Sub Sonics

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12 years 9 months ago #10349 by Hot Lead Zapper
Replied by Hot Lead Zapper on topic Re: Sub Sonics
It sounds like the bolt carrier is short stroking, which is typical for subsonic rifle ammo. I’m guessing the felt recoil is similar to a 22lr rifle…? It’s possible to use weaker recoil spring and hydraulic buffer to address the issue, but I found over time that to becomes unreliable.

If you continue to use whatever low pressure subsonic load you’re using (“hint” need more ammo spec data) you may need to have the barrel port enlarged to increase pressure so your bolt will move the full range of motion, but the upper will only function properly with subsonic ammo there after. Also expansion gas chambers and/or adjustable gas block would assist in fine tuning over-all subsonic platform. Supersonic factory ammo with enlarge barrel port will cause excessive recoil and wear.

I would recommend a machine shop that specializes in EDM porting. Electrical Discharge Machine will not create burrs within the rifling of the bore after port has been enlarged. DPMS 308 factory gas port is .088 and you will need to increase to .100 for reliable bolt cycle.

The max range of subsonic will be extremely limited regardless of false reports stating target engagement at 300yds and beyond. Typically a customize sight or rear scope mount elevation would be needed to allow target to stay within sight picture beyond 100yds. The subsonic projectile trajectory rapidly descends after 75yds below the average scope adjustment compatibilities.

Ideally an YHM Cobra 45acp suppressed pistol can achieve similar results compared to subsonic 308, 300 blackout 7.62X35, 7.62X25 and 7.62X39.

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12 years 9 months ago #10356 by Flowhanan
Replied by Flowhanan on topic Re: Sub Sonics
Going off of faded memory I do belive that that load data was cci magnum primer 8.5g trail boss powder, & 155g Hornady moly coat BST boat tail. However, I havn't fired subs in the auto loader in a long time due to the non cycling ability. I do use them with my 700 sps tactical. You are totally correct with the ballistics. The bullet path is less than desirable. They are fun to fool around with though. I'm having problems with the bolt returning to the locked position after firing. With my YHM can and hand loaded ammo ranging around 2900fps the bolt will cycle but the forward assist has to be used at times to continue follow up fire. I think it has to do with the fact I replaced the factory collapable stock with the Mag-pul stock. I did order the extension tube from DPMS. Gut feeling says that the spring and buffer are the wrong combo as I used what came with the carbine.

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12 years 9 months ago #10379 by Hot Lead Zapper
Replied by Hot Lead Zapper on topic Re: Sub Sonics
Don’t shoot your rifle until you’re able to replace the buffer and recoil spring with correct length of both.

You need to check your lower for possible damage. I’m sure the bolt carrier’s gas key was bottoming out against the rear of the lower if you were using the collapsible length buffer. It’s common for people to make the mistake of using carbine recoil springs in full length buffer tubes, which would create the symptoms as you described, but it’s extremely important to have the correct length buffer to protect your lower.

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12 years 4 months ago #11839 by mtrmn
Replied by mtrmn on topic Re: Sub Sonics
I just ran across this old thread.
In my experience, you're going to have a VERY hard time getting subsonic ammo to cycle any AR action. When you get enough gas and pressure to cycle the bolt, you will have exceeded the speed of sound with your projectile.
I have a heavy buffer/spring in my 308 to slow the cycling from overpressure with full power ammo and the gun works very well with or without the suppressor. But the bolt only moves about an inch or less with subsonic ammo.

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