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rsterne
I have had a request to draw up a 6mm (0.243″) BBT, based on the shape of the .257 cal 90 gr. BBT…. Here is the proposed dimensions….
Although the Kolbe Twist Calculator calls for a 9″ twist, the nearly identical 154 gr. in .308 cal works in a 14″ twist…. The equivalent in .243 cal would be an 11″ twist…. so I have no doubt that this bullet should work in a 10″ twist…. Obviously it would also work in the 9″ twist suggested by the Kolbe Twist Calculator….
If you are interested in seeing this bullet get into production, please add your name to this thread…. I plan to ask Al to make it only in the RG2 and RG4 moulds, which will allow casting Flat Nose, Cup Point, or Hollow Point bullets just by changing the pins supplied…. I would like an RG4 in Aluminum, please….Bob
rsterne
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owencoors1
Hi Bob,
Wow, you are fast! I read all that you wrote and it’s in parallel with my thoughts. I’ve been thinking a lot about this since I asked you to make a 6mm BT HP for the 1:9″ rate of twist barrel. I think what I’m wanting is something like the 308-176gr BT scaled down to .245″. Can you do that? I don’t think this 76 grain is useful, for my barrel. I originally bought the 1:9″ twist 6mm barrel intending to use the RCBS 95 grain mold bullets. I think that this 73 grain HP BT will overstabilize in my barrel.I’m actually thinking that what I need would fall somewhere between the 308-176gr-BT and the proposed 308-200gr BT, scaled down to .245″. If you drew up the scaled down 308-176gr-BT though, I’d go for that. But maybe the majority of 6mm/.243 shooters are using 1:10″ or 1:11″ twist rates and making a bullet for 1:9″ twist wouldn’t sell. I don’t know.
To be honest, Owen, I think there is zero chance of this bullet being “overstabilized” in your 9″ twist barrel…. I added a clarification (in red) above…. On the other hand, I think there is a very good chance that a scaled down .308 cal 176 gr. may not be properly stabilized, or marginally so (which would reduce the BC)…. I have one report of the 200 gr. .308 cal. BBT not flying straight at 200 yards in a 10″ twist, which is the equivalent of a 7.9″ twist in .243 cal for the 98 gr. scaled down version…. (In case you don’t know, for identical bullet shape, twist rate is proportional to caliber)…. The holes in the 200 yd. target showed that the 200 gr. BBT was yawing…. Although it was not tumbling, the 10″ twist was definitely not fast enough…. although it worked great in an 8″ twist .308 cal….
The other problem is driving an 86 gr. bullet (that is what the scaled down 176 gr. would weigh) fast enough in a .243 cal airgun…. I don’t know what kind of FPE you are expecting to generate with your 6mm…. but I have a 6mm with a 28″ barrel, and running regulated at 2800 psi it will not reach 950 fps with a 73 gr. bullet, even running full power…. which I would never do because it uses wayyyyyyyyyy too much air…. Tuned for decent efficiency, I only get 875 fps (125 FPE) with that bullet…. Yes, I can get a bit more velocity by running 3600 psi, but I am sure that even that pressure would not drive an 86 gr. bullet much over 900 fps, if it could even reach that…. Running regulated at 2800 psi, a 60-65 gr. bullet is the optimum for my gun, I actually get 127 FPE at better efficiency than with the 73 gr…. If I had the 73 gr. HP version of this BBT, I might consider running it unregulated, as it would be the equivalent SD to using a 90 gr. bullet in a .257 cal….
Long story short, I don’t think there would be enough interest in anything heavier than this 6mm bullet to generate a Group Buy….
Bob
owencoors1
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