters. AB is now on the third volume (not edition - so each one is completely different) of these slim and expensive books and one imagines many additional exercises and volumes will appear in due course.
In addition to Applied Ballistics for Long range Shooting, The other really useful book if you shoot lots of things and like playing with lots of bullets is his 'Ballistic Performance of Rifle Bullets' now on its 3rd edition and covering several hundred bullets, both sporting and match - drawings, dimensions, form-factors, BCs, optimal rifling twist rates.
Applied Ballistics now offers all their stuff on Apps and Podcasts through a subscription based service as well as running workshops in the US and offering Custom or Personal Drag Models provision mostly for the ELR people using Doppler radar based data. Old-fashioned farts like me prefer the books, but you can spend a LOT of money with AB even on old fashioned print.
The Science of Accuracy - A Ballistics-Focused Academy for Long Range Shooting
Stalker62
Well-Known Member
My last attempt about reading up on ballistics was lost in translation, and did not end well...
enfieldspares
Well-Known Member
This
Says STALKER1962 - So just a tip for anyone else who is about to make the same mistake, (of course no one would). Make sure your ammunition has the 'legs' to get out there
When the ONLY bullet of factory 174 (in fact 180 grain weight) you could get for reloading.303 British was Speer's 180 grain round nose soft point we also fount that, too, at distance would drop like a stone compared to the same loading of powder but with a spritzer profile bullet of the same weight. So not only the "legs" does it need but also the "nose".
At one hundred or two hundred yards I am sure that Speer bullet slew caribou in Canada in wagon loads. But at Bisley or Kingsbury at five and six hundred yards it had the ballistic properties of a half a brick. And was not at all able to correspond with the rifles military sight graduations for five and six hundred yards if sighted to correspond with them at two hundred yards.
Says STALKER1962 - So just a tip for anyone else who is about to make the same mistake, (of course no one would). Make sure your ammunition has the 'legs' to get out there
When the ONLY bullet of factory 174 (in fact 180 grain weight) you could get for reloading.303 British was Speer's 180 grain round nose soft point we also fount that, too, at distance would drop like a stone compared to the same loading of powder but with a spritzer profile bullet of the same weight. So not only the "legs" does it need but also the "nose".
At one hundred or two hundred yards I am sure that Speer bullet slew caribou in Canada in wagon loads. But at Bisley or Kingsbury at five and six hundred yards it had the ballistic properties of a half a brick. And was not at all able to correspond with the rifles military sight graduations for five and six hundred yards if sighted to correspond with them at two hundred yards.
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