Freebore question

[MadDog]

Hello, I have a 300 Whisper upper that I have not had much luck getting to shoot accurately. It is an SSK upper, 10" heavy match grade barrel with a 1 in 8 twist. I have tried a multitude of loads without achieving any decent accuracy. I have tried 240gr 220gr and 190gr SMKs. The 240s and 220s do not stabilize. They are not keyholing completely but seem to hit at about a 45 degree angle. Accuracy is terrible. The 190s mostly stabilize but some will impact the target at an angle. Best group with the 190s was still over two inches at 100 yards. The others were too big to bother measuring.

I have tried faster and slower (I have only shot subsonics in this gun). I have tried 34 different loads plus Corbon and SSK ammo.

I have communicated with J.D. Jones a couple times and tried his suggestions but still having problems.

I have been trying to come up with some explanation of why this upper will not shoot. Recently I purchased a Hornady OAL gauge. I also made a chamber cast.

My question concerns the correct amount of freebore for a chamber. When I used the OAL gauge, the cartridge OAL with the bullets touching the lands was around 2.4". The 190gr SMK was almost out of the case neck.

When I measured the chamber cast, it was .245” from the case mouth to the very beginning of the lands.

Is that normal for a 300 Whisper? If not, could that be causing the problems?

Here are a couple pictures of the chamber cast.

Any input would be appreciated.

Joe

[pud-knocker]

nice chamber cast.....if nothing else you should be able to settle the long running correct brass length debate for the 300 whisper....is it 1.355 or 1.400

I have a 16" SSK AR, it shoots ok, around MOA on a good day....from what I found and through a couple talks with JD, the 1:8 twist is on the edge for the 220s, but if you spin them any faster and tighten up the groups it takes away from the tumbling damage potential....

I found that if I load the ammo longer than mag length and single feed letting them cram about .010 into the lands I got much better groups....but that defeats the use of a semi-auto.....crimping also seemed to help, similar to bottoming out on the lands, seemed to make the pressure build more consistant....

[MadDog]

Originally Posted by pud-knocker

nice chamber cast.....if nothing else you should be able to settle the long running correct brass length debate for the 300 whisper....is it 1.355 or 1.400

I have a 16" SSK AR, it shoots ok, around MOA on a good day....from what I found and through a couple talks with JD, the 1:8 twist is on the edge for the 220s, but if you spin them any faster and tighten up the groups it takes away from the tumbling damage potential....

I found that if I load the ammo longer than mag length and single feed letting them cram about .010 into the lands I got much better groups....but that defeats the use of a semi-auto.....crimping also seemed to help, similar to bottoming out on the lands, seemed to make the pressure build more consistant....

Thanks for the response. I may try your suggestions. Single feeding doesn't seem much fun but it would be interesting to see how seating the bullets into the lands would work.

I have had a similar conversation with JD. I would be pretty happy with MOA on any day at this point. If I can't get it to shoot any better than what it is doing now, I may scrap the barrel and try something else.

[Alleycat]

Is this a new or used upper? Have you chronographed the loads? What was the velocity? What does the crown look like?

[MadDog]

Originally Posted by Alleycat

Is this a new or used upper? Have you chronographed the loads? What was the velocity? What does the crown look like?

I bought it new about a year ago. I initially tried for loads in the 1050 fps range. When that didn't work, I tried working up and down with no better results.

The crown looks fine. I have tried to think of any reason why this upper will not shoot. I have talked to JD Jones a couple times and to be honest, he was not a lot of help.

I was hoping that the chamber cast and OAL gauge would give some answers. I am not sure if it has. It seems a pretty long jump to the lands but maybe not all that unusual. If loaded to mag length, they would have to jump almost .15"

I keep thinking that it could be something in my reloading process. I am no expert at reloading but have loaded 308, 280AI, 45LC, and 45-70 with reasonable accuracy. Besides, the corbon and SSK loaded ammo didn't do any better.

I really had hoped to be able to shoot 240gr subsonics exclusively with this upper. If it will not do that I will try something else. It seems from reading here and other places, that people are able to achieve reasonable accuracy from short barreled AR uppers with 1 in 8 twist. I'm not sure why this one will not.

Any thoughts on it?