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Posted: 5/17/2013 8:27:18 AM EDT
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I recently helped my father build a rifle to his specs. Specs are as follows:
20" HBAR SS 1:8 Match Spikes upper Ni/B treated Spikes lower with Magpul CTR Stock Ceramic coated M16 BCG Heavy Buffer Geissele SSA trigger Doublestar picatinny gas block Magpul PMags (exclusively) During test firing, we had repeated failure to lock back on the last shot. The rifle feeds, fires and ejects. Just no lockback. Magazine does engage bolt hold open on manual retraction. When testing with a standard BCG and H buffer, rifle did lock back. Standard BCG and standard buffer did lock back. When testing with the M16 BCG and standard buffer, rifle did not lock back. I was leaning towards the theory that the combination of the heavier BCG and H buffer are far too heavy and that the BCG simply doesn't travel far enough to allow the magazine follower to engage the bolt stop. But a military rifle would be running that combination and still function in semi-auto, correct? (Please correct me if I am wrong) So now I'm leaning towards gas system as the culprit. Looking for input and advice. How would I check the gas system? Is there a simple way to check the alignment of the gas port to the block? (I thought about running compressed air down the tube and checking return through the barrel.) Everything looks correct visually. Any help is greatly appreciated. |
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Quoted:
Quoted:
I recently helped my father build a rifle to his specs. Specs are as follows: 20" HBAR SS 1:8 Match Spikes upper Ni/B treated Spikes lower with Magpul CTR Stock Ceramic coated M16 BCG Heavy Buffer Geissele SSA trigger Doublestar picatinny gas block Magpul PMags (exclusively) During test firing, we had repeated failure to lock back on the last shot. The rifle feeds, fires and ejects. Just no lockback. Magazine does engage bolt hold open on manual retraction. When testing with a standard BCG and H buffer, rifle did lock back. Standard BCG and standard buffer did lock back. When testing with the M16 BCG and standard buffer, rifle did not lock back. I was leaning towards the theory that the combination of the heavier BCG and H buffer are far too heavy and that the BCG simply doesn't travel far enough to allow the magazine follower to engage the bolt stop. But a military rifle would be running that combination and still function in semi-auto, correct? (Please correct me if I am wrong) So now I'm leaning towards gas system as the culprit. Looking for input and advice. How would I check the gas system? Is there a simple way to check the alignment of the gas port to the block? (I thought about running compressed air down the tube and checking return through the barrel.) Everything looks correct visually. Any help is greatly appreciated. There's your biggest tell, either the bolt catch is out spec or the mag is. Did you try a couple different kinds of mags? I have had this issue with some metal mags, The follower would slip up past the bolt catch, my solution was to tweak the follower and mag body. What Runngunner is saying is that the bolt catch functions as it should when the bolt carrier is manually retracted to the rear. |
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Quoted:
What Runngunner is saying is that the bolt catch functions as it should when the bolt carrier is manually retracted to the rear. ahh my bad for some reason I read does not, you kind of have to run through the whole system, try and seal off the barrel and run compressed air through the tube to see if there is a leak in the gas block area, then you can check the carrier to key mating push in bolt and cover gas vents and blow through key, a little lube or soapy water helps show leaks. also check to make sure the carrier is not binding in the upper at all and that the gas key to gas tube is not binding at all. failing those try a regular carbine buffer with the m16 bcg |
| It seems like the deciding factor is the M16 bolt carrier. Perhaps it isn't the added weight of the bolt carrier, but rather something "defective" about the bolt carrier itself. The weight difference in the bolt carriers is more than likely less than the buffers. It locks back fine with the H buffer and AR-15 bolt carrier, but will not with the M16 bolt carrier and standard buffer (and this combo is probably lighter). |
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Ammo is Winchester 69gr match. I will follow the recommendations above and we will see. Appreciate the input guys. I used to have an account here. The info on this site helped me to build my very first AR (an IDF-ish clone). But that email is long gone as is the original username. The people on this forum are some of the finest, most helpful people I've encountered on the web, and I thank you all for that.
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| Is the BCG new? Hand cycle it using the CH a number of times. Clean the barrel with a chamber brush first to get rid of any grease. Then I usually lubricate the BCG and just hand cycle it like 50+ times as I'm watching TV or something. Then make sure you are using quality, factory 556 ammo to break in the BCG. Don't use weak/cheap 223 or steel cased ammo at first. Run about 100 factory brass ammo first. |
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If the problem child is the M-16 carrier alone, then suspect that you have a problem with the key on it, with best guess, a leak between the key to carrier surfaces. You can test them just holding the bolt closed, and using compressed air down the key to check for a leak between the two.
With the carrier being brand new, just send it back for a replacement for one that is not leaking instead. |
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