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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Changing My Thinking About Slugs

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rsterne:
One very cool thing about airguns is that I never stop learning something new, or at least a new way of looking at something!.... There have been a few things happening lately that have changed my thinking about airgun slugs and the velocity we shoot them at.... Here are the "new" developments....

It turns out that the G1 drag model we have been using to calculate the Ballistics Coefficient of our slugs is not a very good fit to the drag curve of the slugs.... This first showed up as higher than expected BC's for our slugs when they were shot at higher velocities.... Manufacturers were claiming high BC(G1) values for their slugs, to the point of being hard to believe.... When tested at 800-900 fps, we got much lower values than claimed.... Then articles started showing up claiming that the faster slugs were shot, the higher the BC(G1) was.... If the drag model being used to calculate the BC (in this case the G1 model) is a poor match, then BC values change with velocity, whereas they should remain constant.... This was a warning flag that something was wrong.... When we started investigating the actual drag of slugs, by shooting them over a LabRadar chronograph, we quickly found out that the drag in the low Transonic region, between Mach 0.8-0.9, was lower than the G1 model.... This was the reason that as the velocity was increased, the calculated BC(G1) increased, because the slugs were being compared to a drag model where the drag was increasing faster than it was with our slugs....

Miles Morris, UK ballistician, calculated what the drag should be doing for our typical slugs (a tangent ogive with meplat design), and confirmed that what we were seeing in fact had a base in theory.... He called this new drag model SLG0, and while it is a work in progress, it gives more constant BC's than does the G1 drag model.... However, it is only available in one Ballistics Calculator, the "MERO" developed by another UK chap, George Conway.... We have found that the RA4 Ballistics Model, which was intended for the .22LR rimfire bullet, is not a bad match for our slugs, and far better than the G1 model.... Slug manufacturers would be doing a great service to their customers if they quoted the BC(RA4) instead of, or at least in addition to, the BC(G1).... Unfortunately the "quoted BC" using the RA4 model is lower than by using the (flawed) G1 model, so they can't brag as much!....

Those who have followed my writings over the years will remember that I have proposed that there is an "optimum" velocity to shoot pellets at to minimize the wind drift.... It turns out that is about 900 fps, plus or minus a bit, depending on the range you are shooting.... Since the G1 and GA drag models have similar drag curves, then the same thing should apply to slugs, optimum velocity for minimum wind drift should be similar to pellets.... Well, it looks like I got that wrong, because our slugs do NOT follow the G1 drag curve.... Follow this thread as I add to it, and you will see why....

Bob

mr007s:
Interested and following along.

splitbeing:
Exciting news! Following with interest in discoveries...

Baco:
following  :o ;D

JuryRigger:
Watching with great interest!  :D
Jesse

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