GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Engineering- Research & Development => Topic started by: Mark Davis on May 10, 2017, 10:45:23 AM
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This is really directed at rsterne, but all comments are welcome in the interest of enlightenment.
I'm new here on GTA, and am aware that you are "Bob" of Bob's boat tail bullets available from NOE bullet molds.
I have purchased a few of Swede Nelson's molds and find them to be high quality, reasonably priced, and all around good stuff.
But I want to make my own .25 cal air rifle mold for a stock marauder. I have a CNC mill and plan to drill and ream cylindrical cavities undersize, then go back in with a homemade "D" shape cutter and finish the cavity with circular interpolation.
What angle of taper is close to optimum for boattail bullets at 800 fps?
Is there a ratio between bullet diameter and boattail length I should be aware of?
Is the radius at the back of the boattail a strong factor?
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Typical boattails seem to be about 8* per side, but boattails are seldom used on short stubby bullets like we use in airguns.... Therefore, conventional wisdom for boattails really doesn't really apply, for taper, length, or base diameter.... A stock MRod has two huge limitations to the power, the 20" barrel length, and the porting diameter.... Even if you hog the exhaust, transfer and barrel ports out as large as you can go, you cannot get beyond about 0.20" diameter, because going larger will have the exhaust port break into the valve cavity, ruining the seat.... There is simply not enough metal there.... At 3000 psi, for a single, full power shot, I don't think you can get to 100 FPE, more likely much less than that.... You are probably aware that at 950 fps, the bullet weight in grains is half the FPE, so you are looking at a bullet of 40-50 grains.... Have a look at my two NOE designs of those weights, and you will get an idea of the proportions you are dealing with....
(http://i378.photobucket.com/albums/oo221/rsterne/NOE%20Bobs%20Boattails/250-41-FN_BT__41_gr_Sketch_zpsbut4czkm.jpg) (http://s378.photobucket.com/user/rsterne/media/NOE%20Bobs%20Boattails/250-41-FN_BT__41_gr_Sketch_zpsbut4czkm.jpg.html) (http://i378.photobucket.com/albums/oo221/rsterne/NOE%20Bobs%20Boattails/250-51-FN_BT__51_gr_Sketch_zpsbe7jvdqc.jpg) (http://s378.photobucket.com/user/rsterne/media/NOE%20Bobs%20Boattails/250-51-FN_BT__51_gr_Sketch_zpsbe7jvdqc.jpg.html)
Like I said, short and stubby.... Those both use a 10* rebated boattail.... At 800 fps, you may be able to use a slighty longer, heavier design.... The new 62 gr. BBT would be 88 FPE at 800 fps, and you won't get there with a stock MRod, IMO....
HTHs....
Bob
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Thanks for your reply.
This will be my first attempt at bullet design. After rejecting the idea of a pin to form a hollow skirt for speed, ease, and keeping the temp stable for casting. Your boattail design Looks like something I could simulate.
My shooting is mostly in sling supported positions, so an endless supply of good pellets has more appeal than a small supply of perfect pellets.
The .25 grain JSBs are pretty good, and I envision my pellets weight to fall somewhere between 25 grain and your 41 grain design.
I might even leave the boattail off, it would make for a stronger reamer.
When I cut this mold it will be 11 cavity, in blocks the same length as Lee six cavity mold blocks.
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Finally getting to reamer design.
Thinking along the lines of a wasp waist, stubby truncated cone, with flat base.
For first attempt, the shape seems easy.
rsterne,
Is a shape with more weight ahead of center a desirable trait?
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Nick at Nielsen Specialty Ammo is the expert on CG position.... and he always seem to be pushing the CG as far back as he can.... I would ask him....
Bob