Cartridge and SD’s

[Bob3700]

Now that a lot of shooters own or at least shoot on e-targets I am curious what all of u have found concerning SD for ur various cartridges at 600 yds.
I shoot 308, 6 BRX, 6 XC and 243 LSR and use sling n irons. The 308 is king of low SD at 600 yds averaging 6-7 fps with a 155 SMK. The BRX is next with 10 fps ( new bbl.) with 107 SMKs, the 243 SLR averages 11-13 fps with 105 Hybrids, and the 6 XC is in the 15 fps range with 107 SMKs. All are loaded with similar techniques, low neck tension/ soft seated into the lands, selected cases, inside neck reamed, fed 210Ms.

All will shoot as many Xs as I can hold but just curious what others are seeing on the Shot Marker.

Is the 308 inherently easier to achieve low SDs?

Bob

 

[D-4297]

I have achieved low SD numbers with both a .308 , and a Mosin-Nagant 7.62 x 54r chambered for a .308 bullet . I think part of it is there is so much information out there about the .308 , it's a much shorter path to get to low SD loads , rather than that caliber being the answer to your question . Many get extremely low SD's with other caliber rounds .

 

[SKWERLZ]

Best SD I ever had was with a 260 with IMR 4451 and Hornandy 147’s back when they were good.
That thing was always below 5 SD with a velocity of 2800.

 

[Bob3700]

I have achieved low SD numbers with both a .308 , and a Mosin-Nagant 7.62 x 54r chambered for a .308 bullet . I think part of it is there is so much information out there about the .308 , it's a much shorter path to get to low SD loads , rather than that caliber being the answer to your question . Many get extremely low SD's with other caliber rounds .

I understand the usual loading info for 308 but the other cartridges have been around for awhile and surely there are “ Pet Loads” that are proven.
I guess I am curious what others are seeing at the 600 yd. etargets with their loads. My goal is always single digitS but there is more to it than just the rigHt powder charge.

 

[Straightshooter1]

Now that a lot of shooters own or at least shoot on e-targets I am curious what all of u have found concerning SD for ur various cartridges at 600 yds.
I shoot 308, 6 BRX, 6 XC and 243 LSR and use sling n irons. The 308 is king of low SD at 600 yds averaging 6-7 fps with a 155 SMK. The BRX is next with 10 fps ( new bbl.) with 107 SMKs, the 243 SLR averages 11-13 fps with 105 Hybrids, and the 6 XC is in the 15 fps range with 107 SMKs. All are loaded with similar techniques, low neck tension/ soft seated into the lands, selected cases, inside neck reamed, fed 210Ms.

All will shoot as many Xs as I can hold but just curious what others are seeing on the Shot Marker.

Is the 308 inherently easier to achieve low SDs?

Bob

I've read and reviewed a lot of data and tests that suggested to me that the smaller calibers are easier to get very low SD's (like less than 5 fps). In fact, I've read in a couple places stating that the smaller calibers were easier to get low SD's, though I still don't quite understand why. So, I'm kinda surprised that you're not getting better SD's with those calibers you have than what you're getting with your .308.

With my .308, I get SD pretty close to what you're getting, maybe a tad higher on average. For my 6.5 PRC I most often get SD's a little better than 6-7 fps. In my case, I always use a MagnetoSpeed and don't use Shot Maker.

 

[David Christian]

Now that a lot of shooters own or at least shoot on e-targets I am curious what all of u have found concerning SD for ur various cartridges at 600 yds.
I shoot 308, 6 BRX, 6 XC and 243 LSR and use sling n irons. The 308 is king of low SD at 600 yds averaging 6-7 fps with a 155 SMK. The BRX is next with 10 fps ( new bbl.) with 107 SMKs, the 243 SLR averages 11-13 fps with 105 Hybrids, and the 6 XC is in the 15 fps range with 107 SMKs. All are loaded with similar techniques, low neck tension/ soft seated into the lands, selected cases, inside neck reamed, fed 210Ms.

All will shoot as many Xs as I can hold but just curious what others are seeing on the Shot Marker.

Is the 308 inherently easier to achieve low SDs?

Bob

@Bob3700 I would not use the data on the shotmarker. The shotmarker was not developed for this type of data to be reliable, it is only anecdotal. Wind conditions and the stability of the backer boards can distort the numbers to a large degree. These are from the same relay, Labradar (3.74 SD) vs Shotmarker (12 SD). This is a 7mm bullet.

 

[Bob3700]

I've read and reviewed a lot of data and tests that suggested to me that the smaller calibers are easier to get very low SD's (like less than 5 fps). In fact, I've read in a couple places stating that the smaller calibers were easier to get low SD's, though I still don't quite understand why. So, I'm kinda surprised that you're not getting better SD's with those calibers you have than what you're getting with your .308.

With my .308, I get SD pretty close to what you're getting, maybe a tad higher on average. For my 6.5 PRC I most often get SD's a little better than 6-7 fps. In my case, I always use a MagnetoSpeed and don't use Shot Maker.

The SD numbers that I have stated are taken off our Shot Marker target at 600 yds. I have not taken any readings at the muzzle.
I am sure that low SD's are a function of consistent bullet release (aka, neck tension) as well as case capacity and powder metering. Ur bullets need to be pretty consistent as well. Massaged Sierra's or Bergers.

 

[Bob3700]

Dave,

I have always been suspect of the accuracy of the chrono data from the Shot Marker. That said, the comparisons from one cartridge to another fired on the same day should at least correlate to one another.

While the unit is not calibrated , one should be able to at least compare the numbers from the BRX to the 308. At least that is my guess.

Bob

 

[Bob3700]

Dave,
One last thought. The labradar is picking up the bullet SD at the muzzle where there are fewer variables. Once that little missile travels six foot ball fields, the variations in the bullet BC and the atmospherics will certainly play a part. I would expect the SD to be lower right off the muzzle. That is my thought anyway.

Bob

 

[D-4297]

Velocities taken at the target are "referential" , at best due to all of the variables involved .The only "actual" muzzle velocities , and SD numbers that may be close to accurate are taken at the muzzle , and they are also just "close" approximations . There really aren't any "Absolutes" in this . Just the actual name; "Standard Deviation" should tell you it is a approximate measurement .