Hmmm, my 8" SBR with suppressor doesn't quite cycle with subs- spring update post #19

Hmmm, my 8" SBR with suppressor doesn't quite cycle with subs- spring update post #19

This is a discussion on Hmmm, my 8" SBR with suppressor doesn't quite cycle with subs- spring update post #19 within the Defensive Rifles & Shotgun Discussion forums, part of the Related Topics category; Here's the particulars: I have a JP adjustable gas block, pistol length 8" barrel and pistol length gas tube, and a Liberty Victory suppressor. That ...

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Thread: Hmmm, my 8" SBR with suppressor doesn't quite cycle with subs- spring update post #19

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    Hmmm, my 8" SBR with suppressor doesn't quite cycle with subs- spring update post #19

    Here's the particulars:
    I have a JP adjustable gas block, pistol length 8" barrel and pistol length gas tube, and a Liberty Victory suppressor. That particular suppressor is 10" long which is a bit big for 300 blackout, but it has to double as a .308 suppressor soooo.....

    Up until yesterday, with the gas block adjustment set wide open and the suppressor mounted, it would not reliably eject the cases .

    Because I prefer clamp on gas blocks, there is no "detent" to correctly position the gas block so you know the gas block port correctly aligns with the barrel port. So yesterday, I pulled the gas block and started looking.

    I was fitting numbered drill bits in the gas block port to determine the size of the port and noticed the bits were pushed off to one side. Somehow the gas tube port and the gas block port were not 'xactly aligned. I carefully examined the gas tube orientation. It looked true as could be. Also it was pinned which should have aligned the tube with the block.

    Well, since it was apparent I was having gas problems, I sized the gas block port and 'reamed' the holes so they would perfectly match. I was delighted to realize that was very likely to be the cause of the problem. I would learn shortly that would help, but not solve the problem.

    To precisely align the gas block port with the barrel port, I inserted a drill bit the same size as the gas block port into the gas block port and used a digital caliper to measure from the edge of the gas block to the edge of the drill bit. Next, I fitted a drill bit to the gas port in the barrel; wouldn't you know it, it's a bit smaller. So I subtracted the radii of the two drill bits and added that value, a mere 0.003" to my edge measurement. I set the caliper to that measurement 0.243" (I think I remember that right) and zeroed the caliper. I then measured the distance from the pin in the barrel to the shoulder of the barrel and saw that it was 0.020" more. Sooo...

    I slid on the gas block, put a 0.020" feeler gauge between the gas block and barrel shoulder to precisely align the gas ports. I also did some careful marking of the rotational position, but I won't bore you with that.

    On to test fire the 'fix'. I loaded one round in the mag and fired. It didn't lock the slide back. I made sure the gas block adjustment was wide open. Still the same result. So I decided it's time to start lightening the buffer.

    I opened the buffer, got the steel spacer dims and chucked some poly in my lathe and turned out a poly spacer. That'd didn't do it. So I made another, and finally a third. Now it's starting to work. I fired three shots pretty quickly and it cycled. The bolt even locks open on the last shot - but that's not the end of the problem. The bolt is catching on the back of the magazine carrier instead of on the bolt catch - you can see it's not in the right position. Strangely enough, if you pull the mag, the bolt stays locked back, but now I think it's moved forward ever so slightly and the bolt catches on the bolt catch, but not in the right place. I tested this and discovered, sure enough, there are two places the bolt can be locked to the rear. That's true for my other ARs as well.

    So what next. Well, a lower power buffer spring. However, now I'm getting concerned. Basically, what I have done and would continue to do with the lighter spring, is tune the lower to a 220 gr subsonic load. That isn't a problem. What is a problem is that if I get the gun set up for subsonic and start shooting supersonic as one is supposed to be able to do, the sub-tuned gun is going to be drastically over gassed for supersonic. Because the 220 gr sub has to be held to a velocity to about 1050 fps (to remain subsonic) the energy of the gas and load is much less than it could be. In fact, a 220 gr subsonic is going to have about 2.5 times less energy than a 110 gr supersonic round!

    Is that a problem? Well, no doubt your gun will cycle both sub and super sonic rounds if you set it up to fully cycle sub rounds. But the supersonic is going to do some hammering on your gun. There will be about 2.5 times the energy for the buffer and buffer spring to absorb. If the buffer is being fully cycled for the subsonic round, guess what's gonna happen with the supersonic at 2.5 times the energy? The bolt speed goes up, the buffer will bottom out and things get hammered.

    So you guys that shoot SBR ARs with both sub and super sonic loads, how do you deal with this?
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    Daaaannnnggg.... I think you just talked me out of building my own AR pistol. lol

    Sorry, no help but I hope you get it figured out.

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    Quote Originally Posted by Norm66 View Post
    Daaaannnnggg.... I think you just talked me out of building my own AR pistol. lol

    Sorry, no help but I hope you get it figured out.
    LOL I hope I didn't talk you out of it!

    I'm hoping we'll get some insight from some that have more experience with the 300 sub and super loads than I have. But, it does seem like there is a huge energy difference and if the gun cycles the sub, it almost has to be taking a beating with the super.

    That's why I put an adjustable gas block on my SBR. I want to be able to turn the gas down a bit for supersonics. Of course I've got to getting it cycling subs before that's an issue.
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    Follow up: Contacted Wolff Gunsprings...

