7mm Calibre in F Class

RAVEN · Post by **RAVEN** » Tue Jul 05, 2005 11:57 pm

I wouldnt say were custom
But I can only think of 3 in aus 6.5 SMK /berger/ lapua
Are there more mat??
Cheers
RB

Post by **Guest** » Wed Jul 06, 2005 6:16 pm

Post by **Guest** » Sun Jul 10, 2005 4:18 pm

Is the 7mm realy superior BC wise? What is the difference in wind drift at say 1000yds between a 6.5 mm bullet and 7mm bullet at the velocities they are launched at? I am still thinking of getting a straight 284 for F class ,but does anyone have wind drift figures for 6.5/284 VS 7mm/284 sierra bullets at the available velocities for each?

bruce moulds · Post by **bruce moulds** » Sun Jul 10, 2005 6:07 pm

when yhe sierra 6.5x142gn came out they understated th bc of it. thousands of rounds of it proved the bc to be higher.
the new 175 seems to be in the same situation. initial reports seem to suggest good things.
also, & i do not understand this, there is some suggestion that heavier bullets are more forgiving with verticle, in a way unrelated to bc or sd.
regards,
bruce moulds.

Post by **Guest** » Sun Jul 10, 2005 6:58 pm

From various figures given about muzzle velocities with SMK175 in the .284Win, I get the impression that 2850fps is about as high as you would go with pressure and its consequences being the limiting factor. You could probably say an equivalent MV for the 142SMK in a 6.5x284 is 3000fps. Now I'm sure some people will talk about 2950 with the 284 and I can come back with 3100 with my 6.5x284. But neither of us would load to that normally.

So, using those figures, and the highly respected figures from the Sierra website, I calculated the following drift figures at 1000 yds (for a 20mph cross-wind) :

175grSMK in 284Win at 2850fps : 137.9 inches
142grSMK in 6.5x284 at 3000fps : 136.4 inches

So, in practice, the considerably higher BC of the 7mm bullet does not give less drift at the ranges we normally shoot, because in practice, it starts off about 150fps slower (and at 1000 yds it is still slower - I get 1527fps vs 1570fps for the 6.5).

It might be interesting to see how some of the quicker 6mm cartridges compare.

Alan.

Post by **Guest** » Sun Jul 10, 2005 7:12 pm

RAVEN · Post by **RAVEN** » Mon Jul 11, 2005 9:47 am

.284 19Lbs Gun
175Gr Prjectile
MV 2850fps
Impulse 3.4lbs/sec
Free Velocity 5.7 fps
Free Energy 9.6ft/lbs

6.5/284 19lbs Gun
142Gr Prjectile
MV 2950fps
Impulse 2.7lbs/sec
Free Velocity 4.6 fps
Free Energy 6.2ft/lbs