DTAC 6mm 115gr Specifications
115_dtac_specs
by Henry Childs

6mm, 115 grain Sierra MatchKing obtained from David Tubb

x

Measured data, external:

114.96 grain weight

1.295" overall length

0.059" dia. meplat

0.190" long boat tail

0.184" dia. boat tail @ 0.028" from jacket base

Ogive projection beyond 0.185" I.D. gage

Ogive projection beyond 0.210" I.D. gage

Ogive projection beyond 0.242" I.D. gage

0.314"

0.399"

0.64"

Measured data, internal, after removing ogive jacket:

Height of top of lead core above outside of jacket base

Core dimensions at top

Core weight

Jacket thickness:

xxxxxxAt middle of ogive xxxxxxxxxxxxxxx0.018"

xxxxxxAt middle of bearingxxxxxxxxxxxxx0.0195"

xxxxxxAt middle of boat tailxxxxxxxxxxxxx0.023"

xxxxxxAt middle of basexxxxxxxxxxxxxxxx0.033"

1.073"

0.125" dia. by 0.084" high (pill box)

82.16 grains

Conditions for calculations below:

7.5" twist barrel

2950 ft/sec muzzle velocity

60,000 psi maximum chamber pressure

10,000 psi muzzle pressure just before bullet exit

30 deg. Fahrenheit air temp. at 20% R.H.

29.92" Hg barometric pressure at 200' AMSL corrected

Calculated results:

Ix = 0.000102 lbm-in2 (mass moment inertia about long axis)

Iy = 0.001464 lbm-in2 (mass moment inertia about transverse axis perpendicular to long axis and through the center of mass of the bullet)

Iy/Ix = 14.21 (higher than most long range match bullets low Iy/Ix ratios tend to promote tighter groups -- example, a 6mm, 66 grain Cheek 100/200 yard BR bullet has an Iy/Ix ratio of 5.33)

 

Center of mass: 0.764" behind meplat & 0.531" in front of base

xxxxxx Boat tail anglexxxxxx10 degrees

xxxxxx Ogive radiusxxxxxxxx11 calibers

xxxxxx Ogive arc anglexxxxx11.97 degrees

xxxxxx Gyroscopic Stability factorxxxxxx1.56 (looks good for 7.5" twist)

xxxxxx Ogive jacket stress at max. chamber pressurexxxxxx60 ksi

xxxxxx Bearing jacket stress at bullet exit from muzzlexxxxxx47 ksi

Jacket stress is given in ksi (1 ksi = 1000 psi) to set stress values apart from gas pressure values. Also, the jacket stress values look ok when compared to the Berger 6mm, 115 grain VLD, a bullet which tends to distort in the barrel or while exiting the barrel, causing it to miss the target completely.

The Berger 6mm, 115 grain VLD bullet calculated ogive jacket stress levels approach 87 ksi for conditions very similar to those given above.

The 6mm, 115 grain SMK with a three grain reduction in core weight would have about the same ogive jacket stress as the 6mm, 107 grain SMK or 56 ksi, under similar load conditions.

The ogive jacket stress in the 115 grain, 6mm SMK could be reduced below the 60 ksi estimated above by changing the boat tail angle to 7 degrees, thus moving the top of the core further into the jacket. An added benefit would be a higher ballistic coefficient.

-- Henry Childs

6/20/2004

 

 

 




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