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Posted: 10/19/2025 2:04:51 PM EDT
[Last Edit: camarofrk][Edited]
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Hello All, I have a 14.5" BCM URG that is giving me some issues. I will try to keep the backstory of the gun short. I shot it with an AAC M42000 silencer for a couple years. In order to keep the gun functional I had to use an H2 buffer, Sprinco blue spring, and a Bootleg BCG set to full suppressed. Even with all of this the can was trying to kill me with gas with every press of the trigger. I decide enough already. I purchase a Radical Defense CS5 Ti silencer. Huge difference. So much so that I can go back to my BCM BCG, H buffer, and Sprinco blue. No/very minimal gas to face and 3-3:30 brass ejection. I shoot this combo for a year and life is good. I'm at my buddies farm last month and my gun goes down. I am shooting some IMI 55gr that I have been sitting on for a while and notice a primer missing from one of the cases. The primer found it's way into the gas key. That sucks, I'll fish it out when I get home. Get it out and go to my local gun club the following weekend to test with new ammo ( Hornady 62 Frontier). This is ammo I have shot many times in training and competition with no issues. I shoot 5 rounds and gun goes down. Primer in the gas key, again. I know my bolt is old so I replace it with a new BCM as well as the firing pin and cam pin. I go to the club today to test. I shot close to 60 rounds and I inspected every single case. No more popped/falling out primers and the cases looked good. Yeah! 1 o'clock ejection with silencer. What? With some of the rounds the brass was trying to go to 12 o'clock and landing on my bench in front of me. I take off the silencer and I'm seeing 2 o'clock ejection. I switch out the BCM BCG with the Bootleg BCG set to full unsuppressed and see the same issue. The ammo I shot was as follows: AAC 55 and 62 Hornady 62 Frontier IMI 55 (same ammo that started it all. It did great) Lake City/Federal M193 The gun with the CS5 silencer feels almost like shooting the M4 2000. This is where I'm confused and frustrated. I just don't know how I go from a very nice low backpressure can to the opposite. Like a switch was flipped. My apologies for such a long post I just want to give you as much info as possible. I'm probably forgetting something. Thank you for the time. Edit: When I say pooped primers the primers did not look like they were blown out by excessive overpressure. There was no powder residue around the base of the case. |
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A little hard to follow... First, you should have left the H2 in it. And perhaps tried an H3. Check to make sure the Sprinco Spring is not cracked and is the appropriate length. Sprinco makes excellent springs but they are a little more susceptible to corrosion, keep them lubed. You need to check head space when getting popped primers and make sure the lugs and chamber are clean. If it's a stock BCM upper you probably could find an adjustable gas block that would fit the BCM gas block dimples, since BCM GBs are not pinned. Take a look at Griffin's new two position GB. Griffin Gas Block How many rounds on the upper? I'm not super sensitive to gas in the face. Is the rifle shooting unusually hard? I run mostly Surefire RC2s, and other higher back pressure cans on Geissele uppers 14.5" and 11.5", and I'm not familiar with the can you are using. 95% sure its not an ammo problem |
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The spring is 11" long and has no cracks or corrosion. I keep it lubed. The can is a low back pressure can and was performing like it with this spring and buffer combo. No to very low gas to face . Edit again: I fully cleaned the chamber, lugs, and bore before shooting today. The gun does seem a little more snappy/punchy than before. It's acting over gassed when it was not before |
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Originally Posted By Homesteader375: Have you cleaned your can? The can has less than 1000 rounds on it. When you look inside you can still see the gold titanium color. I'm not discounting the can yet but it is just odd that the gun went from grassed well to over gassed between two shooting sessions. I have a theory as to what is happening but I'm waiting to see what the headspace check is. |
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Originally Posted By camarofrk: The can has less than 1000 rounds on it. When you look inside you can still see the gold titanium color. I'm not discounting the can yet but it is just odd that the gun went from grassed well to over gassed between two shooting sessions. I have a theory as to what is happening but I'm waiting to see what the headspace check is. Yeah, at 1k rounds the can seems very unlikely. This sounds like excessive backpressure vs headspace. Hard to really say without looking at casings. |
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Originally Posted By Homesteader375: Yeah, at 1k rounds the can seems very unlikely. This sounds like excessive backpressure vs headspace. Hard to really say without looking at casings. I have most of the casings. They look good. I don't, or didn't, see any signs of overpressure. I'll look at them again in better light. It was overcast and raining the day I shot. |
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Originally Posted By camarofrk: The gunsmith went over it and found nothing wrong. Both headspace gauges passed. I don't understand how a low back pressure can turns into a high back pressure can. I bought the damn thing to keep gas out of my face. It was. Now it isn't. Gas port erosion? Eyeball it with a bore scope and see if the port has opened up. You may need an adjustable gas block for Christmas. |
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I sent the can back to Radical Defense to be looked at a couple weeks ago. Waiting to get it back. I doubt anything is wrong with it. I have not shot the gun since I sent it back. When I get it I'll shoot and give a report. It was just an odd sequence of events that lead to this. Primers coming out of cases. Replaced bolt. No more primer issues. Then heavily gassed with low pressure silencer. Even without the silencer ejection was no longer at the 4 o'clock it used to be. The gunsmith did say the gas port looked like a diamond shaped football. He opened it up to .076. I believe BCM is .075. Accuracy is still really good but a new barrel may be coming up. |
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Originally Posted By Dakota_Don: Gas port erosion? Eyeball it with a bore scope and see if the port has opened up. You may need an adjustable gas block for Christmas. The gunsmith opened the port up to .076. I think BCM is .075? Either way I have a Bootleg adjustable carrier I used for my M4200 silencer. I may have to use it for this. Or a new barrel? Good barrels don't seem to cost too much. |
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