New 10" barrel, bullet stability problems

[snipecatcher]

It's threaded 1/2x28, the same as the Shilen barrel I had before it. The first time out, I shot it with a Noveske pig to ensure it would cycle, but I took it off after that to eliminate it as a variable. I went to the range again today and shot at 50 yards for the first time. I shot at a posterboard with no backer. Same thing. Still getting off center holes.
-Dan

[snipecatcher]

Well I've finally found a bullet that will stabilize at subsonic velocities: the 110 V-Max. I loaded some from 1400 down to 950 fps and all are making round holes. They have a calculated stability factor of 5.2 at 1000 fps. I also loaded some 175 matchkings in the 1400 fps range. These also made round holes. They have a stability factor of 4.2 at 1400 fps. It appears that I need a calculated stability factor of at least 4 in order to shoot the 300 BLK at subsonic velocities. As of right now, things aren't looking good.
-Dan

[Hoser]

Im thinking it isnt an 8 twist or maybe something along those lines...

Do you know anyone with a borescope? Might be a good idea to take a look in there and check things out.

[TCCrewchief76]

Originally Posted by Hoser

Im thinking it isnt an 8 twist or maybe something along those lines...

Do you know anyone with a borescope? Might be a good idea to take a look in there and check things out.

I was just gonna recommend that. Take a cleaning rod with a line marked down the center line. Mark the rod just as it starts to turn, and when the centerline rotates back to the top again, remark. Measure the distance between the marks, and this is your twist rate.

Kevin

[Titleiiredneck]

Originally Posted by snipecatcher

It's threaded 1/2x28, the same as the Shilen barrel I had before it. -Dan

Quick question, ya think it could be because the muzzle walls are so thin with the .308 bore vs the 1/2 od? Why I ask is had having a simmilar problem with one of my sbr uppers in 9mm. Thin walls can cause warpage when threaded, in which case it may cause the bullet to wobble when leaving the muzzle, just a thought.

[snipecatcher]

Interesting. That thought has crossed my mind too. I know 1/2x28 generally isn't recommended for a 30 caliber bore, but people still do it, and even on 9mm. The problem with that is that my 30 caliber RIFLE suppressor is threaded 1/2x28. That's just what the manufacturer uses so it can be put on a standard AR. It does bother me how thin it is, but I've been told it is fine by the suppressor manufacturer. As for the twist rate, the barrel maker measured it down to the 1/4", so I trust his word that it is a 7 twist.
-Dan

[amafrank]

Originally Posted by Titleiiredneck

Quick question, ya think it could be because the muzzle walls are so thin with the .308 bore vs the 1/2 od? Why I ask is had having a simmilar problem with one of my sbr uppers in 9mm. Thin walls can cause warpage when threaded, in which case it may cause the bullet to wobble when leaving the muzzle, just a thought.

I don't think the thread diameter and pitch are causing the issue. This thread is in use on many other .308's with no problems at all. 9MM is a larger bore and is getting much closer to the thread root so in a high pressure rifle I could see a possible issue there but not in the .308. In reality the 1/2-28 works very well in 9mm for the most part so the smaller .30 cal should have no issues whatsoever. I think there is either a problem with the barrel or the twist rate and the simplest solution is to do as others have noted and check the twist. If he's down to 110gr bullets to get a straight hole than something is seriously wrong....its not just a muzzle thread issue.


Frank

[snipecatcher]

I did try measuring the twist rate, but it is pretty tough to do when you only have about 7" of rifling to work with. I ended up with something close to 9", but I'm not sure if the measurement I got is accurate.
-Dan

[TCCrewchief76]

Originally Posted by snipecatcher

I did try measuring the twist rate, but it is pretty tough to do when you only have about 7" of rifling to work with. I ended up with something close to 9", but I'm not sure if the measurement I got is accurate.
-Dan

OK, I'm confused. How is there only 7" of rifling in a 10" barrel? Maybe this is your problem... Just kidding! Perhaps mark the cleaning rod on the top and bottom, and double the twist rate when you measure the rod at half-rotation.

Kevin

[snipecatcher]

Good idea. I did it that way, five times in a row, and all 5 measurements were really close. I took the average of those and ended up with about 8". Still, there has got to be something else. Even a 9 twist should stabilize a 175 Matchking at 1000 fps.