Effect of bullet passing through mach transitions?

[Canuck Bob]

I have been wondering how cast bullets in the 30 cal range behave when shot slow enough to pass through the mach transition say between 50 and 100 yards?

My only experience is 22LR and it does seem to effect groups. I light loaded my 444 many years ago and 50 yards it grouped well. It sprayed after 75 yards. That rifle was very accurate with hunting loads.

I ask because I would prefer my loads are accurate to 100 yards even though it appears that I'll be shooting mostly at an indoor 50 yard range.

 

[fiver]

stay above or stay below all the way to paper.
buffeting will cause the bullet to wobble or veer and lose stability.
a more streamlined bullet can fight it longer especially if it is spinning quick enough, but sooner or later it can't.
the streamlined nose helps it get through the barrier easier and faster by keeping the air wave in a smaller more centralized area on the nose.

 

[Ian]

Boat tail bullets seem to do better when the wave passes them up, but not very many cast bullet shooters use boat tail bullets. The Lee 309-230-5R is one that tolerates the subsonic transition pretty well, IF you spin it fast enough to stabilize it in the first place, which is best done with a one-turn-in-7" ROT barrel. Generally, subsonic transitions play hoc with bullet stability.

 

[fiver]

even in a 7 twist at full low speed you can watch the groups double every 50 yards.
if you speed them above sonic you kind of defeat the purpose of having a sub sonic load...LOL but increase the accuracy over distance.

 

[Canuck Bob]

With my new interest in plain base I wondered how bad could this phenomena be. Turn a 2 moa 30-30 to a 3 moa. It seems from above it is not a linear result. It seems things get worse over distance which makes sense as rotation slows.

This has me rethinking my plan for the Win 94 32 Spl 16" twist.

 

[fiver]

think about it as the forward momentum overcoming the rotational stability.
the buffeting is just the front of the bullet pushing against the air.
so you have something flying along spinning like a slightly bent screw going into wood at some point.

 

[Ian]

This has me rethinking my plan for the Win 94 32 Spl 16" twist.

Either use really, really light bullets or load it full tilt. This is why I'm not a member of the Flat Earth Society who rebarrel everything with super-slow twists....doing so limits your options.

 

[fiver]

yeahit does [chuckle]
I tend to not even bother to ask/measure/care what the twist rate is.
I just try stuff till I'm happy and get what I want, or give in and do what the rifle says it wants.

 

[KHornet]

Well stated Fiver, well stated!!!!!!

Paul

 

[RBHarter]

Your experience with the 444 is exactly what happened to me with a 45 Colts in a carbine . Shoot golf balls at 75 miss the berm at 105 ......

Go as fast as you can as slick as you can supersonic with something like the 312-155 Lee or #45 Cramer .

Calculate your local mean or minimum supersonic . Load as heavy as twist will allow with the slickest bullet you can get .
The 230&247s are great but most true 30s struggle with them . While it is unbelievable I have a Savage 06' that will shoot the 230 NOE just not as a sea level sub . On the other end of the scale I have a 308 Savage that doesn't give a rip what what shape it is if it's over 175gr it will shoot a group that looks like pie plate sized slotted spoon . I did throw some 312-230s down range in a Savage 30-30 but the 308x410 holes at 50yd stopped that after the first 5 .

The answer to the above is the 06' has a special ordered in 65' 1-8.5 or a mistake screwed to the front of it .
The 308 is a 1-12+ 7?' and from 140-168 jacketed the only adjustment needed is vertical . It will shoot the 301618 172 gr PP bullet inside 2"
The 30-30 is a 80s 340 with a 10" twist , as a result it runs out of case and pressure before the heavy is stable .

I'd love to petition Al for a 230 shaped 200 , suspect there's not a lot of demand for such a beast .

 

[Ian]

Weird. I have three different .308s, one with a ten twist one with a 11" gain twist, and one with a fixed 11 and they all three shoot the Lee 230 clear on out past 200 yards pretty well. Not MOA, but close. Starting subsonic, too.

 

[RicinYakima]

Just two little tidbits for your consideration: a 173 grain .30 caliber jacketed bullet from a military 30/06 looses 0.1% of rotational speed at 1000 yards. Secondly, from spark photographs, the leading edge of the front driving band of a cast bullet has more air resistance below the speed of sound than over the speed of sound, as the shock wave passes to the rear.

 

[freebullet]

I've done a lot of reading about transonic bullet flight...

My og efforts to understand the details behind it all stemmed from 168bthp vs 175bthp in 308's. The books & net don't always jive with my targets....so,

Only thing I can tell ya for sure is sometimes it just works, sometimes it don't. I deal with it by going shooting to find out. The target never lies, but often gives clues.

 

[fiver]

the plain base 200 would be ideal in the 300 BO especially in the Ruger and other rifles with 1-8 twists.
the 1-7 rifles would eat it up and keep it going to @200yds if run right on the edge of making the crack sound.

I haven't pushed the 230 much past as slow as a few grains of pistol powder will make it go.
I switch over to a gas checked 165 and hold medium speeds with it then swap over to jacketed 110's at 2400.

in the 32 win I'm pushing mine a little too far right now, it's in the what 'was I thinking' recoil area and needs backed down a click or two.
but out to 50 yds I would bet a gas checked 120gr bullet would do pretty darn well, even at 10-1100 fps.
the recoil would be minimal at most, it would be like shooting a light loaded 30 carbine round.

 

[Canuck Bob]

This question was prompted by a quick study of 22LR CCI tables. It struck me that 22LR bullets need to be Stinger or Velocitor speeds or they hit transition at or before 50 yards. The best chance at 50 yard accuracy was wanted so it was easy to solve for 22LR, start sub-sonic.

Then while researching plain base I used the Hornady Calculator to research staying super sonic to at least 100 yards. Using simple numbers (BC .02, 150 gr. bullets) 1400fps was marginal for avoiding the transition zone and 1500 ideal at 100 yards.