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?

[Cornholio]

Are you using the SSK suppressor?

Does your barrel have the two ports near the muzzle?

Do you have a .30 cal bore rod to run down your upper with suppressor attached to check for clearance?

What is the diameter of the suppressor's aperture?

[Artful]

Can you use your bore scope to check for baffle strikes? or build up of materials? does the fowling on the end of the suppressor look uniform?
Any loose baffles? or loosness between suppressor and barrel (ie does it change position when you are shooting?

Have you used this lower with other uppers and gotten good accuracy from it? Have you tried this upper without the suppressor on it?

[John3]

No suppressor - still waiting for my Form 4 approval.

Barrel has no ports - JD Jones said it is more accurate that way ?!?

I don't have a Stoney Point OAL gauge for 300 Whisper, and I haven't yet measured bullet run-out, etc. I'm just going to send this thing back to SSK and let them trouble shoot it.

3 of my buddies bought Whisper uppers with me, and we will mix/match try them out this weekend, including super-sonic ammo, hopefully.

Mine does run fine on full auto using the factory Colt 653 buffer (light weight) and spring. I had SSK modify 14 magazines, 20's and 30's, and they all seem to run just fine.

[Artful]

We await the results of you group shoot, hopefully by comparison it will show what is going on here.

So you did a group buy of uppers - can one ask what the price came out too?
I have to put mine together and am interested in which way to go (buy parts and assemble or purchase ready made.

[John3]

We did a group buy of 4 SSK uppers and suppressors, through a local Class III dealer. The MSRP price for the upper is $1,100, and the suppressor $635 (extra $35 for the Black finish). The discount my dealer got for this order was very small, maybe 20%. It was about enough to cover his paperwork time, shipping and sales tax, and we paid him the SSK MSRP as a net delivered price (including the local 8.25% sales tax). The dealer might have made $50 or $100 on each set, but that's fine with me, since I want to keep him happy, too. Not many of those guys around.

My thinking on paying the big bucks for the SSK upper was simply to avoid problems and save time getting it up and running. I don't mind paying more for someone else's trial & error to determine the correct gas port size and placement, feed ramp configuration, etc.

I also wanted a minimum length suppressed upper for my M16, so that implied the 10" barrel with 5" of the 12" suppressor overlapping the barrel, for a net suppressed barrel length of 17" or so. This is because the 300 Whisper in sub-sonic reaches its optimal velocity at 10" or so, and any extra barrel length is not only wasted, but detrimental to accuracy, because of the slow bullet taking extra time in the longer barrel. I am restricted to using the upper on my M16 lower (or I could have registered a lower as an SBR). I don't mind the extra SSK suppressor weight, since I will use this for full auto fire, and my experience with suppressors is that size = quietness, and weight = robustness.

Also, hopefully, JD Jones will stand behind the product and fix any problems. He has personally answered every one of my email questions, although sometimes a bit tersely. So far, he and his people have been reasonably responsive, although it is amazing to me that they don't take the time to post a FAQ with the top twenty or so Whisper questions on their web site. I must have been the 100th person to call and ask "how much for Black finish", "how much does the suppressor overlap the barrel",or, "what alternatives work for rail tubes", for example.

In fact, I will post the info I got from them. Might save someone else some time and hassle.

[Cornholio]

I also wanted a minimum length suppressed upper for my M16, so that implied the 10" barrel with 5" of the 12" suppressor overlapping the barrel, for a net suppressed barrel length of 17" or so. This is because the 300 Whisper in sub-sonic reaches its max velocity at 10" or so, and any extra barrel length is not only wasted, but detrimental to accuracy, because of the slow bullet dwell time in the longer barrel.

I hope this isn't true. I commissioned a rebarrel job on my Rem 700 to Whisper with a 1 in 8" Douglas with a barrel length of 22" past the recoil lug.

I have a small .30 cal suppressor which seems to work best with lower pressure gas. So rather than use a short barrel (which would probably work well with a large volume suppressor that telescopes over the barrel), I figured on using a longer barrel to lessen the gas pressure hitting the suppressor. I'm willing to deal with the added weight and bulk if it means quieter performance. But I'm not sure if I'm willing to accept diminished accuracy/precision...

