New 10" barrel, bullet stability problems

[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.

[Alleycat]

Are you shooting it with the threads bare? If you have a thread protector, shoot 3 shots with it on tight and 3 shots with it off. It's important that it's a protector and not a brake or something else that will put downward pressure on the threads. Just a little experiment.

[amafrank]

I don't think the thread protector will make any difference at all. I've fired a number of threaded rifles with no protector or suppressor and never seen any difference between doing that and having something threaded on. I've never seen any wobblies caused by having no thread protector or can either. I have seen some problems when the crown isn't square to the bore or even though.

Frank

[snipecatcher]

I don't have a thread protector. Like I said, when I got the first barrel (8 twist), I shot with a Noveske pig because I didn't know if the rifle would function. I then took it off and got the same results. I then put on the 7 twist barrel and have been using it ever since. I have not used any type of muzzle device on this barrel. I may take "the pig" with me to the range next time with the 'blast baffle' removed just to see if there is any difference. Anybody want to try the 8 twist barrel and see if you can do any better? I don't have a gas block for it, but I'd be willing to send it to one of the members on here who has a good bit of experience with this type of thing to experiment with if I can't get something to work with the 1680 powder.
-Dan

[rumlover]

did you ever borescope the barrel? Perhaps there's a burr somewhere, a warped barrel, or manufacturing mistake at the gas port. Your loads should all be working fine.

[N310toN170]

Any chance of getting a couple < 150 Grains loaded?

I usually load up a bunch of 150 FMJ BT's with cannelure and lightly crimp into the cannelure. This allows the brass to be fully fire formed at the shoulder as opposed to the soft squeeze put on by the 300/221 die when converting .223 brass. While the brass is being formed by higher pressure loads, I've seen some really strange behavior.

Once you're using fire formed brass that has been full length sized (and optimally checked for neck thickness), if your barrel won't stabilize 110-150 hypersonic then you know its a manufacturing issue that may or may not get better with use. If they do stabilize try progressively increasing bullet weight, while decreasing powder charge.