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Posted: 7/3/2013 8:26:44 PM EDT
| can a 1/7 twist barrel cause 55 grain fmj 223 to keyhole? |
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First, what ammo are you using. Second, is the muzzle clean (or cleanable)? Keyholing due to the barrel happens more due to erosion or damage at the muzzle than at the chamber, but typically throat and muzzle erosion are at essentially the same level through the life of the barrel unless something happens to either one other than just wear from firing. Undersized bullets, poorly monitored loading, and bullet damage before loading can also cause keyholing. And for that matter, shooting through grass and weeds can cause 55s to be destabilized.
I am probably not alone in putting quite a bit of faith in a fairly new Sig barrel, whether it's a plain M400 barrel or one of their fancier 516s'. With 1500 rounds through it, some people would state that your rifle is just getting properly broken in; I seriously doubt that your barrel is worn to the point that it is really contributing to a keyholing issue. But crud built up at the muzzle CAN cause problems and "inexpensive" ammunition that leaves a lot of crud can cause build up in a relatively short period of time. |
| Are you sure it is keyholing? Some people confuse keyholes with the paper ripping a certain way because it is not flat against the backstop. And if it is truly keyholing, I would think all, or a vast majority, of your rounds would have done so. Not 2 out of 100. Especially if it was hardware related. |
| Try different ammo first 1500 rounds is not enough to cause wear that would contribute to key hoping. Shooting heavy bullets in a 1-12 twist will cause this but the reverse is not true. Improper cleaning or other abuse that results in a chipped crown can cause this as well. The other question is what sort of groups are you shooting? Any thing more than 2 inches at 50 yards would cause me some concern( again confirming with more than one ammo brand) |
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Quoted: Yes a 1/7 barrel can have a 55gr bullet tumble, but there are too many variables to list. Are you shooting steel cased ammo? Wasn't there a run of that a while back where the bullets were undersized and didn't stabilize in anything? Please educate me on a scenario where it is the 1:7 twist that actually causes the keyhole? And not carbon fouling at the muzzle or throat erosion? Edit to state I was talking about with the 55gr ammo mentioned at 50yds.
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| I had 4-5 keyholes out of the last 1000rd lot of silver bear 62 gr JHP that I shot. It happened out of 3 or 4 different uppers, including at least 2 1/7 twist barrels and 1 1/9. Definitely the ammo. Given that it was Russian I assumed it may have been 5.45 bullets that found their way into .223 cases. |
| Im shooting pmc bronze 55 grain. never ever will i shoot a steel case through my ar. i take very good care of my rifle and i never let it really reach high enough temperatures to cause major throat erosion. the paper had a perfect bullet shape hole in at the point of aim. did a little research online and so far as i can tell the 1/7 twist seems to be the culprit or maybe i came across a bullet that was slightly off center/ |
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Quoted:
Quoted:
Yes a 1/7 barrel can have a 55gr bullet tumble, but there are too many variables to list. Are you shooting steel cased ammo? Wasn't there a run of that a while back where the bullets were undersized and didn't stabilize in anything? Please educate me on a scenario where it is the 1:7 twist that actually causes the keyhole? And not carbon fouling at the muzzle or throat erosion? Edit to state I was talking about with the 55gr ammo mentioned at 50yds. Mechanical damage like: Flash hider or compensator not in line with bore and bullets striking/rubbing on it when fired. Flash hider tightened too much and threads stretched. Pinned on flash hider pin beat into hole causing dimple of barrel to be pushed into bore. Pinned on flash hider attempted to be forced off causing pin to displace material into bore. Gas block set screw holes drilled too deep and screws over torqued damaging bore. Crown damaged by dinging with steel cleaning rod causing jacket to be deformed when bullet leaves barrel. Or have an under sized bullet like silver bear or whoever did and it tumbles out of everything it is shot out of. ETA- It has been noted that a clean/undamaged 1/7 barrel will not have any problems stabilizing a 55gr projectile. Since the OP's barrel is undamaged the final finding was the bullets were being skipped into the target. |
| I did recently remove the flash hider to clean the crown of the barrel but when i tightened it i just held the barrel with my hand and used an armorors tool to tighten the flash hider..could it be over torqued like that just by hand and holding the barrel with the other hand. |
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Quoted:
I did recently remove the flash hider to clean the crown of the barrel but when i tightened it i just held the barrel with my hand and used an armorors tool to tighten the flash hider..could it be over torqued like that just by hand and holding the barrel with the other hand. You don't over tighten, you will be fine, but as mentioned you do not need to remove the flash hider to clean. The over torque is for the guys that go two revolutions on a crush washer and put a ring in their barrel. |
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Quoted: Quoted: I did recently remove the flash hider to clean the crown of the barrel but when i tightened it i just held the barrel with my hand and used an armorors tool to tighten the flash hider..could it be over torqued like that just by hand and holding the barrel with the other hand. You don't over tighten, you will be fine, but as mentioned you do not need to remove the flash hider to clean. The over torque is for the guys that go two revolutions on a crush washer and put a ring in their barrel. If you can get to the carbon build up at the back of the flash hider at the crown of the barrel the flash hider need not be removed. I have seen some rifles(first hand) where the flash hider contained the carbon build up deforming the crown and the only way to clean it thoroughly was to remove the flash hider. Not a normal course of maintenance but if the barrel and crown are not routinely cleaned and allowed to build up you may need to pull the flash hider to restore accuracy if this is the source of your problem. |
| I have had PMC 55gr bronze ammo hit the target sideways inside of 50 yards out of my 1/7 CMMG. That same rifle will shoot other 55gr fine and shoots 62-75 gr fine. I have another 1/7 CMMG that shoots 55gr PMC just fine. My other 1/7's shoot it fine. It just happens with that one 1/7 CMMG and only with 55gr PMC ammo. |
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