[John3]

JD Jones told me there is no difference in accuracy, velocity or reliabilty in 300 Whisper Corbon 220gr sub-sonic between 10" and 16" barrels, and that the velocity is about 40 fps higher in 14” barrel. I guess this implies that 14" is the end of the powder burning cycle, and that the round starts to slow back down due to barrel friction after that. He also stated that bullet T-I-B (Time-In-Barrel) was not an issue with any barrel length for 300 Whisper in AR15 uppers, but that T-I-B is an issue with all .510 Whispers and with .338 and .300 in light rifles (i.e. Contender). These must be "very firmly and consistently gripped" to repeat POI.

No mention of any suppressor considerations. He did state that there are 2 schools of suppressor theory: Low Pressure/High Volume and High Pressure/Low Volume, and that the LP/HV design works best for the Whispers.

In spite of JD's claim ("T-I-B is not an issue for any length AR15 barrel"), I know from personal experience that slow bullets are very sensitive to any slight barrel movement during firing, including any recoil-induced movement. You may not see this effect from a solid bench rest, but holding off-hand, sitting, leaning against a tree, etc. will cause the POI to change in seemingly random ways. This has been consistently my experience with slow pistol calibers (357 Mag, 44 Mag, 45 Colt) and sub-sonic .308 rounds in 16" or longer rifle length barrels. Because of this, I opted for the shortest possible barrel (10").

[Artful]

[QUOTE=John3]JD Jones told me there is no difference in accuracy, velocity or reliabilty in 300 Whisper Corbon 220gr sub-sonic between 10" and 16" barrels, and that the velocity is about 40 fps higher in 14” barrel.
In spite of JD's claim ("T-I-B is not an issue for any length AR15 barrel"), I know from personal experience that slow bullets are very sensitive to any slight barrel movement during firing, including any recoil-induced movement./QUOTE]

I'll second that from experiance with pistol shooting - follow through is very important with slower velocity firearms (pistol, muzzle loaders etc).

While tha crosshair settled on target and the cartridge was setoff and the gun is in recoil on a 22-250 or .223 the bullet is out the barrel on slower guns it's still in the barrel and if you don't hold it on target (follow through) you'll have lots of near misses (some not so near! :rolleyes:)

For myself I'll go the 16" to say the paperwork and expense :p

Anxiously awaiting shoot comparison with you fellow Whisper shooters.

[Cornholio]

I figured that with a long, heavy barrel and the super-light recoil of a subsonic Whisper, especially with a suppressor, that there wouldn't be much recoil-induced movement.

[Artful]

3 of my buddies bought Whisper uppers with me, and we will mix/match try them out this weekend, including super-sonic ammo, hopefully.


John did you ever get together and see if it's your upper or something else? :confused:

[John3]

I only got to test one other upper from our order, and it was shooting 4"+ groups, too. I sent mine back to SSK on August 22nd, and am waiting for resolution. I'll post here when resolved.

[John3]

My SSK .300 Whisper 10" bbl upper is now shooting <1.5" groups consistently at 100 yards, with Corbon 220 gr. factory ammo, and the SSK suppressor installed. SSK replaced the barrel, but it still wasn't shooting well without the suppressor.

Here's what I learned:


1. I get 4-8" groups without the SSK suppressor, off a benchrest. I'm thinking there is some kind of perturbation going on with the gas system, when the heavy suppressor is not there to calm things down.

2. I get better groups (3"?) without the suppressor with the gas turned off, but they are not consistent.

3. It takes about 30 shots to "condition" my clean barrel for best accuracy. Why? No idea, but JD Jones insists this is necessary, and it works. I now just leave my upper dirty, although I don't feel good about it. Thank goodness the barrel is stainless.

4. This combo (Corbon sub-sonic with suppressor) is all I really care about anyway, so I am pretty happy.

5. Except . . . now that I have the damn thing shooting right, no one has the Corbon ammo for sale. This gun has become the most popular one with my guests for mid-day plinking at the deer camp. It is a real kick, and very quiet, but a case of ammo doesn't last very long, especially with full auto bursts.

Next step is to get back to my Leupold SPR scope tests, now that I have this baby up and running right. I'll do this as soon as I can find some more ammo, and post results under the "Which Scope is Best?" topic. So far, it is dead on at the 100 and 200 yard aiming points, but I haven't tried the 300 and 400 yard aiming points yet.