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Posted: 11/24/2004 9:16:31 AM EDT
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I recently received a match grade stainless steel barrel and started the process of breaking it in. I won't go into the barrel manufacturer at this point as it is probably not pertinent to my question. Upon inital cleaning, I noted that the feel of the gas port as the patch and jag passed by it was a little more pronounced then my chrome lined barrels. Also, the patch and jag would often hangup on the gas port as I was pulling the patch back through the barrel stopping the cleaning rod. I assumed that this was due to the unlined barrel. After 2-3 break in rounds this past weekend, I decided to look down the muzzle to see if I could tell why the port felt rough. Everything looked fine except that the port wasn't top dead center (TDC). It's roughly 15-20degrees left of TDC and I can't tell if it was drilled on the proper angle in relation to the center of the barrel or drilled at 0 degrees in relation to the barrel extension just left of center. The barrel came with a low profile gas block installed. The block is TDC and the rifle appears to function properly (I only fired 3 rounds as I froze my ass off). So, I'm just wondering if I can expect any other problems or complications being setup this way? Thanks! |
| I would think that if the gas port was drilled 20 degrees of of TDC, the rifle would not function. Now if it were drilled off center of the bore centerline, the rifle may function, but I would expect problems. I have a barrel where the gas port is off no more that 5 degrees and the upper has functioning problems (this is one of my experimental/project uppers). Without seeing exactly what you are seeing inside the bore, it is very hard to say whether there is a problem or not. Other than a burr from the drilling, as long as it is functioning OK, I'd continue to shoot it and monitor it. Pay attention to see if the burr goes away after a few more rounds. |
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I have an unlined chromoly barrel that has a definite "snag" when passing jag&patch through bore. After years of shooting benchrest, this wrinkled my brow. I have swabbed bores on a few AR's other than mine and they all, regardless of barrel steel or lining, have some degree of "snag". I am really looking forward to your reports on functioning, after it warms up for you. We got 8" of snow last night here in KC. |
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Mongo, I thought the same thing. If the port doesn't line up with the gas block, then the rifle won't cycle. But, you bring up a good point. Since it does function (at least 3 single shots), I would tend to believe that it was drilled at the right angle, just off center. Which I'm not sure if that puts me better or worse off. The gas port feels a little more rough then my chrome lined barrels when cleaning away from the chamber, but not bad. The port may be right on top of a lan which would snag the patch a little more. I've just never had a gas port grab a patch when pulling the patch back through. And it's not every patch, so I assume it's catching a corner or something. I will update the weekend after next when I get the next chance to shoot. The last time out, I wasn't prepaired for the cold weather. No snow, but a 15 degree temp difference between my home and the range in the hills is what did me in the last time out. The barrel is cleaning quick, so a couple more single shots and it's on to three shot groups. I'll be able to test accuracy as well to see if it's where I would expect it to be. Thanks for the help. |
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On your gas block, look exactly 6 o'clock opposite where the gas port is. The diameter of this hole (at 6 o'clock) is probably much larger in diameter than the gas port in ther barrel itself. For example, if the gas port in the barre Ithat you can not see) is say .063", and the 6 o'clock hole in the gas block is .125" (1/8" inch), then this much larger diameter gas block "gas port" does not have to be dead-on the barrel's gas port to allow for a sufficient flow of gas to run the gun. Noe that being said, the barrel's gas port "should" be at dead 12 o'clock if any type of proper fixturing was used to drill and ream it. So this would make be suspect that the feed ramps are not at dead 6 o'clock either, and/or the barrel alignment pin at 12 o'clock, because the exact location of those physicaal point control where the gas port goes after headspacing and everything else. By the way, the proper way to make the gas port with a minimum burr on the inside edge is to undersize drill it and then ream it to actual size. This was obviously not done in your barrel, or you are unlucky enough to have your gas port breaking through on the corner of a land of rifling, which in theory would create the worst kind of burr. These gas port burrs are why you military Armorers should never believe in those "barrel straightness" gauges you use when inpsection barrels. Always try de-burring with a new GI bore brush (i.e., tight fitting) mounted in an electric drill for a couple on minutes before re-droping the gauge and posssibly wasting $$ ordering a new barrel. Been there done that 100's of times. Also have the t-shirt, bumper sticker, ball cap, and tatoo. Happy Holidays to all... |
Great info. So the barrel straightness rod might wreck the barrel if there is a burr? Or does the bore brush remove the burr and then its ok to drop the rod? and Happy Holidays! |
No, droping the gauge through the barrel will not hurt it. But all too often, military armorers will assume its bent and replace it unnecessarily using our tax dollars and their own limited unit funds for such maintenance costs. The bore brush + black & decker saves about 90% or better. Then I would bore scope to see. Usually there was a little mound of gilding metal accumulated at the gas port opening that just could not be removed. Then I would target the rifle to see if it still possessed the ability for the Marine to qualify, inspect brass for signs of high pressure, etc.. If it did, we noted on the rifle's 11003 card the anomoly and re-issued the rifle "barrel as is" with my signature as Ordnance Officer. |
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I shot the barrel yesterday with mixed results. On the plus side, the rifle cycles properly. I fired two single shots and the bolt locked back as expected. I also shot two three round groups with out any failures to feed or eject. However, the groups were not that impressive. The first is just under 1.25MOA and the other is 1.5MOA. Well outside the MOA or better that most people claim. I does appear that there is a burr on the gas port. After your responses, I took a closer look and I can see some metal. And after 11 rounds it hasn't gone away. There is another post going around talking about gas port burrs and it seems that the typical solution is to polish it off with JB's. I like JB's, but am not comfortable using it alot. So, I will contact the barrel manufacturer for recommendations to proceed. I'm wondering if the burr could be causing the accuracy problems? With the uneven gash in the jacket, could that cause the bullets to spin improperly? Thanks for all your help! I wouldn't have thought to check for the burr otherwise. |
1.5 moa is fine if your barrel is not floated, nor a hi quality tube to start with. |
I don't think so, as long as the cant is minimal. By that I mean look at the diameter of the hole under the gas block that was created when the gas block/FSB/whatever was drilled up into the cavity where the gas tube traverses. This diameter is usually 2 or 3 times the gas port's. Gas leakage or restriction somewhere in the system is much more likely the problem if you piece firestick is "short stroking". Testing to see if the bolt comes back enough to activate the bolt latch is one method. Another is to hold the gun "lightly" at your side so it is moving rearwards after you fire as the bolt/carrier group moves rearwards as well. If the fired round ejects and the next round is fed, your "cant" issue is no longer a gas pressure issue. Sorry it took so long to respond, but 60+ hours a month at work (KAC) is taking its toll on my outside activities. |
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