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Posted: 10/6/2006 8:27:25 PM EDT
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I am looking for some advice. We recently converted some military M16A1’s into entry weapons for our tactical team. We installed 11.5 inch barrels and collapsible stocks with all the necessary carbine components. The weapons will fire perfectly in semi mode but have intermittent stoppages in auto mode. The weapon will fire and cycle in auto mode firing several rounds but will then stop. A round will be in the chamber and the hammer appears to be in the fired position. Cycling the charging hammer the weapon will fire again a few rounds and stop the same as before. The weapons have been thoroughly cleaned and rotated numerous magazines so we are sure these are not issues. We suspect the problem may be with the auto sere and or the trigger group. Is this possible or are we looking in the wrong area? Anybody got any ideas |
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Do you know which buffer y'all are using? Sounds like bolt bounce. You should try one of the heavier carbine buffers, like the H2, H3, 9mm or a rate-reducing buffer like the MGI or Enidine. If my assessment is off-base, I'm sure some other helpful folks will chime in. |
Nah, sounds about right. At least according to what I learned at Armorer School. |
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Yes, you have bolt carrier bounce. That happens when someone shortens a barrel to 11.5 inches and opens the gas port to make sure you do not get short-stroking. However, if the port is opened too much, it drives the bolt and carrier rearward with too much velocity. The springs compresses and sends them back with the same force as it was collapsed. The bolt carrier trips the auto sear but is traveling so fast that the bolt engages the lugs, the bolt carrier slams into the barrel extension, and then the bolt carrier rebounds to the rear unlocking the bolt, and the hammer finally drops. So, you end up with the hammer down and a live round in the chamber. Usually you will also have a light primer strike. You can solve the problem by putting in a heavier buffer such as the Colt "C" buffer or an AAC. You can also first try the Fat Boy gas tube from PRI, and that may fix the problem by itself. I like to use the Fat Boy gas tubes first, so I do not have to get a different buffer. But, if push comes to shove, you can use both the Fat Boy and the heavier buffer and the combo should fix any bolt carrier bounce problem. It is more accurate to refer to this as bolt carrier bounce because it is the bolt carrier that bounces off of the barrel extension not the bolt. This term is used in Black Rifle II as well. Charles Tatum Alamo Professional Arms |
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I also reccomend heavier buffer in shorty carbines. I run H2's in my shorty full autos. Also check the timing. On a shorty it is best if the sear is tripped as close as possible to the time the bolt locks into place. You can check timing by inserting the back of a drill bit into the ejection port. Ease the bolt carrier forward and insert the drill bit at the front of the ejection port between the port and the front of the carrier. If the sear is tripping too early you can see by how much by the size drill bit that will fit between the port and the carrier. I like around .015 on 11.5 uppers. Timing can be adjusted by removing metal from the part of the carriers that trips the sear. A lttle at a time please. Try the heavier buffers first. Do not over buffer the weapons. If an H2 works stick with it. If you have to, go to an H3. If that does not fix the problem check your timing. If the sear trips too early a shorty just will not run. Make sure your springs are in good condition at all times. |
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Chief, I think I can help you. This is what our company do. I think it is a combination of the endine and the gas port hole on the barrel. Check the gas hole on the barrel under the front sight and the size should be .08 and use the regular or the H2 heavy buffer. It will fire full auto but the rate of fire will be very high at around 900rpm. We do have our own rate reduction system if you want to reduce the rate of fire but you have to make the rifles run reliably first before you reduce the rate of fire. We converted hundreds of M16A1's with triangular handguards to 11.5" barrel. Our webpage is www.ferfrans.net and bushmaster, STag, ARMS knows us. The way to measure the hole is a gauge that looks like a pin with different sizes. Your local machine shop can figure it out. Please do not do any alterations on the lower or upper receivers. How many rifles did you convert? |
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As an aside, we had some replacement uppers from Bushmaster that had issues. Turned out that their "mil-spec" M-4 uppers were a long way from spec in that they had no feed ramp cuts and the gas ports were an incorrect size...plus they were not 1 in 7. The FSB was also too short and wouldn't zero. Learned to be a lot more specific about what we needed after that. Colt or LMT for me from here on out. |
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