Is it me?

[rab]

Any of you technically savy types can chime in....

I changed my ammo from PMC Bronze 55gr for something better. I now shoot mil surplus FMJSCBT 62gr LC "Grn Tip". They seem to fly tighter. I zeroed for 100. Good tight groups. However, my 50yd impact is 6-8" low? Is there that much difference or is it me? Does any one have or know of the trajectory table for CAR/M15 using 62gr ? I heard that the FMJSCBT stuff will fly very different but that much and at that short a distance?

Stag Arms Mdl 1
16" Chrome lined
EoTech 516 w/5XMako

I shoot bench, standing, kneeling and prone with pretty much the same results. I didn't notice with my old ammo. I may have improved some since last year....
I would have shot both distances with the irons, but I can't see quite that small taht far.
thank you ahead of time......

 

[Skyyr]

Both of your questions/experiences are quite common.

For one, depending on your barrel twist-rate, different weights of bullets will behave differently. Some barrels like lighter rounds, some like heavier rounds. Yours seems to like heavier. I'd keep trying different weights until you get your best groups - that will be the weight your barrel "prefers." As a rule of thumb, the faster the twist-rate, the heavier of a round you want.

As for your low groups, let's go back to physics. Heavier rounds of the same caliber/load have more mass, so they don't accelerate nearly as fast as lighter loads. Because of this, they're slower. Remember that gravity pulls everything down with equal force. Because of this, heavier rounds drop more over distance than lighter rounds because they're traveling slower horizontally, but "falling" at the same rate as the lighter round. This is why your groups are much lower at 50yds. Just re-zero your rifle using your new ammo. After doing so, if you go back to the lighter rounds, you'll notice they'll be high at 50yds.

The advantage to using heavier rounds is that even though they're slower and drop more over horizontal distance, they keep their energy longer because of their extra mass. This means that, out past 400-500'ish yards where the lighter loads would run out of steam due to air resistance and arc into the ground, the heavier rounds will have a "flatter" trajectory. They're also less susceptible wind and don't get blown off course as easily.

Hope that answers your questions!

 

[Jimro]

6-8 inches low at 50? It's you.

There is no way m885 ammo can be dead on at 100 and that low at 50 unless you mounted your optics about a foot above the bore.

So it has to be you.

Jimro

 

[Skyyr]

6-8 inches low at 50? It's you.

There is no way m885 ammo can be dead on at 100 and that low at 50 unless you mounted your optics about a foot above the bore.

So it has to be you.

Jimro

That seems about right (maybe a BIT low) if he zero'd at 100.

Instead of a 100 yd zero, use a 25yd or 50yd. Those are the standard military sight-in distances and the most versatile.

 

[olyinaz]

The point of impact at 50 yards should not have changed 6-8" if you're still zeroed at 100 yards, and if it truly did (I'm skeptical) then the only thing I can think of is that you've got some sort of weird barrel harmonic going on.

If you really want to see what your rifle can do buy some Hornady and Federal Premium ammo for it (various bullet weights) and see which one takes the gold. I bet you'll be surprised when you find the "sweet load" for your rifle.

Oly

 

[Jimro]

olyinaz,

Barrel harmonics cannot explain how a fast moving cartridge like a 223 could go from 0 inches at the muzzle, to negative six inches at 50, and back up to 0 inches at 100.

Barrel harmonics only affect how the bullet leaves the muzzle, not how the bullet acts in flight.

Jimro