Need help with .308 projectile drop

[another_luser]

Good afternoon folks.
I’m fairly new to scoped rifle shooting and just purchased a Savage 10 in .308 with a Nikon 3-9-40 scope. I zeroed it at 100 yards (indoor range) and was getting 1.5 inch groups with American Eagle 150 grain boat tail. I switched to 168 grain Zombie Max ammo, and all of my shots (three groups of four rounds each) hit about 7 inches low at 100 yards. I switched back to the American Eagle and the next group was back on target. All shots were from a bench rest.

My rifle experience consists mostly of Ruger 10/22 at 25 yards and Ruger Mini-14 at 100 feet, neither scoped. (My eyes won’t let me shoot more than 100 feet any more, hence getting a rifle with a scope.)

Any idea if I am doing something wrong?

Thank you,
Another_luser

 

[BoilerUP]

Heavier bullet=lower velocity=point of impact below the lighter, faster bullet.

 

[chrome_austex]

Yeah, different ammo is going to group at a different POI. Its not all velocity, some of it is when the bullet exits the barrel as the barrel is whipping with the shockwave.

Sometimes faster ammo will hit lower and vice versa.

7" is a big difference at 100y, but I suppose its possible. Are you shooting off a bipod?

 

[allaroundhunter]

7" is a big difference, but were the groups just as good as the lighter rounds?

 

[TonyAngel]

Don't tear your hair out over this, as long as you're getting acceptable groups for the rifle. Fact is that different ammunition acts differently.

 

[jmr40]

7" is a lot more than I'd expect. I doubt if you are doing anything wrong as long as you are getting decent groups.

The 150 gr bullets are leaving the muzzle about 100-150 fps faster than the 168 gr bullets. Hornady shows 2700 fps for the 168 gr bullet and Federals website shows 2820 fps for the 150 gr bullet, but remember all guns are different, the actual velocities in your gun could be very different.

I shoot both bullet weights in both my 308 and 30-06 and there is no measurable difference at 100 yards. At longer ranges the heavier bullet drops more but the difference is less than you'd think because the heavier bullet is more aerodynamic. I've tried some 130 gr bullets in the 308 and they are about 2" higher at 100 yards with the same scope setting.

 

[another_luser]

Thank you for the replies.

chrome_austex, I was using a plastic rest (not sure the brand as it was range-supplied) and not a bipod.

Perhaps the best bet is to get a third type of ammo and see what the results are as this rifle may not like Zombie Max. Or perhaps it was a bad batch; now that I think about it, there seemed to be less recoil with the Zombie Max, and I kind of expected more recoil considering the heavier bullet.

Thank you again for your input.

Sorry, forgot to mention, the Zombie Max groups were acceptable (2 inches, ish), just way low...

 

[adelbridge]

First off, I want to know where you found a 100 yard indoor range, that is an ideal spot to zero in a rifle. I get about 1.5" drop switching between 168 and 150 grain bullets. 7" would point to an underloaded round. I dont know much about Zombie Max but maybe they load them down for their intended application (profit). Once I ran a box of managed recoil 7mm rem mag and I only got a 3" drop.

 

[Driftertank]

my guess would be underloading (QC issue), a freak harmonic issue, or operator error (did you shoot both with your stance/hand position th same, the gun in the same position on the rest, etc. ?)

Try a different 168 load. If the recoil feels ok, and it still hits way low, maybe a harmonic. If it's closer to your original 150gr aimpoint, i'd say likely a QC issue with ammo.

Just my 2 cents (and that may be all it's worth).

 

[henschman]

Since both are acceptable size groups, I would just chalk it up to the different loads having different POI's, which is common. A 7 MOA difference between loads is a lot, but still within the realm of possibility. As one other said, it has to do with at what point in the recoil impulse/barrel whip the round exits the muzzle. If you plan on doing some shooting with both loads, just write down the MOA difference, and make the necessary clicks on your scope when you switch loads.

 

[wally]

Sorry, forgot to mention, the Zombie Max groups were acceptable (2 inches, ish), just way low...

Try different ammo. Most .308 Savage 10 users report better groups with 168 gr ammo than with 147 gr. Getting ~2" groups with ammo the price per round of the Zmax is IMHO terrible.

Different ammo shoots to different POA/POI, but yours seems a much greater difference than most. Report back if you try a good 168gr "hunting" ammo.

 

[kludge]

7" at 100 yards would have me asking questions too.

First I would look at the scope to see if it moved, but you want back to the previous ammo and it shot to the same POI, so that is out.

Next I would look at the rest. The rifle might be bouncing on the rest. Try a sandbag on the rest.

 

[No4Mk1*]

The actual difference in drop of the 2 loads comes to 0.2" with the 168 grain lower using a ballistic calculator. Any other difference is caused by barrel harmonics, technique, or some other variable at the shooting bench/ scope, etc.

 

[wally]

The actual difference in drop of the 2 loads comes to 0.2" with the 168 grain lower using a ballistic calculator.

Ballistic calculaors make for nice software, but its unsual for different brands of ammo the same bullet weight to shoot that close POA/POI

Without chrono measurements from his rifle for each load the ballistic calculator is just a high tech guestimate of what things might be like in the absence of barrel harmonics etc.

Interesting mystery. I hope he tries some different ammo and gets back to us.

 

[another_luser]

Thank you for the replies.

I went to the range today (adelbridge, the range is in Crestwood, KY) with Winchester 150 grain power point and Remington 150 grain core-lokt, in addition to the American Eagle 150 grain and another box of Zombie Max 168 grain.

The Winchester and American Eagle were both on target; the Remington was five inches low with a good grouping of 1.5 inches (good for me, at least). The 2nd box of Zombie Max was again seven inches low, but it does have the same lot number as the first box. The only place around me that had any other brand of 168 grain was selling them at 39.99 a box, and 2.00 a round is a bit out of my price range for plinking.

I’ll chalk it up to this rifle being very particular about the ammo it likes. I can’t complain, though. At least it likes the cheap stuff.

Thank you again for your replies.

another_luser

 

[BoilerUP]

Shooting at OpenRange, eh?

You might poke around online and order some 168gr and 175gr Federal Gold Metal Match...probably run $20-21/box + shipping.