Airflow Around a Meplat

[rsterne]

I have been asked why it is that a bullet with a Meplat can have less drag than a Round Nose.... Here is a diagram that hopefully will help you understand that....



When the air flows around the nose of a bullet with a flat on the front (Meplat), it piles up in a zone of eddies and turbulence called the "Stagnation Zone".... The air in that zone is effectively being carried along with the bullet, and provides a smoother nose shape for the rest of the air to flow around as the bullet cuts through it.... This makes the bullet act longer than is really is, with the nose working as if it was elliptical in shape.... This effect is represented by the blue lines in the above drawing.... The red line represents a hemispherical nose, which will not have any significant stagnation zone, so the radius at the very front where the air first starts to split to go around the nose is blunter, and therefore has more drag....

This is valid only for Subsonic flow (below about Mach 0.8 or 900 fps).... above that the air in the stagnation zone starts to compress (which is why Transonic and Supersonic flow are called "compressible flow").... When that happens, the smooth "streamlines" begin to break down, and the drag starts to increase rapidly until shockwaves start to form when the local airflow exceeds the speed of sound.... Eventually, once that happens, a large Meplat becomes, quite literally, a drag, although it may still have no more drag than a round nose, and often less.... The longer the nose, and the smaller the Meplat, the less the drag once you are at Mach 1.2 and above (truly Supersonic), although a small meplat, such as found on a HP Spitzer (10-20%), can actually have less drag than a sharp point, even at extreme velocities....

Careful matching of the radius of the Ogive and the diameter of the Meplat, can extend the smooth Subsonic flow up to about Mach 0.9 (1000 fps), and this is of course desireable if you want to push the velocity into the 900-1000 fps range.... I studied this in some detail using a program called McDrag, and determined that for longer bullets, that occurs when the angle where the Ogive meets the Meplat is at about 20 degrees to the centerline.... This is not always possible with short, blunt bullets, of course.... without the Meplat getting so large that it's drag takes over.... In those cases (including the example above), the best compromise is where the Meplat isn't too large, and the ellipse formed by the stagnation zone is an efficient shape....

Bob

[Tater]

Great stuff as usual Bob. Thanks for that diagram and explanation.

[K.O.]

Hi Bob I am thinking about getting this Lee .22 Bator and hollow pointing it for use with the .22WMR barrel (Marlin Micro-groove)  and hollow pointing would help to bring it down to  mid 40g weight.  I read a couple comments about it dropping at about 50g-51g pre gas check and lube on the cast boolit forum.

http://www.midsouthshooterssupply.com/item/0000622BATOR/.22-Caliber-55-Grain-Double-Cavity-Special-Order-Mold-C225-55-RF-by-LEE-RELOADING-PRODUCTS

I wonder if that same principal applies  to a shallow hollow point and deeper hollow point?

also I used these figures to use the Border Barrels twist calc, I think I got it right?

Bullet diameter  .224
Bullet length      .52
Nose length       .109
Meplat diameter    .120
Either Base diameter   .224
Or Boat-tail angle (degrees)
. . . and Boat-tail length 
(Optional) Barrel twist   16

I used 11.4 for Specific gravity...

I have read of it stabilizing in a 16 twist  and do have a bit of faith iin the Micro groove barrel BUT... if  not I could shorten the gas check base by hand to until it stabilized and then have it milled... I think calc says .045 would do... and honestly  getting down closer to 40g might help with only a 22" barrel and stock .25 Mrod power...

I can not believe my practice seasoned .22 lr 25$ barrel is looking good... chucked it up in my delta 17" drill press (250 rpm) ground it down close to 9/16 with a 4" angle grinder(cooling with a bowl of water and wash cloth frequently)(sparks say good amount of carbon by the way).

then 9/16 spacers between two files to finish to size... so I think I will chance it with the .22WMR... then just need the oring seat... and such...

So how does a deep or shallow hollow point act in longer and shorter subsonic boolits?

[Roadworthy]

Thanks, Bob.  Really good stuff.  Technical explained for the common man's understanding.  I don't know how you do it but I do really appreciate the insights.

[rsterne]

As I understand it, there is no practical difference in the Form Factor between a Hollowpoint and the equivalent size Meplat, as the air in the HP cavity is stagnant and travels with the bullet.... They have a lower BC simply because they have a lower SD from the reduced weight in the same shape....

A 55 gr. bullet in .224 cal is getting up there in SD, you will need lots of pressure and barrel length to get a decent velocity (depending on how you define that, of course).... My .224 cal Hayabusa shot 41 gr. bullets in the mid 1000s with a 29" barrel on 3000 psi and bore-sized porting throughout.... It has a 14" twist, and in Europe they recommend a 15", apparently it has marginal stability in a 16".... I have some 60 gr. bullets, but they are for a 10-12" twist, so have never used them....