    I realize Wolff Gunsprings is not an AR builder, but they do know how their gunsprings are being used and why.

    I gave them a call and described what I had and the problem of cycling I was having. I then asked if this was a common problem or something unique to my build. She unhesitatingly said, "No, it isn't unique, it's a common problem and guys are putting reduced power buffer springs in their ARs to resolve the problem."

    I said thanks and ordered two. I guess I'll see now. But it almost has to be the problem. The standard buffer weight and spring is designed for a rifle load; the 220 gr sub is no where near a rifle load in terms of energy, i.e. 464 ftlbs, well at my velocity from the 8" barrel. It makes perfect sense - a load with 2.5 times less energy than a rifle load - it just isn't going to work the action - unless......you change the action.
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    Talked with a Lt Col Weapons tester/sniper about this stuff a few years ago (his last name was Savage - go figure). He told me people tend to want to go lighter on springs/buffers, but the real solution is to get a heavier buffer as the bolt is cycling too fast with the additional backpressure from the suppressor. Of course I don't know how his setups compare to your's - think he mostly dealt with M4's, but it may be something to try. If it works, your gun won't get hammered quite so much as in the other direction.

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    Turn the gas port closed more for full power loaded once you get it to properly cycle? Did you already drill out the gas port on the barrel ?
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    I have an AR pistol with a YHM suppressor.

    I'm sorry to say, I haven't had good results with sub/super sonic ammo either. I have been hesitant about changing out the buffer for the same reason you point out. I have had cycling issues as well.....

    Just curious, but how are the decibels with your Liberty can? And have you gave them a call?

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    [snicker]...he said "gas problems" [snicker]

    (sorry, couldn't resist)

    Hope you get it running right
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    Shooting any type of 'reduced load' in just about any semi-auto always has the potential to not cycle reliably. Yes, you can 'tune' the gun for the reduced loads, then it's not set properly for full power loads. The degree to how much springs and ports are adjusted of course varies, may not need a lot in some situations.

    You may have to configure it for one range of loads at a time. That it the easy solution as I see it.

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    That is what I gathered from other sub sonic shooters as well. Your AR is either set up for normal rounds, and is a single shot/manual action with subs, or subs only.
    Sticks

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    You can't have the best of both worlds. ARs were never designed to be pistols or to shoot subsonic ammo. It sometimes takes a lot of tweaking to get these running right.
    Good luck

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    Quote Originally Posted by nedrgr21 View Post
    Talked with a Lt Col Weapons tester/sniper about this stuff a few years ago (his last name was Savage - go figure). He told me people tend to want to go lighter on springs/buffers, but the real solution is to get a heavier buffer as the bolt is cycling too fast with the additional backpressure from the suppressor. Of course I don't know how his setups compare to your's - think he mostly dealt with M4's, but it may be something to try. If it works, your gun won't get hammered quite so much as in the other direction.
    Different problem here. On a supersonic load, i.e. standard M855, etc. the suppressor does in fact increase operating pressure, so reducing the gas adjustment would help. Subsonics are a different problem.

    With subsonics, the muzzle energy is about 2.4 times less than a supersonic round. There's just not enough energy/gas pressure left to cycle the action.

    Quote Originally Posted by rocky View Post
    Turn the gas port closed more for full power loaded once you get it to properly cycle? Did you already drill out the gas port on the barrel ?
    I started with the gas block closed - it's not easy ejecting rounds with no gas to kick them out!

    I have not drilled out the gas port on the barrel. Will that be necessary? I did that on another barrel on a different gun (same problem) and it didn't resolve the problem with subsonics. I guess I was 'saving' drilling the barrel port as a last resort, but I'm all in if you think that will help???

    Quote Originally Posted by 47Gabe View Post
    I have an AR pistol with a YHM suppressor.

    I'm sorry to say, I haven't had good results with sub/super sonic ammo either. I have been hesitant about changing out the buffer for the same reason you point out. I have had cycling issues as well.....

    Just curious, but how are the decibels with your Liberty can? And have you gave them a call?

    Cheers...
    I think there's a lot more to tuning the pistols and pistol length SBRs. They just don't have much dwell time and that's a factor.

    I can shoot subs without ear protection with the Liberty Victory and it's probably not quite as loud as a 22 rifle. The spec on the can is 32+ dB. The measured dB from a 220 gr sub from a 16" bbl was 121 dB which is waaaay below the 140 dB where ears are in danger, nearly a factor of 9.

    However, the dB level from a barrel shorter than 16" will be a higher due to the higher exit pressure at the muzzle.

    I have not called Liberty, but that's a good idea. I'm sure they've seen this problem.
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    Here is exactly the problem I'm having - in slow motion even!!!

    Notice the 249 gr does cycle rounds, but does not lock the bolt back in the proper position. Compare the position of the locked bolt on the sub and supersonic ammo.

    BTW, the 400, 800, 1600 fps is frames per second, NOT feet per second. Slow motion is produced by shooting video at high frame rates and then playing it back at standard frame rates, typically 24-30 fps.

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    Quote Originally Posted by rocky View Post
    Turn the gas port closed more for full power loaded once you get it to properly cycle?
    I agree with Rocky, you should be able to turn you gas down on full power with the lighter spring and have it eject properly. Don't turn the gas off just down one setting.
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