Thank you to our advertisers!
Bob's Boattails Drag Calculations
Select Gate
READ GTA FORUM RULES BEFORE POSTING
Welcome New Members
GTA Forum Help Desk
GTA Announcement Gate
Airgun Legislation Actions/Information
Boss's Corner
Dealer Area
GRiP "Gateway to Airguns Review Program"
Airgun Repository of Knowledge
Airgun Content Creator Videos
Airgun Event Videos
Air Arms Airguns
AirForce Airguns
Air Venturi Airguns
Artemis/SPA Airguns
Barra Airguns
Beeman Airguns
Benjamin Airguns
Cometa Airguns
Crosman Airguns
Daisy Airguns
Daystate Airguns
Diana Airguns
Evanix Airguns
FX Airguns
Gamo Airguns
Hatsan Airguns
JTS Airguns
Macavity Arms Airguns
Pinty Airguns
Umarex Airguns
Vintage Air Gun Gate
Weihrauch Airguns
Support Equipment For PCP/HPA/CO2
All Air Gun Accessories Gate
3D printing and files
Optics, Range estimation & related subjects
Scopes And Optics Gate
Tuners
In Memoriam
GTA Contributing Members
Air Gun Gate
BB Guns and Such
"Bob and Lloyds Workshop"
American/U.S. Air Gun Gates
European/Asian Air Gun Gates
PCP/CO2/HPA Air Gun Gates "The Darkside"
Projectiles
Air Archery
Air Guns And Related Accessories Review Gates
Hunting Gate
Machine Shop Talk & AG Parts Machining
***Pay It Forward***
Buyer's, Seller's & Trader's Comments
Bargain Gate
Back Room
Member Classifieds Gate
Hobbyist Classifieds Gate
Target Shooting Discussion Gate
Target Match Rules
Shooting Match Gates
Field Target Gates
The Long Range Club
100 Yard Match
Discussions By States
Welcome,
Guest
. Please
login
or
register
.
Did you miss your
activation email
?
1 Hour
1 Day
1 Week
1 Month
Forever
Login with username, password and session length
Home
About
Help
Old GTA
Gallery
Search
Stats
Login
Register
Advertise Here
GTA
»
All Springer/NP/PCP Air Gun Discussion General
»
"Bob and Lloyds Workshop"
(Moderators:
Rocker1
,
ezman604
,
amb5500c
) »
Bob's Boattails Drag Calculations
« previous
next »
Print
Pages: [
1
]
Go Down
Share This!
Author
Topic: Bob's Boattails Drag Calculations (Read 1655 times))
rsterne
Member 2000+fps Club
GTA Senior Contributor
Posts: 27130
GTA Forums Person of the Year 2017
Real Name: Bob
Bob's Boattails Drag Calculations
«
on:
June 08, 2023, 10:51:30 PM »
I have asked British Ballistician Miles Morris to run my current Bob's Boattail designs through his program to calculate the Drag Coefficients.... Here are the results for the six different length designs.... I have included the common G1, G7 and RA4 drag models for comparison.... Note that the commonly used G1 drag model is a very poor choice for all these slug designs, as it predicts about 25% more drag at 1000 fps than occurs with most airgun slugs....
Note that below Mach 0.8 (900 fps) the two shortest BBT designs, the 1.5 calibre and 2.0 calibre long versions, have virtually the same drag.... They are a good fit to the RA4 drag model (originally intended for the .22LR bullet).... Here is another chart, showing the velocities over Mach 0.8 in expanded scale for more clarity....
It is interesting to note that the longer designs, 3.0 calibre, 3.5 calibre and 4.0 calibre long BBT, are a near perfect match for the G7 drag profile, which makes sense, as it is intended for boattail bullets!.... They are also very close in drag to each other, with the 4.0 cal. long BBT having only a bit less drag that the 3.5 cal. long or 3.0 cal. long designs.... The middle length, 2.5 calibre long BBT, is a decent fit to the G7 drag profile, but Ballistics Coefficients calculated using that would have quite low values (not an issue if you use G7 for the trajectory calculations).... Scott Hull, who has been shooting a 7mm version of that slug (weighing 106 gr.) has found that using the RA4 drag model works very well below Mach 0.95 (1070 fps), at distances out to 500 yards, so that would be a good choice for the 2.5 calibre long BBT.... For those of you not familiar with my BBT designs, here are 3D printed models of them, all to the same calibre (in this case 51mm)
....
These Drag Coefficients are calculated, not measured, but Miles feels they should be within 5% of the actual drag.... I am in his debt for taking the considerable time to run all these through his ballistics prediction program....
Bob
Logged
Coalmont, BC, Canada
The only thing necessary for the triumph of evil is for good men to do nothing.
Stand up for what you believe in, my friends!
jackssmirkingrevenge
Sharp Shooter
Posts: 818
yes
Real Name: Jack
Re: Bob's Boattails Drag Calculations
«
Reply #1 on:
June 09, 2023, 12:18:46 AM »
Quote
all to the same calibre (in this case 51mm)
I look forward to your next
big bore project
Logged
QC
"You have to make many weapons, because this field is long and short life"
rsterne
Member 2000+fps Club
GTA Senior Contributor
Posts: 27130
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Bob's Boattails Drag Calculations
«
Reply #2 on:
June 09, 2023, 12:41:41 AM »
Jack, it might be possible to shoot those plastic models, but with the many hours to print, ammo supply would be a problem, and the BC would be terrible because of the light weights.... The models were .257 calibre, scaled up almost 8 times for printing.... Each 0.2mm thick layer in the print represents 0.001" in length on the .257's.... The 4.0 cal. long model is just over 8" tall.... They sure do look nice sitting on my bookshelf, though....
Bob
«
Last Edit: June 09, 2023, 12:53:29 AM by rsterne
»
Logged
Coalmont, BC, Canada
The only thing necessary for the triumph of evil is for good men to do nothing.
Stand up for what you believe in, my friends!
Print
Pages: [
1
]
Go Up
« previous
next »
GTA
»
All Springer/NP/PCP Air Gun Discussion General
»
"Bob and Lloyds Workshop"
(Moderators:
Rocker1
,
ezman604
,
amb5500c
) »
Bob's Boattails Drag Calculations