I tried the NAA 30 gr. bullet in a Microgroove barrel, and you couldn't hammer them into the bore without destroying them, you certainly won't get a 0.225" bullet in there.... Don't forget to reduce the SG of the lead somewhat if you hollowpoint the mold when calculating the stability.... About half the theoretical correction seems right because the HP cavity is near the middle, and it's the outside that has the most effect on the stability.... If the stability isn't marginal, you don't even need to worry about correcting for the HP cavity....

Milling off the gas check rebate may reduce the accuracy for two reasons.... When casting a .22 cal bullet it is difficult to get a perfect base, distortion from the sprue plate is common.... and the rifling tends to create tiny "fins" that protrude past the base of a flat base bullet, causing accuracy problems.... Most guys using bullets in PCPs look for gas-check designs for these reasons....

Bob

[K.O.]

Thanks Bob I probably should have just started another thread...

This is a long term project I need to finish paying off the Mrod before starting to pick up the casting and sizing equipment so I am just looking at options right now...

I am profiling the Marlin 22lr right now but not for slugs... And I think that is the one you wanted to use with the NAA 30g...yep the NAAs are for the .22WMR cap and ball...

My "seasoned" Marlin .22lr slugged at .22 maybe .198  (to bad they do not sell just .22 lr leads) so yep neither the .225 Bator or the NAA will fit that barrel and I do not know if they could be sized to fit... maybe... I do think that it will make an excellent pellet barrel for large headed pellets... it looks like the Eun Jin round nose fit perfectly

I need to pick up some NAA slugs to see how they fit I do think They will work in the 22WMR barrel it slugged at .224 and the Bators should size to .224 pretty well..?

"
Milling off the gas check rebate may reduce the accuracy for two reasons.... When casting a .22 cal bullet it is difficult to get a perfect base, distortion from the sprue plate is common.... and the rifling tends to create tiny "fins" that protrude past the base of a flat base bullet, causing accuracy problems.... Most guys using bullets in PCPs look for gas-check designs for these reasons....
"
well here is the hopeful part the gas check is .085 long so if I mill each bullet by hand by .045 it might get rid of some distortion, loose a few grains of weight. and still leave enough gas check at .04 long to help...  also the Ballard style rifling that most are using seems to be more likely to cause fins than the Micro groove... The second twist graph (above) is with a length of .475 (with only half the gas check milled off)...is a .04 gas check shank worth having ill it help... well I would like to find out

I will post the twist graphs for it milled all the way off(and 11.4 sg and 10.4 sg also) and I bet it will be about a 1:17 twist to make it happy...

Bullet diameter (inches) .224
Bullet length (inches) .435
Nose length (inches) .109
Meplat diameter (inches) .120
Base diameter (inches) .224
Boat-tail angle (degrees) 0
Boat-tail length (inches) 
Barrel twist (inches) 16
Secant nose radius (calibres) 0
Specific Gravity (gms/cc.) 11.4 & 10.4

[K.O.]

"
A 55 gr. bullet in .224 cal is getting up there in SD, you will need lots of pressure and barrel length to get a decent velocity (depending on how you define that, of course).... My .224 cal Hayabusa shot 41 gr. bullets in the mid 1000s with a 29" barrel on 3000 psi and bore-sized porting throughout.... It has a 14" twist, and in Europe they recommend a 15", apparently it has marginal stability in a 16".... I have some 60 gr. bullets, but they are for a 10-12" twist, so have never used them....
"

Yeah velocity that is the  kicker but in the end it is a cheap experiment $30 or so...

the 1:16 twist will save a couple fpe over the 1:14 twist... maybe?

I would be happy around 825/61.5 to 850/65.7  (fps/fpe) I think should still out do a pellet at range I think...

I think a lot rides on how successful of a diet  it gets if at about that same 41g well a 7" shorter barrel for 200 fps less...I do think the faster you go the more it takes to go faster so maybe it will make it... I do have 10 & 15 lb hammer springs...

? 

I keep buying $3 of lottery tickets a week in hopes of being able to do the experiments quicker...

[PakProtector]

The top lines of .22 LR ammo by Eley use a flat nose, termed EPS. This stuff is high sub-sonic( 1050-1080 fps ). It shoots quite well...
cheers,
Douglas

[Cal]

I've often wondered why we are not all shooting Keith style wadcutters.
Lots of similarities,....   but the bore contact area.