Canadian Airgun Forum

I have been asked why use boattails in my slug designs for airguns.... It is a popular misconception that they are used exclusively for high velocities and long range, while in fact their greatest effect is when the projectile is Subsonic.... Since it can easily be done when making bullet moulds, by simply not making the mould cavity as deep to eliminate the boattail portion to produce a lighter, flat-based slug, the shorter of my Bob's Boattail designs (those 1.5 or 2.0 calibers long) are also made in a flat based version.... I thought it might be instructive to compare these FB slugs to their BBT parents....



The red line is the 1.5 caliber long BBT, and the dotted red is the FB version of it.... The blue line is the 2.0 caliber long BBT, and the dotted blue is the FB.... The G1 and G7 drag profiles are shown in black for reference.... These Drag Coefficient curves were calculated using Robert McCoy's "McDrag" program, which is also used by Kolbe, and JBM Ballistics.... Note that Cd is independent of caliber, whereas the BC of larger caliber slugs of the same shape will be higher, because of their greater Sectional Density....

You can immediately see that the boattails have less drag, particularly below Mach 1.... Also of note is that all of these slugs have more drag than the G1 drag profile when Supersonic.... This is because of the Meplat (flat nose) common to airgun slugs, compared to the Spitzer shape of the G1 standard projectile.... I thought you might find the theory behind my Bob's Boattails interesting.... The designs have been refined over the years, and NOE is in the process of introducing moulds to several of my new versions....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I have couple boxes of .25 cal 44 gr and 46 gr each boattails. For target rings shooting at 100 meters last summer I could not reach the precision I was expecting.
I have a theory that the speed to spin ratio was maybe low, promised myself to give it a last chance this season.
I was shooting these somewhere about a 920-950-980 fps range with a 1:18 twist rate liner, next I will try much higher speed somewhere about 1000-1050 to spin these boattails faster.
The MK2 cannon is in bits and pieces on the table in front of me, today the last critical part finally arrived.

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FX Impact MK2 PP - heavy modded
Edgun Leshiy2 - heavy modded
and a lot more scopes with great glass

My initial series of Bob's Boattails probably had too much taper, which combined with the rebate (step down behind the rear drive band) was creating flow separation and that flow was not reattaching before it reached the base.... British Ballistician Miles Morris suggested I reduce both, and all my new designs have a smaller step behind the drive band (in fact it is the same diameter as the smaller center portion), and less taper to the boattails, particularly on the shorter ones (such as your 44/46 gr.).... Not all sizes have been made, but the first one produced by NOE is a 106 gr. 7mm, and it is shooting well out to 500 metres, and the BC was 0.27 at 980 fps, showing that the boattail is working as intended....

You should not need to increase the RPM on the slug, in fact it should work fine in a slower than 18" twist....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I still need to test my Condor at full power with the 51gr. BT's, Bob. I've only shot them at 50 meters using a 4-10 setting on the power wheel.
Seems to me, the ROT is 16" on the Condors IIRC. Pretty sure my FX Dreamline won't drive them fast enough at any setting. I also have some of the
lighter 26.8 and 29gr. NSA slugs to try. I'll likely moly coat those as well.

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Best Wishes
Daryl

Seems counter intuitive that the shorter projectile is offering a higher BC. Are these calculated or measured numbers?

Things have changed a bit since I've gotten back into the game.

I did all of my rabbit (snowshoe hare, actually) and grouse hunting for the last 4 years Pre-Krovid with my .22 and .25 Talons. Shot dozens and dozens of both, out to 85 yards or so with the .25, and 50 with the .22. All with Preds.

To be fair, these are not really Talons, but heavily modified (Talon Tunes stuff c. 2015), and pretty much duplicate Condor velocities. The .22 sports a 12" barrel for quick and easy truck exit (most rabbit stuff up here is road)

You've really perked up my interest in the slugs and boattails. I'll be visiting Daryl soon and check out the molds and so on . . . . . you very likely will have another customer!

Boattail cast slug in .25? Crazy, man!!! That's a slam dunk!

The 51gr., at a mere 25 yards, made an oblong hole for 5 shots. Out at 50yards, they were out almost an inch, but that was at a low setting on the wheel.
I must try them at higher settings.

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Best Wishes
Daryl