14.5" Midlength Gas - What gas port size?

[Joeymac]

I have a 16" ballistic advantage premium series midlength .308 barrel I'm going to cut down to 14.5" and then pin/weld a suppressor mount onto. I have an adjustable carrier key on the BCG to regulate gas. I will be firing a mix of suppressed and unsuppressed, but my goal is reliable cycling of most ammo when unsuppressed. My plan is to adjust the gas key for last-round-bolt-hold-open using some weaker powdered stuff; maybe wolf, MFS, or some milsurp ball ammo. That way pretty much anything should cycle suppressed or unsuppressed. 

 

One thing I don't know is if I'm going to have to open up the gas port after shortening and threading. What should the port size be on a 14.5" midlength .308?

[98Z5V]

First pinned topic in this section:

https://forum.308ar.com/forum/65-308-ar-what-you-need-to-know/

Welcome.  Hit up the intro section and tell us about yourself when you get a chance.

[Joeymac]

21 hours ago, 98Z5V said:

First pinned topic in this section:

https://forum.308ar.com/forum/65-308-ar-what-you-need-to-know/

Welcome.  Hit up the intro section and tell us about yourself when you get a chance.

I had looked there first. There were no Midlength 14.5” .308 barrels listed, unless I missed something. 

All the 308 barrels listed are over 16” except one listed as an Armalite 13.5” @ 0.081” ... which might be considered the closest info to what I’m seeking but the barrel length is off by a whole inch after the gas block (significant dwell difference) and ar10 has tubes are different length than DPMS/308ar gas tube lengths anyways. I suspect that the armalite 13.5” middy at 0.081” would suggest yet a smaller hole for a DPMS pattern 14.5” midlength. 

[jtallen83]

2 minutes ago, Joeymac said:

I had looked there first. There were no Midlength 14.5” .308 barrels listed, unless I missed something.

Drill up one size at a time until you get the desired function. Then of course add your data to the thread so the next guy will have a baseline, it's a work in progress so to speak 

[Joeymac]

That would be a super pain the butt.

A) Pinning the Muzzle device so disassembly .

B) the range is 30 min from my shop

for what it’s worth, the current gas port checks out at 0.072-0.073” with pin gage method. (Ballistic Advantage 16” midlength Premium Series 416R 1:10 twist .308win)

[jtallen83]

11 minutes ago, Joeymac said:

That would be a super pain the butt.

Not really that big a deal. You should be able to loosen the gas block enough to get to the port. A bit set, some lube, a wooden dowel, and a cordless drill are all you need. You can do it

[98Z5V]

32 minutes ago, Joeymac said:

I had looked there first. There were no Midlength 14.5” .308 barrels listed, unless I missed something. 

All the 308 barrels listed are over 16” except one listed as an Armalite 13.5” @ 0.081” ... which might be considered the closest info to what I’m seeking but the barrel length is off by a whole inch after the gas block (significant dwell difference) and ar10 has tubes are different length than DPMS/308ar gas tube lengths anyways. I suspect that the armalite 13.5” middy at 0.081” would suggest yet a smaller hole for a DPMS pattern 14.5” midlength. 

Not gonna make a difference.  Drill that thing at .0.080", and it'll run.  Well.  You're splitting hairs, and worried about minute things, but you're not considering the amount of gas that the .308 Win round can produce.

You know this already, math wise.  You're 0.072~0.073" right now, on 16" barrel that you're gonna cut down by 1.5".  If you're worried about it, and don't want to be over-gassed, then just split the difference.  72/73 thou on 16", 81 thou on 13.5", and you want 14.5".  Run it up to 77 or 78 thou and be done with it.  It's rocket science - but it's not.  There's plenty of gas in this cartridge.  I'd bet it'll run just fine, right now with it's 72/73 thou, cut back to 14.5"...

For the dwell time discussion - you ever seen what a rifle-gas 16" .308AR looks like?  There's not alot of dwell time, and you cut that gas port at 0.100".  You're overthinking dwell time on the 308. Let's say you go too big, and jack it all up - add an adjustable gas block and cut it back down.

Now, since we're just spewing out info for you, drag your ass into the Intro Section and tell us about yourself...  It's right here...

https://forum.308ar.com/forum/22-introductions/

My $0.02.  Take it for what it's worth.

Edited June 2, 2018 by 98Z5V

[Joeymac]

Following up for posterity:

I took it to the range and it went great. Lubed it up, ran a lubed patch down the bore, and then started shooting. It cycled everything and locked back on everything. 100 rounds (25% suppressed) and no FTF, FTE, jams or hiccups. No stopping to clean or lube either.  The brass looks clean and not mangled. It shot pretty flat with the just the brake and no silencer, although the blast was sizable. 

