Corbon 220gr Sub-sonic Velocity & Accuracy

[John3]

I am newto this forum, and just got a new SSK .300 Whisper M16 upper, w/10" barrel and suppressor.

Corbon 220gr sub-sonic is chronographing at 1035fps, high 1050, low 1020, with an SD of 10 fps. Corbon claims 1040 fps, so my results are very close.

This is outstanding considering how small these powder charges are.

This level of velocity variation should add only about 1/2" of error at 100 yards (according to my Oehler Ballistic Explorer Program).

I am not happy with the grouping, however.

I'm getting 4"h x 2"w groups, using a Leupold 20x scope.

The high shots are not related to high mv's, and the low shots are not related to the lower mv's.

Anyone have any thoughts?

[d-mon]

Is your barrel free floating?Is your barrel brand new? Did you go through a proper barrel breaking?
Some time, when I recrown a muzzle, it can take me up to 30 shots before recovering descent accuracy.

ND

[John3]

ND, thanks for your questions.

Barrel is new Shilen 1/8 twist, 10". About 60 rounds through it. Cleaned with Kroil & JB bore paste down to bare metal every shot for first 10, every other for next 5, then every 5 for next 20. I use a Hawkeye bore scope to confirm bare metal.

Still gets 4" h x 2" w groups at 100 yards.

[Artful]

How are you supporting the gun when firing the groups?
using Bipod or machine rest or sand bags or ?

Have you tried any other ammo? - is this ammo out of all the same lot?

Is this with a hot barrel or going from cold to warm - have you tried to keep the barrel as close to cold as possible and group it?

[John3]

Artful, thanks for your insightful questions.

"How are you supporting the gun when firing the groups?
using Bipod or machine rest or sand bags or ?"

I'm using a Wichita rest with rear Protector sand bag, same position on bags every shot.

"Have you tried any other ammo? - is this ammo out of all the same lot?"

I've only tried one lot of Corbon 220gr, but chronographing every shot and it is fairly consistent.

"Is this with a hot barrel or going from cold to warm - have you tried to keep the barrel as close to cold as possible and group it?"

I need to try some of these scenarios - might help to better define the problem. Even with a warm (not hot) barrel, grouping is out of whack.
This is a heavy contour 10" Shilen barrel, very stiff, free-floated. I checked the muzzle crown for damage.

Also, I've tried 2 different scopes, both carefully leveled for each shot.

The odd thing about this problem is the vertical nature of the grouping.

I will probably just send the upper back to SSK for resolution.

[Artful]

Do you see any gaps in the mating of you upper to lower? or other flex area's? I agree 4x2 is larger then expected of a quality barrel on an AR platform. But out of curiosity I would try a box of super sonic's to see if they shoot any better. Have you checked the jump from the loaded overall length of the Corbon to the rifling to see how far it has to jump? or put one in the chamber and removed to see if it's being pushed into the rifling?