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Posted: 7/24/2010 7:05:12 AM EDT
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Setup for the gun is:
10.5" Noveske SS in 5.56, the provided gas block and tube, A2 flashhider CMT upper and BCG Standard CAR buffer and spring (the ones that came with my VLTOR EMOD kit) The issue I believe is the gun is under-gassed and short stroking. I tried it out with low pressure civi stuff (remington umc .223 and similar) and it would fire and cycle fairly well, but it would not lock back on an empty mag, and would occasionally close on an empty chamber (meaning it did not cycle far enough back to strip another round correct?). Of course I got it soaking wet with CLP, and tore apart the BCG and checked to make sure everything is in working order: Gas key is tight, gas rings look good and are still tight inside the bolt carrier, extractor is firm and in good shape. I checked the gas tube and block and they seemed fine as well. I then tried some Fed XM193 5.56 and the gun immediately started shooting better, more reliable, would lock back on empty mag *most of the time* and fewer instances of closing on an empty chamber. It did not solve all the issues, but they happened less often. So the higher pressure ammo immediately made a big difference. This to me means that the system is not getting enough gas to function properly, correct? With both types of ammo I tried an H2 buffer and this only magnified all problems. Going back to the CAR buffer made the gun shoot better with fewer malfunctions. Again leading me to believe it is under gassed and adding weight to the buffer was slowing it down even more. What I want to know is: Considering that all relevant parts on the gun have been checked and seem to be in perfect shape, am I correct in my assumption that the gun is under-gassed and this is causing the short stroking? Now what are the solutions to this? I know I can have the gas port opened up a little, or I could add a KX3, or both. Is there anything I'm missing here? Anything else I should try before dropping off the barrel at my smith... What is the stock gas port size on a Noveske 10.5 SS? And what should I have it opened up to? Thanks for the help in advance Chris (crappy picture, yes I know the CH is missing in this pic and the front sight is backwards ;)) 
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My take based on everything you listed is that the gas port is too small and not enough gas is getting sent down the tube or the gas block hole is not lining up properly with the hole in the barrel, basically restricting gas flow.
This is why the guns runs better with hotter ammo and a lighter buffer (less mass in the system to cycle back). The standard colt gas port hole for a 10.5 is 0.093" or a #42 drill bit. However it is my experience that you don't need to go up to this size to get a 10.5 to run properly. I personally usually start at a #46 drill bit (0.081) which is the size for a Colt 11.5. Drill, reassemble, test fire. If you need more gas than I go up a drill bit size (really it's down a number 46 to 45 to 44 etc) until the gun functions. Most 10.5 I have worked on will run with a #44 dril bit (0.086) or smaller. If I am running with an adjustible gas tube or block I usually just drill a #43 (0.089) and adjust the tube/block down as needed. You can always test your theory by buying one of thoes Oly 5 inch CAR flash hider things pretty cheap to simulate larger barrel length or if you know somebody with a suppressor, test with their can attached and I bet it will cycle with no issues. If I were you, I would pull the gas block, measure the gas port hole, and if it is undersized go up a notch bigger and retest. Hope this helps James Austin, Texas |
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Was the upper pre-assembled or did you put it together yourself/have it put together?
I'd say double check that the gas block port lines up with the barrel gas port. If so, then you seem to have the right idea about the system being undergassed. ETA: Sorry, noticed you said that you checked the gas tube/gas block and said it looked fine. Did you disassemble it to check it? |
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I have almost the exact same set up as you. I built my upper with a CMT receiver with the 10.5" Noveske barrel. I got my BCG (Auto carrier) from Noveske, but I think CMT makes them for Noveske. I used a Noveske lower with a standard spring and an H buffer. I ran XM193 for the first 300 rounds to break everything in. I have run PMC, PMC Bronze, S&B 193, and 55gr Wolf with out any issues. I am at a little over 700 rounds now. How many rounds do you have through the gun? The only difference I can see is the buffer weight. I never tried anything else. Is your carrier Auto or Semi? The way I understand it, you increase the weight of your buffer and carrier to increase dwell time of the pressures in your barrel so the bolt does not unlock to early. It is near impossible to install that gas block wrong, so I can't see that being a problem. Do you have another BCG to try? I would not try drilling out your gas port until a last resort. Most likely that is not the issue. Call Noveske first to find out what the size is supposed to be. Once you drill it, you cannot make it smaller. My focus would be on the BCG, or buffer and carrier weight combo.
Jeremy |
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Thanks for the help so far guys-
I have tried a different bolt group in the gun with much the same results, it was from a buddy of mine's working 10.5" so I'm quite confident it's not the BCG. And as Jkrammes said, it's pretty much impossible to put a noveske gas block on wrong, it automatically indexes due to the dowel pin used along side the set screw. I did eyeball it (without taking it off) and it is as close to perfectly vertical as I could imagine it being so I'm quite confident it's aligned properly. I'll try and pop off the gas block and check the size and have it opened up if need be. Those noveske's are so damn hard to get on and off though, haha it's a pain in the ass. Thanks for the advice on gas port size as well, I don't want to go any bigger than necessary! Though suppressors are not legal in the [not so] great state of MN so it's not much of an issue I guess. |
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Ok popped off the gas block and found a gas port that is, to the best of my abilities in measuring (crappy caliper... but good enough) I have a gas port that is right around .067 -.070. Looks like I'll be having it opened up a bit.
Thanks again for the help, I'm sure this will solve my problems. Chris |
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Just so you know, measuring a hole with calipers is not the way to go. You will never get an accurate measurment. Pin guages is the way to go. So using a #42 or #43 drill bit would be a better way to guage the size of the hole. Use the back end of the bit, not the part with the flutes. FYI, with wire guage bits the larger the number the smaller it is. So #43 will be smaller than #42. Alot of the bits will have the decimal equivaliant printed on the package. Find one that fits and you will know the port size.
Jeremy |
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I would contact Noveske. The smallest port that runs is best. Lots of range with gas ports from different manufacturers.
I have a RRA barrel that was opened to .078. On the other end, I have a Rainier Ultra Match 7.5" that runs great with the factory .056. I have no answer as to how it runs with that small of a gas port but I've talked with several owners of the set up and they all run. I also run a 9mm buffer, M16 carrier and BCM extractor up grade. |
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