 With no silencer it puts brass in a pile about 10 feet away at around 3:00-4:00. Which for an AR15 would be perfect, I'm not sure about an AR10/LR308 though. And with the silencer attached and no other adjustments it actually deposits brass about 4 feet away a the 1:00-2:00 position... which I think that forward thrown brass means slightly overgassed, although that wouldn't be surprising considering the addition of the silencer. Nevertheless, it still functioned great anyways even with a silencer. I'm surprised it ran so well with no gas regulation at all for either.  

Magical setup: .073" Gas Port, 14.5" Midlength Barrel, Plain Gas Block, Regular Bolt, Slash Heavy Car Buffer (5.6oz). 

It ate Freedom 155gr Amax Reman, Gemini 155gr Amax Reman, MFS 150gr Steel, PRVI 175gr SMK, German DAG surplus, and Malaysian L2A2 Surplus. Locked back on all of 'em. At this point, I don't even think it's worth playing around with tuning the gun using the adjustable gas key & lighter buffers. Although I might switch over to an A5/AR10 carbine tube though with a Slash XH Carbine Buffer (8.5oz) and Magpul FCS for a slightly longer LOP. 

Look at the sweet concussion in the gif image... the can nailed to the post 5 feet away is rattling and you can see the camera itself being shook 10 feet away.

 

[Chuck Plissken]

On 6/6/2018 at 12:01 AM, Joeymac said:

Following up for posterity:

I took it to the range and it went great. Lubed it up, ran a lubed patch down the bore, and then started shooting. It cycled everything and locked back on everything. 100 rounds (25% suppressed) and no FTF, FTE, jams or hiccups. No stopping to clean or lube either.  The brass looks clean and not mangled. It shot pretty flat with the just the brake and no silencer, although the blast was sizable. 

 With no silencer it puts brass in a pile about 10 feet away at around 3:00-4:00. Which for an AR15 would be perfect, I'm not sure about an AR10/LR308 though. And with the silencer attached and no other adjustments it actually deposits brass about 4 feet away a the 1:00-2:00 position... which I think that forward thrown brass means slightly overgassed, although that wouldn't be surprising considering the addition of the silencer. Nevertheless, it still functioned great anyways even with a silencer. I'm surprised it ran so well with no gas regulation at all for either.  

Magical setup: .073" Gas Port, 14.5" Midlength Barrel, Plain Gas Block, Regular Bolt, Slash Heavy Car Buffer (5.6oz). 

It ate Freedom 155gr Amax Reman, Gemini 155gr Amax Reman, MFS 150gr Steel, PRVI 175gr SMK, German DAG surplus, and Malaysian L2A2 Surplus. Locked back on all of 'em. At this point, I don't even think it's worth playing around with tuning the gun using the adjustable gas key & lighter buffers. Although I might switch over to an A5/AR10 carbine tube though with a Slash XH Carbine Buffer (8.5oz) and Magpul FCS for a slightly longer LOP. 

Look at the sweet concussion in the gif image... the can nailed to the post 5 feet away is rattling and you can see the camera itself being shook 10 feet away.

 

Good to hear. I was late to chime in but have a now 14.5" pinned AAC flash hider which was cut from a Bear Creak arsenal 16" mid. Had a .075" gas port size. Ran a 4.8oz Tridentis buffer with no issues on federal 147-149 nato loads. 

I'm now going to ditch the 4.8oz and go with the heavier 6.5oz when I start going suppressed. 

I have played around with the standard rifle tube and buffer. I used tungsten weights to make it 7.8 oz and can keep going heavier of I like. It doesn't fix over gassing but delays the bolt speed to extract much easier and smoother.

[Joeymac]

I've always heard with AR15s, for max reliability, to "run the heaviest buffer your rifle will lock back on". Something about more momentum moving helps it power through fouling and such. I'm sure It's not the lightest recoiling option, but I can understand why more momentum helps reliability. 

I've actually got an A5 Tube en route and a Slash XH buffer, plus I've got plenty of H/H2/H3 buffers around. I'm going to give my RCA Adjustable gas key a try and tune the gun to cycle the XH buffer perfectly. Then, if I can achieve it,  I'm going to step down buffer weights until it cycles unsuppressed perfectly. this means that I should have the gun setup for prefect cycling suppressed and unsuppressed with only the silencer install and buffer swap. 

And if the strategy doesn't work, then I can fall back the configuration in the video and have a perfect cycling gun unsuppressed and a decent cycling (slightly overgassed gun) suppressed. 

I just want to avoid having to tinker with gas settings every-time I change ammo or the suppressor.

Edited June 8, 2018 by Joeymac