Very interesting. Could you list the pros and cons of this caliber when compared to .257 and .224? That Arsenal copy of 257420 sounds like a sure bet for a long range bullet.
I could write a long article and end up saying very little.... in reality, there is no overwhelming reason to choose 6mm (.243 cal) over either .224 cal or .257 cal.... much like there is no overwhelming reason for .20 cal pellets, compared to .177 or .22 cal.... All the arguments about why one caliber is better than another are a matter of which compromise you prefer.... In the broadest, and most general terms, consider the following....Larger calibers make more FPE, but require more air to do so (their efficiency is marginally better, however).... More FPE makes them more suitable for larger game....Larger caliber bullets have a higher Sectional Density, for any given shape, so for that shape, have a higher Ballistics Coefficient (because of the square/cube rule)....The higher the SD, the longer the barrel, or the higher the pressure, it takes to achieve a given velocity.... important if we want to stay in the 900 fps range....To achieve 950 fps, using 3000 psi and a 24" barrel, requires (with current technology, on air) the SD to be under 0.17.... the further under, the easier it is to achieve.... Conversely, as we go to a smaller caliber, for a given SD, bullets get longer and slimmer, and therefore tend towards less form drag, and a better BC....For target shooting, or Varmint hunting, at long range, a high BC is better.... and in addition the FPE requirements are relatively low.... favouring a smaller caliber....Wind drift (when shooting in a limited velocity range such as in the 900s) is almost entirely dependant on the BC.... higher BC equals less drift....Wind drift is minimized, at ranges out to 200 yards or so, when the muzzle velocity is around 900 fps.... and in fact is as low as the same bullet at about 3000 fps.... There are probably more things I could add to that list, but looking at those, caliber choice, most importantly, depends on the use.... If the goal is to shoot small game, or targets, at long range.... and being limited by the physics to optimum velocities in the 900s (low 1000s at most).... then you can make an overwhelming argument that the smaller the caliber, the better.... provided you have enough downrange FPE to engage your intended target.... This means that all things being equal, a .224 cal would be hard to beat for most Varmint airgunning.... So, why is the .257 cal the "go to" caliber for this task (I don't think anyone would currently argue that).... IMO, the biggest reason is the availability of suitable components.... ie barrels and bullets that work together....Probably the most widely used combination in the .257 is the Lyman 257420 (SD = 0.158), in a 14" twist barrel, shooting between 950-1050 fps.... It casts at about 73 gr. in pure lead, is deadly accurate, and within that velocity range has 145-180 FPE of energy at the muzzle, and maintains about 115-130 FPE of that at 100 yards.... It hits hard, but requires a pretty powerful gun to get into the mid 900s, things like a modified Condor running relatively long-ish barrels and/or high-ish pressures.... There are more powerful PCPs, shooting even heavier bullets, in order to get a better BC, the most notable being the 92 gr. "Noble" (named after our own dyotat100).... but with an SD of over 0.20 that requires even more push (longer barrel and/or more pressure) to achieve similar velocities.... One could make the argument that the additional FPE required is wasted on small targets, although if Predator hunting is your forte, it could be just the ticket....If we look at .224 cal, it is sadly neglected and under-represented, for basically one reason.... Most of the barrels are too slow a twist to stabilize the longer projectiles we would like to use.... The most common barrel available is only a 14" twist, which limits your bullet choice to under about 50 gr.... While pushing that weight at 950 fps gives you 100 FPE, adequate for most work, and equalling a subsonic .22LR, we can't use those specialized, long lean bullets we would like to.... In fact, there are VERY few moulds available in .224 cal (that will work in a 14" twist), the best out there at present is probably the Lyman 37 gr. 225107 (SD = 0.105).... Pushed at 950 fps, you get 74 FPE at the muzzle, and are down to about 54 FPE at 100 yards.... To get a bullet with an SD comparable to the 73 gr. 257420, we end up requiring 55 gr.... which then dictates a twist rate of about 10".... Yes, you can get that from LW, if you want a barrel with a 1.26" OD, but TJ's, for example, have nothing faster than a 12 twist, and that is .223 cal.... So, the .224 cal remains neglected in airguns, from lack of a readily available combination of barrels and bullets.... Also, .22 cal bullets are just that much harder to cast well than .25 cal....The reason I looked at 6mm was to see if there was a combination of "stuff" that would make sense for what we "need" for the job of Varmint control and target shooting.... I was surprised to find that there is pretty much an ideal combination sitting there waiting for somebody to put it together.... TJ's have a 10" twist barrel that is 0.243" groove and 0.237" land.... and Arsenal sell moulds for the "Bowman" 60 gr. (SD = 0.145), which is a scaled down 257420.... If there is any bullet that stands a great chance of working well, that would be my bet.... While it would work fine in a 14" twist, it won't be turning too fast in that 10".... and the faster twist allows for heavier, longer bullets to be used as well.... if and when the caliber develops a following, and transitions to higher power airguns.... With the SD, you should be able to drive it into the mid 900s with a 24" barrel at 3000 psi, and still maintain reasonable efficiency.... Another possibility is the 73 gr. NOE (SD = 0.177), which needs a 12" or faster twist, and a bit more power to drive it.... Then, of course, I looked at designing a BBT, and scaled down my .257 cal 78 gr. and came up with a 65 gr. bullet (SD = 0.157) with a design twist of 11".... For those comtemplating a less powerful build, the NOE 46 gr. (SD = 0.111) would provide the opportunity to use a regulated PCP at lower pressures, without the need to have too long a barrel....Make no mistake, a 6mm shooting the 60 gr. Bowman is still a powerful PCP.... At 950 fps, it will have 120 FPE at the muzzle, and about 90 FPE at 100 yards.... just about exactly duplicating the power of a .22LR.... A HP version would be devastating at those power levels, without being overkill.... and would use significantly less air than a .257 cal of comparable performance (wind drift and trajectory).... Is it necessary, no?.... Is it "better" than a .224 shooting a 55gr. pill would be for Varmint hunting, no not really, although a bit more FPE.... However, it would be a lot of fun, not too hard to put together from available components, and would certainly do the job.... Do we need any more reasons to try it?....Bob
The 46 gr. NOE 245107 bullet would be near ideal for that project, Kirby.... You should add your name to the list of those interested in an Inventory Re-run on the NOE website....Bob
I could write a long article and end up saying very little.... in reality, there is no overwhelming reason to choose 6mm (.243 cal) over either .224 cal or .257 cal............ CDo we need any more reasons to try it?....Bob
The other thing I like about TJ's barrels is the light weight.... I can stiffen them up with a Carbon Fibre sleeve and end up with a barrel that is stiffer than a typical PB barrel at a fraction of the weight.... Virtually all of the PB barrels I have seen are 1.20 - 1.26" in diameter, and must be turned down to the profile you need.... Even so, they weigh a LOT, even ones that are thinned down to 9/16" or so at the muzzle.... Why start with a barrel that is intended for 50,000+ psi when we only need 1/10th of that strength?....Bob
Make no mistake, a 6mm shooting the 60 gr. Bowman is still a powerful PCP.... At 950 fps, it will have 120 FPE at the muzzle, and about 90 FPE at 100 yards.... just about exactly duplicating the power of a .22LR.... Bob