Different areas of grouping w/different ammo?

[Patty]

Anybody know why my STG58A places groups in different areas of the target with different ammo? German may be a nice tight group in the center, Radway places them 3-4" low and left. All group well for surplus, but in different places on the paper. Any ideas? Thanks.

[Talyn]

Patty,

Germanic FALs prefer german ammo thus your StG groups better with the German ammo. If you had a L1A1 then RG would group better since they're both English. Just kidding! :) .

RG might not be as hot as the German ammo thereby grouping different. I had some German MAN ammo once and it shot alot better from my FALs than RG does now. My opinions of course.

Talyn

[Upside Down @ 100 MPH]

It is because of barrel harmonics. The muzzle of the barrel oscillates when the gun is fired. Two loads with different velocities will impact the target differently because the muzzle is in a different place respectively for each load when the bullet exits the muzzle, changing point of impact. The severity of the POI change between loads depends on how much the barrel "whips". Lighter barrels do it more because they flex more.

[Blag]

An added note is that the speed of sound in steel is around 16,000 feet per second, which is about 6 times .308 muzzle velocity. There's no way a bullet can outrun the barrel vibration from the noise of a shot.

Here's more about barrel vibration than a normal person would want to know -

<A HREF = "http://www.vnij.com/books/appstudies/rifle/rifle.html">Simulation of the Vibrational Response of a Rifle Barrel During Firing</A>

[Upside Down @ 100 MPH]

A warning.....pop a couple of aspirin before trying to read Blag's link. Man is it technical, I got a headache from trying to understand the math portion. :eek:

Good reading though.

[farranger]

Radway shoots lower in all my and my friends guns, I think it might have lower velocity, anyone know if this is true?

[Dirtfarmer]

Hooo- How about this one...search through here, and you'll see that Radway-Green typically chronos HIGHER than other surplus.
(Search velocity, maybe...)
Most notice that it seems to require more gas to function than other, but that seems to be to a different pressure curve, (which impacts the perceived recoil due to impulse-time as well..)
However, higher velocity loads remain in the barrel for a shorter time, so if the bullet weight are the same, the faster ones should impact lower.
It is against conventional wisdom, I know.
Goodwill,
-Dirtfarmer