300blk accuracy issues?

I have a 300blk rifle (just one, so unfortunately can't compare to a different gun). The BEST groups I can get it to shoot right now is about 4 MOA at 100yards. More vertical stringing than horizontal, normally.

Rifle specs:
Factory AAC 12.5" barrel/extension, gas block, and BCG
Upper assembled by a professional gunsmith.

Ammo (I HOPE the issue is here, but certainly not sure):
-Hornady 150 FMJ
-SMK 125gr
-Barnes 120gr TAC-TX
-Barnes 110gr TAC-TX

All loads so far have been done with various charges of H110, and factory new, primed Remington brass. Since it was already primed, I just took it out of the box, loaded, and shot it. I bought 500 pieces of this brass and am not quite through them yet.

Scope on the rifle is a Nikon P300blk in a Larue LT140.
I think I can feel a very..... very..... VERY minor bit of wiggle between the scope and the front "ring" on the mount. Just throwing that out there as well, though even if present, I doubt it would lead to groups this big.

I'm sure my hold isn't perfect, so chalk up about an inch to that if you want.

Like I said, the BEST groups I can get with the Barnes are about 4". The other bullets are bigger.

What should I be checking? I have some new Hornady brass as well, but haven't tried it yet. I don't know anything about the primers that are in the rounds I'm using so far (other than that they have all gone bang).

It just seems like I should be getting better accuracy off a bench, with a scope, and with good bullets... what do you think?

243winxb said:

Needs fixing?

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I suspect it is so minor that it is not a major contributor to whatever is going on, but, I suppose I can't rule it out at this time.

So no one thinks the cases not being sized could be an issue?

I'll have to figure out some kind of shim for the front ring in order to get it super tight I guess.

I've run them up to the velocity specs stated in the factory loads; no apparent changes in accuracy across the velocity range that I could see.

Thanks for the input all. I will figure out a way to tighten the scope and try again.

Yes that is the style of mount I have. I just basically took a bigger wrench and torque the crap out of the bolts on the Rings more than I could before. I don't think I can feel any movement now. I'm about to head off to the range and try again.

Ok, as far as I can tell the scope is now completely locked down. I think before, like someone said, it might have been moving 1/1000th or something.

Unfortunately I can't say I see a difference on paper so far. All the shooting here was done with the 110gr TAC-TX and 19.06gr H110. Typical 100 yard groups:

The one on the left is a 5 round group with one somewhere off the paper. Changed he POA a little lower for the one on the right.


Unlike before I did some 200 yard shooting this time, which actually resulted in something I wasn't expecting; if anything I think all of this 200 yard performance looks slightly better than it does at 100:

200yd Group 1:



200yd Group 2... uh... I'll chalk this up as some anomaly:



200yd Group 3:



I guess I'll try some SMKs with the scope tighter but I don't have any reason to expect success right now.

"Wiggle" was not the right word. It had barely detectable movement if I pressed it side to side against the front of the objective bell. Now it doesn't.

I will move the scope and mount combination to a rifle I know shoots better and see if I can isolate it as a cause or not.

Why have a 12.5" barreled gun (is his a pistol, or SBR?) with a scope on it? That's a gun for tactical, fast-pointing application, needs a red-dot or some sort of wide aperture, quick targeting sighting device. I'd be very happy hitting body-mass size targets at 100 yards with that rifle, rather than attempting to cut 50-cent piece size groups.

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I'm no rifle shooting expert, but always heard that if anything, short barrels are more accurate due to less flexibility... is that no longer a thing?

My 5.56 SBRs don't seem to have a problem demolishing the level of accuracy displayed above. Yes, they are short range guns, but I still like to see what I can actually make them do in terms of accuracy before putting a reflex type sight on them.

LoonWulf said:

If you have other scopes and mounts, swap the whole rig and try again, you shouldnt have to crank on a mount to get it tight on a scope dont discard scope/mount as an issue yet. Also even if you had a good smith assemble the parts, there maybe a fitment issue that he didnt notice, or was beyond the scope of a simple upper assembly. Check thru it again just to be sure.

Ive had issues with accuracy in my blackouts related to ammo also. I run h110 as well, and it needs to be right at max for me to get decent groups.
The second thing ive found is that blackouts are usually throated for LONG bullets, even 150 pointed are usually a way off the lands. Its my personal opinion that this means that cases and bullets MUST be square. Id honestly talk to Bart about case prep for concentricity. ive followed some of his replys in threads and used some of the advice to good results in my blackout. Lastly if nothing seems to work try the heavier bullet weights, ive got some 208s 220s and 245s i could share some of if youd like.

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Thanks... I've been loading for max length of about 2.25 so they will fit in any magazine, but could potentially go a little longer for Pmags only. The "long jump if you are loading for AR15" seems to be a recurring theme, but probably not much you can do about that if your mag length and barrel throat are what they are? I have no experience with trying to adjust runout, etc. But sounds like it could potentially help.