7mm Bob's Boattails

I've been giving some thought recently to something in between .25 and .30 cal.... I can hear you saying "why bother", but it is a matter of what barrels are available in what twist rates, and how to get the longest, heaviest bullet that has the highest Ballistics Coefficient that you can use in a gun that will function on 3000 psi without going to a 30" or longer barrel.... and also the amount of air per shot (shot count), which is related to the FPE.... This thread provides some reference material.... http://www.gatewaytoairguns.org/GTA/...?topic=68763.0 .... Here are some of the things I pondered....

At 3000 psi with a 24" barrel, the heaviest bullet you have much of a chance of driving to 950 fps is one with a Sectional Density of about 0.168.... That means the following maximum weights, and the corresponding FPE level at 950 fps:

.224 cal - 59 gr. = 118 FPE
.257 cal - 77 gr. = 154 FPE
.284 cal - 95 gr. = 190 FPE
.308 cal - 112 gr. = 224 FPE
.357 cal - 150 gr. = 300 FPE

In order to get the best BC with a bullet of that SD, you want the best possible Form Factor, and that goes through a minimum (ie the least drag) with bullets between 2 to 2.5 times as long as the caliber.... For my Bob's Boattails, that means weights in the following ranges:

.224 cal - 42-54 gr.
.257 cal - 63-81 gr.
.284 cal - 85-109 gr.
.308 cal - 110-139 gr.
.357 cal - 171-216 gr.

This means that the best BC should be found between .25 and .30 cal for bullets with an SD of 0.168, and smaller or larger bullets should have slightly more drag.... To have that SD, you need nearly the heaviest bullet in .25 cal and the lightest bullet in .30 cal, in between you have more choices.... So, 7mm is looking like the sweet spot....

Now let's look at twist rates.... You can get .257 barrels in 14" twist (25-20) and 10" twist (25-35) from TJ's.... The 14" twist will only handle a BBT that is 2 calibers long (63gr.) in .25 cal, if you go longer than that (to get the maximum SD), you would have to go with the 10" twist.... In .284 cal, TJ's have a 14" twist which will work up to about a 100 gr. BBT.... TJ's have two liners in .308 cal, a 10" twist (no problem) and a 20" which is a bit too slow for a 110 gr. BBT, although it should work in a .308 cal LW barrel.... Lothar Walther do have a wide selection of PB barrels, but they are 1.26" diameter and very expensive.... So, if I choose the right weight bullet, I can design one specifically for the 14" twist TJ's barrel.... and that's exactly what I decided to do....

http://i378.photobucket.com/albums/o...ps7d756679.jpg

This design is based on the pattern that Veral Smith at LBT made to cut the molds for my .250 cal 52 gr. BBT.... It uses the same boattail and nose patterns, with a longer center section and two grooves instead of one.... I see no reason why Veral could not produce a mold for this bullet.... The order number should be # 28-96-BST.... The calculated subsonic BC is 0.32, so it should have excellent retained velocity and energy at long ranges....If I were to try this project (and I may, next winter), I would use a 26-28" barrel from TJ's.... They are 0.278" land, 0.284" groove, 6 lands with a 9/16" OD although other ODs may well be available....

Bob