Originally Posted by Buffalobob
The old M118 special ball bullet which was a173 gr FMJBT was stable through the transonic. The bullet was made by the Lake City Arsenal and is not to be confused with any SMK. You can probably find a few on Ebay. They were also loaded in 30-06 as the M72. Why they were stable is not clear to me but I believe the boat tail was very shallow. The dies that were used to make the bullet wore out and the QC became erratic and production was terminated.
That bullet was designed, tested and put into production in the 1920's for long range machine gun use. Made available to civilians through the DCM for competition, it proved a very accurate long range bullet. In the 1930's tests were made with the .30-06 match ammo using it and the arsenal doing it learned that bullets made without the crimping groove shot more accurate; so no more crimping groove and crimped case mouths for the .30-06. It was made up through the 1980's.
No way was the original set of jacket forming dies used in the coin, cup, trim, core and final-form stages making those bullets used for 60+ years. Several hundred sets of dies were probably made. And its boattail was virtually identical to Sierra Bullets' 190 grain and heavier HPMK's. It's BC was equal to Sierra's original 180-gr. FMJBT match bullet and almost up to what their 190 HPMK bullet has.
The reason the M118 round with the old 173-gr. FMJBT bullet was replaced with bullets using Sierra's 175-gr. HPMK bullet was the bore and groove diameters of the M14's 22-inch barrels shooting them. Interesting note is that when Lake City Arsenal came out with the M852 match round with a Sierra 168 in it, the same subsonic problems happened with M14's 22 inch barrels. Near worn out or those with larger bore and groove diameters didn't shoot either bullet fast enough to keep it super sonic through 1000 yards. In Winchester Model 70 .308 Win. target rifles with their 26-inch barrels, even near worn out ones, shot either bullet fast enough to stay supersonic through 1000 yards.
That 173-gr. FMJBT bullet in M72 .30-06 match ammo stayed well above the transonic point well past 1000 yards.
No fast-flying, long boattail bullet maintains accuracy in transonic flight. The reason the old military 173 FMJBT machine gun bullets weren't stable past their transonic point was less than 1000 yards was they weren't shot fast enough for their ballistic coefficient of their aerodynamic shape.
Sierra Bullets well knows what happens when a small change to a bullet's shape causes problems. Case in point is their 180-gr. HPMK bullet used in competitioin. The US Army Active and Reserve Rifle Teams were champions at long range with their M14NM's shooting M118 match ammo whose 173 bullet was replaced with a Sierra 180 HPMK. Winning matches and setting records was the norm with it. Then Sierra changed the boattail to a shorter one with the same shape as their 168 HPMK. The new "short-tailed" 180 HPMK had less BC and no longer shot accurate past 900 yards in 22 inch barrels. The Army Teams convinced Sierra to retool and make them 180 HPMK's with the original "long-tailed" design with a longer boattail bullet. They continued to do well with it but that bullet was never again sold commercially. Only the short-tail version was available at retail.