Sierra 123 gr SMK vs. Lapua 123 gr

[LRRPF52]

sititunga,

Have you looked at some of the slower powders for your bullet and barrel length? It seems that I am getting about as much velocity as I can on the edge where brass life suffers more with that load. If you even out the pressure curve with the available barrel length you have, it should be easier on your brass, bolt, and throat. I'm pretty sure I felt some loose primer pockets after that batch, and that was probably only the 3rd loading of that brass when I loaded last.

There was a ton more information about this on the forum before it was destroyed by someone. It had each bullet categorized in the reloading section in a very helpful manner, with lots of real-world experience from forum members trying several different powders, COAL's, barrels, etc. It would have made a great source for a reloading book specifically for the Grendel.

When I did my initial load development, using the same powders, brass, and bullets as AA, it seemed to work out that I had about 18 fps loss in velocity per inch of barrel length with the Grendel, but I've heard others say it is more like 20-28???

I know that my 30.0, 30.5, and 31.0gr loads with the 100gr NBT's felt nothing like the 120gr and 123gr loads...the recoil was almost non-existent. I'm kinda excited to see what happens with the Elite Ammunition 98gr solids. They will be very forgiving with pressures as there is much less bearing surface with the driving bands. If I can get them to go 2750 fps, they should extend my effective range to maybe 700.

LRRPF52

[noone]

I think many bullet manufacturers deliberately use differently computed BC's just so it IS difficult for us to easily compare them. Uninformed shooters end up comparing apples to oranges because they are comparing two different BC's and don't know why their ballistic computers don't match up to actual results.

If your bullet's BC is truly a G1, and your computer uses a G7 BC, then it will be sort of in the ballpark, but the further out you go, the bigger the error will become.

[LRRPF52]

I think many bullet manufacturers deliberately use differently computed BC's just so it IS difficult for us to easily compare them. Uninformed shooters end up comparing apples to oranges because they are comparing two different BC's and don't know why their ballistic computers don't match up to actual results.

If your bullet's BC is truly a G1, and your computer uses a G7 BC, then it will be sort of in the ballpark, but the further out you go, the bigger the error will become.

BC's are only to plug into a theoretical drag model anyway, and the BC of the exact same projectile going 2500 fps is different from when it is going 2900 fps. However, when I plug in the BC's for the projectiles I'm using in a theoretical model, I'm usually dead-on in the elevation trajectory out to 700m. The radar data really comes into play for the big boy calibers like .338 LM-.50 BMG at distances past 1300m.

The G1 and G7 data will get you very close at the distances where the Grendel shines: out to 600yds. I always use G7, but it probably doesn't matter that much. Lapua's radar data will get you within 11cm at 1500m with the .338 Scenars.

LRRPF52

[bwaites]

The G7 data that Bryan Litz is producing for the VLD/Semi VLD (like the Scenars and long SMK's), really seems to hold up better than the G1's at distances past 500 meters in my experience.

The G7 data accounts, at least in part, for the reduced velocity at distance, and the computations done with my 7mm WSM have been spot on from 500-1200 meters.

The variable speed G1's that Sierra has used cause the ballistic computers to have nightmares, and my Iphone/Ipad version of Ballistic FTE gives some of the weirdest numbers I've seen for the 107 SMK. I know they aren't accurate, but the BulletFlight and Isnipe software must use some composite and their numbers are spot on.

[JASmith]

...The variable speed G1's that Sierra has used cause the ballistic computers to have nightmares, and my Iphone/Ipad version of Ballistic FTE gives some of the weirdest numbers I've seen for the 107 SMK. I know they aren't accurate, but the BulletFlight and Isnipe software must use some composite and their numbers are spot on.

Sierra has used the variable-speed G1 for a very long time. They were among the first, if not the first, major bullet manufacturers to both realize the single-value G1 doesn't work all that well for modern bullets and to do something about it. At the time, the G7 coefficient wasn't in vogue.

Remember that the "ballistic coefficient" attempts to wrap up the whole subsonic-transonic-supersonic drag curve into a single number. It can be done for one bullet of a particular shape. The mapping gets less accurate as the bullet shape changes even slightly. This will be true also for bullets that are scaled from one caliber to another. Details like meplats, corner radii, etc. are not transferred very well from one caliber to another. They are, however, important to the drag function.

Bottom line -- how well do velocities and trajectories by JBM for Sierra multi-value BC's map into those with G7 coefficients for the same bullet. When you find a difference large enough to care about, find a way to test and do it!

(I haven't done the comparison yet, either, but hope someone does and tells us about it!)

Cheers!