.257's - hype, or the magic caliber?

[PelletsForPests]

There's been a lot of talk about .257's, and I was wondering if anyone can explain why they would be preferable over say a .308 given the fact airguns can send either caliber down range at 950 fps+?

The obvious answers are that a .257 will have a slightly higher shot count and cheaper ammo, but aside from that, I want to know if I've missed something while comparing a couple posts and ballistic tables on popular lead cast ammo.

I know (assuming one is using high grade ammo) the higher you go in caliber, the better BC's you will get based on inertia if nothing else, but does the .257 have an inherently better sectional density or something that makes it ideal for shooting lead cast projectiles?

[Zebra]

There is no such thing as a magic caliber but.... It is easier to achieve tighter groups with smaller caliber ammo. A clover leaf with .257 projectiles would measure shorter CTC than a .308 clover leaf.

People that compete in FT or BR typically prefer .177 over 22cal for the same reason.

[farmerjoe99]

I think part of it is that so much has already been documented on .257
with a easy to acquire and known accurate combination of barrels and bullets.

[K.O.]

yep it is pretty much about available barrels and molds/boolits available... 

Heck I would love to see a slow twist .270 with some 65-90 grain rounds

or maybe a  slow twist .243  (in other words a .244 Rem)...

But for now I would be happy for more .22 and  .25 acp molds... just because when sending the lighter stuff 30-50 grain it can still be done with a hand pump...

More fpe means accuracy at longer distance if managed right but really starts getting hungry for air quick so in the end what compromise do you make is what matters...

[rsterne]

I have a slightly different take on airgun calibers than most, when it comes to bullet shooters.... With modern day PCPs you can pretty much hit 950 fps, which is the sweet spot between trajectory, wind drift, and efficiency....  For any given pressure and barrel length, there is a maximum Sectional Density you can use to reach that velocity.... The Ballistics Coefficient is the Sectional Density divided by the Form Factor, which is basically how "slippery" the shape is.... For a given SD and basic shape, the bullet length is a constant.... Therefore, the smaller the caliber, the lower the FF you will get for a given SD, so the better the BC....

This favours the smaller calibers once the bullet leaves the muzzle.... However, in terms of internal efficiency, larger calibers tend to have the edge, producing more FPE for a given amount of air.... Then you have to look at the purpose of the round.... IMO, .308 cal is the trade-off between Varmint/Predator rounds and Big Game.... It is also the tipping point where the bullets start to get draggy as you go larger.... So I consider .224, .257, and 7mm to be primarily Varmint rifles.... and .357, .408 and .458 cal to be for Big Game.... leaving the .308 usable for both, but IMO ideal for neither....

If you are punching paper, the larger you go, the more air you use, and what is the point, you're only killing paper.... Since for a given barrel length and pressure, the SD is limited to get a decent velocity, you may as well go with .224 or .257 cal.... There are more bullets available in .257 cal, and they are easier to cast.... You can get barrels with a twist rate that match .257 airgun needs easier than you can .224 barrels.... so by the process of elimination my choice for target or varmint is the .257 cal.... There are bullets available from about 65 gr. to over 100 gr. and barrel twists to match.... and you can drive them fast enough with the right combination of barrel length and pressure for the weight you choose....

Bob

[PelletsForPests]

Thank you Bob. I had failed to put together that form factor essentially just pertains to drag. Also, I hadn't considered that at some point a larger "boolit", despite having increased terminal velocity and inertia, loses accuracy as it's counterintuitively more succeptsble to atmospheric inconsistency due to external inefficiency that increases exponentially with friction