GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on July 03, 2015, 02:32:20 PM
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My basic design is called a "Rebated Boattail", used by Corbin and Lapua, and has several advantages over a conventional boattail.... The rebate leaves a clean, accurate edge where the bullet exits the crown, instead of the taper of a conventional boattail, which is hard to get concentric (square) at the parting surface.... It also provides a better gas seal, and deflects the gasses to the side rather than around in front of the bullet.... There is an excellent discussion on the Corbin website at.... http://www.swage.com/ftp/rbt.pdf (http://www.swage.com/ftp/rbt.pdf) .... The fact that Lapua, with their excellent reputation, also use the rebated boattail, convinced me this was the way to go....
When casting bullets, the sprue plate tends to distort the base of the bullet slightly.... With a flat base bullet, where that is the last part to leave the crown, it's like having a bad crown, releasing the gas unevenly.... This distortion tends to be worse on smaller calibers.... Big Bore Airgunners have been finding that gas-check designs, without the gas check, tend to be more accurate because the corners of the last driving band are sharp and square to the bore, providing a clean bullet release.... When I researched the rebated boattail idea, it became instantly obvious to me that it would cure the problems associated with flat base distortions at the same time.... Like most people, I thought that boattails were used for high velocity, long range use.... I was surprised to find out that the boattail is actually equally effective at reducing drag in the subsonic range.... Just look at these three Drag Profiles....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20Coefficients_zpsmvh8fg13.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20Coefficients_zpsmvh8fg13.jpg.html)
The G1 profile is a flat based Spitzer, while the G7 is a Spitzer Boattail design.... It has 40-50% less drag below 900 fps than a flat base bullet.... I was convinced this needed to be explored further for airguns.... Now reality set in.... Our bullets tend to be short and fat, and the larger the caliber, the more that applies.... Bullet mold manufacturers are very limited in what they can produce, unless you get a custom "Cherry" made, which is very expensive.... The most readily available custom molds at a reasonable price are from Accurate Molds, and they have several restrictions on their designs.... They won't make anything smaller than .30 cal, or make a Meplat smaller than 0.18", the base has to be at least 0.255", and the base can't be less than 0.060" smaller than the largest diameter.... Obviously, this severely limited my design choices, but there are still several in Tom's catalogue, including a .300 cal 70 gr., .308 cal in 79, 95, and 110 gr., .357 cal in 109, 124 and 152 gr., .408 cal in 200 and 252 gr., and .458 cal in 280 gr.... By necessity, the boattails are short, and the base large, making them look much like a tapered gas check feature.... The BC is better than a flat base of the same design and weight, but not by a lot (maybe 20%?)....
For the smaller calibers, I had to turn to Veral at LBT Molds.... Veral uses a "Tracer Lathe", which bores the mold cavities following a pattern that is 10x size, like a Pantograph setup.... He didn't have any rebated boattail patterns, so six of us got a group order together and he made one to my specifications.... The first mold he produced was for a 51 gr. .250 cal design.... It has turned out to be very good in quite a few PCPs, at power levels around 100 FPE.... The setup that Veral has enables him to make caliber and weight changes easily, so now that he has the boattail pattern, he can make other designs.... I have since designed a .224 cal at 41 gr., .257 cal at 63 and 79 gr., and a 7mm (.284) at 94 gr.... The two latter designs are both turning in sub-MOA groups in the hands of American long range airgunners, although like with any cast bullets in airguns, the devil is in the details (getting the diameter to match the bore)....
It was never my intention to make any money from this.... All I want to do is further bullet design for long-range airgunning.... The designs and molds are in the public domain, and all I ask is that if someone makes them commercially, they acknowledge my effort.... My latest concept, as yet untested, is a low-friction, bore riding design, intended to be easier to chamber and to have less critical tolerances to variations in bore diameter between barrels.... Here is an example....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/308%20cal%20BBT%20152%20gr_zps0blk15f4.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/308%20cal%20BBT%20152%20gr_zps0blk15f4.jpg.html)
Think artillery shell with a boattail.... AFAIK, this design could be turned into a mold by Veral at LBT with his existing tooling.... I am hoping this is the future of long-range airgunning.... All we need is some adventuresome soul to try it and find out....
Bob
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thanks bob, great post,i got the bbt 110 mold from accurate mold few weeks ago, great design, very very accurate out of mt rs2 and shin sung 357 201. now i want to get the 458 280 gr and once i find LBT molds am ordering the .250 51gr mold, glad you share so much of your time and knowledge with us on this sight,and on other ones as well,when i think smart, i think bob when it comes to air guns and ballistics's. thanks again brother.your a one of a kind. ;D
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I may just be that adventuresome soul! I like it!
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What is Veral's turnaround time on a custom mold?