The AirGun Guild

General Category => Airgun Ammo => Topic started by: steveoh on January 14, 2019, 08:48:28 AM

Title: Ballistic Coefficient of Arsenal 257420 FP, and Boat Tail ?
Post by: steveoh on January 14, 2019, 08:48:28 AM
I bought the Strelock Pro app last night and was busy setting up a variety of airgun/ammo/scope combinations. Was working on the new spiffy Airforce Texan .257 and tried to find Ballistic Coefficient of the Arsenal 257420 FP and Boat Tail. I found charts for Lyman 257420 which is the design I believe Arsenal used. But these charts are all at centerfire velocities. I'd think that air rifle velocities will change the BC. True?

I'm sure Bob will chime in, as the boat tail version of this bullet is his design.

Title: Re: Ballistic Coefficient of Arsenal 257420 FP, and Boat Tail ?
Post by: steveoh on January 14, 2019, 09:20:50 AM
Well looky here:

https://www.gatewaytoairguns.org/GTA/index.php?topic=129816.0

Bob answers the question no matter if it's here, the GTA or ??

Quote
I ran across this tonight, and it appears to be a fabulous resource.... http://www.accurateshooter.com/ballistics/bullet-database-with-2900-projectiles/

It has over 3900 bullets, including many cast bullets.... You can access the data by caliber, and sort by manufacturer, weight, length, or other factors.... It gives the Sectional Density (SD), and the Ballistics Coefficient (I assume G1) at different velocities (if known).... As an example, here are the results for the popular Lyman 257420....

SD = 0.148
BC above 2200 fps = 0.129
BC above 1800 fps = 0.152
BC above 1300 fps = 0.18
BC above 800 fps = 0.20
BC below 800 fps = 0.15

I have only just begun to explore this database, but wanted to share it with all of you....

Bob
Title: Re: Ballistic Coefficient of Arsenal 257420 FP, and Boat Tail ?
Post by: rsterne on January 14, 2019, 09:54:15 AM
When in doubt, for flat based, FN bullets of the same general shape and proportions as the 257420.... the BC for us to use at airgun velocities is usually somewhere between 80-100% of the SD.... Assuming a weight of 70 gr., a .257 cal bullet has and SD of 0.151, so the subsonic BC should be between 0.12 and 0.15.... A short boattail like that used by Arsenal will usually add about 0.1 to that, so for the boattail is it likely between 0.13 and 0.16.... Only shooting it from your gun will pin it down any closer than that, IMO....

IRC, Doug Noble once told me that he uses 0.13 (or was it 0.14) for the BC for the flat based 257420.... BC is proportional to weight, so if your bullet is heavier (or lighter in the case of a HP or boattail) the BC needs to be adjusted accordingly…. My Lyman mold drops at 73.4 gr. with pure lead, but the HP Erik did is big, and they are only 66.0 gr.... I think the Arsenal version of the FB, FN 257420 is slightly lighter than the original Lyman, from what I understand.... for sure the BT will be lighter....

Bob