300 Blackout Accuracy Problem

[Beckett]

I recently aquired a 300 Blackout upper from CMMG (16 inch barrel, 1:8 twist) and ordered a bunch of the Remington 220 grain subsonic Ammo. Took it to the range, sighted it in and am getting 1 inch groups at 50 yards and 4 - 5 inch groups at 100 yards. I have put about 40 rounds down the barrel.

Im trying to figure out if this is normal or not. Should I try other ammo, be patient till the barrel is broken in better or what? Anybody have similar problems or any suggestions?

[snipecatcher]

Post up a picture of the bullet holes at 100 yards.

[Beckett]

I wish I could but I didn't keep the target. Didn't think about it.

[ds762]

I'd try different ammo .. if that doesn't solve the problem you have a bad barrel.

Did you contact CMMG to see what they say about it?

[LouBoyd]

What is the elevation (above sea level) and the approximate temperature where you were getting 4-5 inch groups? Was it windy? What roughly were the shape of the groups?
Strung vertically? Strung horizonatally? random? Single flyers?

This is where a chrongraph would be handy.

Cold can give higher air density and reduce stabilty (unlikely this time of year but your profile doesn't show where you live).

High temperature can give higher chamber pressure and velocity driving the bullets into the transonic region causing instabilty and (usuallly) random point of impact errors which may increase more or less than linearly with distance. .

Launch angle error will generally be linear with distance (like a loose scope mount).

Wind will typically give horizontal stringing more than linear but not with the square of the distance.

Velocity variation will usually cause vertical stringing with the square of the distance.

Some people keep targets with small groups. All those tell you is that you're doing everything right. A useful test target will have 5 to 10 shots each marked with the time it was shot and its chrongraphed velocity. Also record the gun, cartridge, bullet model, elevation, temperature, and air density (or barometric pressure and humidty). Those are worth keeping.

[Beckett]

I obviously need to shoot another group to gather more information. (In Dallas, TX by the way)

We were shooting 1/2 dozen guns, letting barrels cool every 3 shots and wind was minimal. Groups on the other 5 weapons at 100 yds were excellent and the 300 Blackout was all over the place. I don't believe temperature, wind or humidity was the problem because that didn't affect the accuracy of the other 5 guns.

I am gathering from the comments that accuracy should be better and I either have equipment malfunction or operator error. Just trying to find out if there was something simple I was doing wrong like using a "known" poor performing ammunition or not breaking in my gun correctly.

Thanks for all the help and suggestions! I'll try again and see what happens.

[Alleycat]

It’s been reported by some that the subsonic ammo from Remington is all super or some rounds are super in some guns. This will cause all kinds of havoc with groups.

[LouBoyd]

Originally Posted by Alleycat

It’s been reported by some that the subsonic ammo from Remington is all super or some rounds are super in some guns. This will cause all kinds of havoc with groups.

That is a potential and likely problem with any ":factory" subsonic ammo. The stability of subsonic bullets is highest between about 1000 and 1025 fps in "normal" conditions.l It's worst in the 1100-1150 fps region. The air flow around a bullet is complex near the speed of sound. There is no other range of velocity where the drag varies so greatly .

Shooting in hot weather may raise the bullet velocity into the transonic range, particularly if the rifle was laying with direct sun shining on a black magazine while other guns were shot. The ammo could have been well over 100F when fired. Just about all powders have a positive pressure vs temperature coefficient.



The G7 drag model (blue line near the bottom is the closest to low drag boattail bullets like the 220 Sierra matchking used in the 300 BLK subsonics. Notice how sharply the drag changes for a small change in velocity between 1000 and 1100 fsp. This is not a chart of stability, but stability is related to the ratio of rotational inertia to the overturning effect of drag.

As an experiment (if you don't have a chronograph available), put some ammo in a zip lock bag in an ice chest, then shoot some groups with the ammo cold and see if the groups improve. If so the cure is to handload or don't let your ammo get hot.

[rsilvers]

Actually we shot the Sierra 220 on Doppler radar and the bullet was closest to a sphere in drag. G1 was second, then G8, then G7. So G7 is one of the worst drag profiles for the 220 when subsonic. Still, G7 was a close match - just not a good as G1.

[Rancid Coolaid]

What optic is on the rifle?

I have a friend with a 16" 300WTF and he has seen not stellar results with his 220-gr factory subsonics (which have, for reference, all been subsonic - in Houston.) I have shot 2 boxes of the factory sub and it is nowhere near as accurate as my rifle's favorite load (8.0 of lil gun over a 208 amax.)

I won't go so far as to say the Remington factory subs suck, but they ain't good - and that has been true of the 2 300s I have shot it through.

Groups will (sometimes) open up if you optic is too low power or too high power, which is why I ask about the optic.

And were groups from bench, prone, or off-hand?

Were the rounds evenly distributed in the groups or were they stringing? Did they string vertically or horizontally?

Lastly, you mention accuracy of the other guns was good, and I bet all 5 others were supersonic. Be aware that small changes - in anything - shooting a subsonic round equal big changes in point of impact. It ain't a fair fight to put a 308 next to a 300Whisper and compare accuracy. Once the 300's favorite loads are established (and how did you break in your barrel?), results should improve.