Bench-Talk

Randy Without trying to sound to stupid, how do you calculate the stability factor (Sg) of a bullet? Ted

http://www.eskimo.com/~jbm/cgi-bin/jbmdrag-5.0.cgi

While many formulas 'work', I have found the Sg calculated at JBM (link provided by Al, above) to be "the cats meow"! You can 'take it to the bank' - right down to the point of bullets being technically "stable", but yawing at such an exaggerated angle the most would say the bullets are tumbling . . .

Here are some examples for the BIB .25 Caliber bullets: at sea level, 3000 FPS MV, and standard conditions(59 Deg. F, 29.92" of Hg and 50% RH), the 88 Gr. BIB is 'ideally' suited (where ideal is Sg = 1.5) to a one turn in 12.3" rifled barrel; a ten twist produces  Sg 2.2. The BIB 110 Gr. FB needs a 1:9.8" twist to achieve Sg 1.5, while a 1:10" twist imparts a quite useful 1.4. Shooting the 110s via a 1:12" twist should prove a "wreck". Anything much below 1.4 Sg should be avoided. RG

Thanks for that link and also for the good info. As far as your theory that the BIB 110 would prove a wreck in a 1/12 barrel. I can confirm that as a ground truth. I have tried it twice. With my Shilen 1/12 barrel they have shoot close to 1/2 inch indoors. With my Bartlein they will just barely stay on the paper, although the holes are only slightly out of round.

When I have witnessed Coalition troops shoot the SS109 62 grain bullets from the M16A1 the bullets went through the car doors sideways at 25 yards so I guess I was expecting a full bullet profile on the target.

I wonder if the Shilen has slightly faster twist than the Bartlein? The Shilen barrel had shot the Fowler 110 into less than 1/4 inch on its initial outing. The Fowler is a bit shorter bullet than the BIB but only by about .015 inch so it should have been unstable. Jef Fowler was surprised it shot so well out of that barrel as well.

I have a Shilen #5 contour 1/10 hand lapped Chromoly barrel that I got at the Shilen Swap meet a year ago for $110 that I want to put on my Ruger and I am curious to see how well that rifle will shoot the 110 before and after.

Just in case you are wondering I do have a 6PPC but what is the fun in doing the same old thing all the time.

"IF you always follow the lead dog the scenery never changes."

Ted

"The Fowler is a bit shorter bullet than the BIB but only by about .015 inch so it should have been unstable." One of the surprising little "secrets" relating to Sg: while 0.015" difference in length looks and seems inconsequential, on a .25 caliber bullet, of a given weight/length/profile, it's worth about 0.3" difference in the twist rate required to attain the SAME Sg - in your example, a substantial boost, but still, pretty lacking. Over time and varying conditions, for bullets in this length vicinity, a 1:12" twist just won't cut it! I have a pair of Bartlien barrels ready to chamber (one a 25BR and the other a 25Hackett Hunter) and a Broughton - all of my other barrels from these makers have proven excellent. I finally got my 25x47 LAPUA barrel shooting very well (Broughton 1:10 - 5C) with a healthy charge of IMR-4007 - despite some poor decisions on my part, it shot well enough to allow a second place finish in our first Hunter tournament. It'll get another test soon - at 100/200 and 300 Yd. Following a frustrating learning curve (something 'new') it is shooting the 110 Gr. FBs quite acceptably now. Having been spoiled by the 30x47s, 30x44s, various 'short' .308s (44 - 47 mm), and 30BR chamberings for nearly 25 years now, I've been finding it quite a challenge to get the same PRECISION level out of the smaller caliber variations on the same case capacities . . . especially consistently REPEATABLE precision: I believe this to be 'effeciency' related, but can't "PROVE" it . . . ;) RG