I have had a few questions (thanks for all the work, Daryl) about how Boattails compare to Flat Base bullets at airgun velocities.... Using Geoff Kolbe's Drag and Trajectory Calculator, I compared the latest .308 cal Bore-Riding Boattail I designed to a Flat Base bullet of the same weight and nose profile.... Unfortuantely, while I was inputting the data for the Flat Base bullet I made an error, which I have since corrected.... The data below, and the comments about it, now reflect those corrections.... My profuse apologies, it was a case of GIGO (garbage in, garbage out)....Here is the Boattail.... The Flat Base has the same nose, but parallel at 0.308" diameter from there back to the base, and the overall length is 0.85" to keep the weight the same....Here are the calculated Drag Curves for the two bullets.... First the Flat Base....Next the Boattail.... Note the subsonic drag is less than half that of the Flat Base, but there is a little spike in the drag at just over Mach 1.... Above Mach 1.2, the Boattail has slightly less drag....When the trajectory and wind drift are calculated for various muzzle velocities, for a rifle zeroed at 100 yards, here are the results.... Solid lines are the Boattail, Dotted lines are the Flat Base....Note that the Boattail has a flatter trajectory and less wind drift at all velocities, although the advantage in drop is small at over 1300 fps.... Subsonic, the Boattail shows an advantage in trajectory, and a huge advantage in reduced wind drift....There are a couple of other things about the wind drift that are interesting.... The least wind drift occurs for the Boattail when starting at 900 fps.... while for the Flat Base bullet the least drift needs a higher muzzle velocity of 950 fps.... However the Boattail always has less drift, and in fact at 900 fps it is predicted to have less drift at 400 yards than the Flat Base at 200 yards.... Refering to the top graph, you will note that in all cases, there is a muzzle velocity where the wind drift is the worst, increasing with range.... For both bullets, that occurs at a muzzle velocity of about 1300 fps at 200 yards, 1500 fps at 400 yards, and 1700 fps at 600 yards.... This is important for anyone considering shooting supersonic in hopes of having more success in hitting distant targets.... If you contemplate doing that, these results tell you that most achieveable velocities, even with Helium, will put you right where the drift is the worst.... To stay supersonic all the way to the target, means you need about 1600 fps to get to 200 yards, and over 2000 fps to reach 400 yards.... Even at those extreme velocities, the wind drift will still be greater than if you stayed subsonic.... These calculations are only valid for this one comparison, but of course it is possible to compare any two bullet by this method, just very time consuming.... I feel that they are typical of what you will find, however, when looking at what are the possible advantages of a Boattail for use in airguns.... One other thing I did not address is that for a given length, Boattails are longer, and the shape also requires additional stability.... They therefore need a faster twist rate to be stable.... For the two bullets in this example, the optimum twist rate, to give a Stability Factor of 1.5 at Mach 1 (worst case), is a 10" twist for the Boattail, while the Flat Base only needs a 16" twist.... If kept below 1050 fps, the Boattail can use a 12" twist, and I have found that in most cases the twist correction for my Boattails is exaggerated, they will work in a slower twist than the calculated one....I hope that this information gives you some insight into the advantages and disadvantages of using a Boattail design in airguns.... I feel that there are significant gains to be made right at the muzzle velocities we are striving for, in the 900-1050 fps range.... It is worth noting what the retained velocities are for the two bullets, both starting out at 1000 fps.... For the Flat Base, they are 865 fps at 200 yards, 761 fps at 400 yards, and 671 fps at 600 yards.... For the Boattail, the retained velocity is 921 fps at 200 yards, 864 fps at 400 yard, and 813 fps at 600 yards... The Boattail is travelling faster at 600 yards than the Flat Base at 300, and over 140 fps faster by the time they are both at 600 yards….I apologize for the earlier error.... Bob
Bob all is ok on paper, but will it shoot as well as this plain base 30 cal has done in many air guns see attachment
jr460.... you're right, of course, I should just give up and crawl back under my desk.... Bob
BOTAIL just do not work that well below the speed of sound
QuoteBOTAIL just do not work that well below the speed of soundI've done the math.... perhaps you should.... the boattail has less than half the drag below Mach 0.85 (950 fps).... Of course I suppose it is possible that all the research done by the military at places like the Aberdeen Proving Grounds may be wrong, and you may be right.... Look at the Drag Coefficient in the subsonic region.... particularly at 900-1000 fps.... These drag models were downloaded from the JBM Ballistics website, and are the official versions....The G1 Model used above.... flat base spritzer....The G7 Model used above.... spitzer boattail....Everybody, including me, thought that boattails were intended for extreme range and high supersonic flight.... because that is their typical use.... One look at those drag curves convinced me that maybe.... just maybe.... we had overlooked something.... I am well aware that boattails, fired from barrels without a fast enough twist to take into account their inherent stability problems just as they drop through the speed of sound have been known to tumble.... The obvious solution, to me, and particularly since we don't have the power to launch them at 3000 fps, is to stay subsonic where they don't have a stability problem.... and have about half the drag of a flat based bullet.... If you want to have a positive input on this discussion, then find some numbers that support your statement that I quoted above.... or some hard data that does the same.... don't just be a naysayer because you have a different opinion without facts to back it up.... You might like to watch this video, they don't mention boattails, but with only 100 fps loss over 300 yards, that leads me to believe they must be boattails....Or read these articles on subsonic sniper rounds that use boattail bullets out to 600 yards....http://www.sadefensejournal.com/wp/?p=751 .... http://www.sadefensejournal.com/wp/?p=1318 .... and lastly, a couple of types of commercially available subsonic centerfire ammunition using boattails for the .300 AAC Blackout, which has a reputation for superb accuracy....http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100http://www.midwayusa.com/product/486942/doubletap-ammunition-300-aac-blackout-240-grain-sierra-matchking-hollow-point-boat-tail-box-of-20Conventional thinking?.... no.... but to me the potential is obvious.... By using Helium, high pressure and long barrels, we can duplicate the performance of the .300 AAC Blackout with an airgun.... food for thought....Bob
Advertise Here
Share This!