Interesting findings while shooting...

[SmokingFrye]

I hunt with a .260 Remington and have primarily fired factory 120 gr Federal NBT's with great accuracy and consistency. It was no different today as I took a half dozen shots from 100 yds and maintained a super tight group through all 6. After this set, I tried a 3 shot series of 140 gr Remington BTSP's out of shear curiosity, and although they too maintained a sub-MOA group, that group was nearly 4.5" lower than the group shot with the NBT's. I maintained the proper time between groups to ensure i didn't overheat the action or barrel as I switched back and forth between the two different cartridges. And the results were identical through it all. I know that different bullets perform differently, but have never seen a 4+" difference between cartridges at 100 yds with this kind of consistency. Can any of our smart rifle/cartridge folks help explain this one to me? The rifle is a Savage 110 with 22" barrel, all factory, and a Nikon 4-12×40 BDC Scope if it matters. I'm open to any ideas as i research this more.

 

[denton]

Simple: Barrel whip.

A shock wave travels much faster in steel than in air, and will arrive at the muzzle before the bullet does. So the barrel is swinging in an arc... probably an ellipse... by the time the bullet arrives.

If you change the amount of time the bullet spends in the barrel, you change the point on the arc where it exits.

Ideally, you want the bullet to exit very near the end of the arc, where the barrel is moving more slowly.

 

[SmokingFrye]

Haven't thought of that. I guess the difference in grain weight and muzzle velocity between the two rounds could be annoying to this rifle. I'd just never seen this issue before (to this extreme) in any rifle i own, regardless of the cartridge fired. 4+" in 100 yds certainly pealed my curiosity.

 

[JTHunter]

In addition to the whipping of the barrel (harmonic vibrations), the extra 20 grains of the bullet will have a slight downward trend. Also, considering test barrels at the ammo factories are usually 24", your 22" could also let the Remingtons "underperform".

The fact that the Remington's are already a "boattail" bullet should have made that weight difference negligible.

 

[CB900F]

Smoking;

Sheer curiosity prompts me to ask if you know the muzzle velocities of the two rounds? I handload for a 6.5 is the reason for the curiosity.

900F

 

[SmokingFrye]

The 120 gr advertise 2950 fps, I clicked them closer to 2900, and the 140 gr Remington cartridges advertise at 2750, but i have not personally clocked those. Your reloads are probably pushing much higher velocities.

 

[CB900F]

Smoking;

Actually, no. I only use one load for my 6.5 Swede & had the barrel throated for it. That's a 140 grain Sierra Gameking that leaves the 22" pipe at around 2725 give-or-take depending on temperature. It is, however, outstandingly accurate, and consistently shoots .5's if I do my part.

900F