Large frame 308 build function help request

[RHutch]

I recently put together a large frame AR and am having issues with bolt hold open and with the bolt picking up a second round.
Thinking my issue is in the receiver extension and buffer but not having put one of these together before, I’m asking for opinion on the fix.
Parts list:
-Zev matched receiver set
-Criterion 16” barrel, rifle gas
-Superlative adj gas block, set wide open for the moment
-JP FMOS carrier and HP bolt
The following I purchased as a kit supposedly for large frame AR setups IIRC
-Vltor RE-A5 receiver extension (measures 7 15/16 inside length)
-Vltor A5-2 buffer (measures 3.995” in length)
-Vltor spring.

When I manually pull the BCG back with the charging handle as far back as it will go, bottoming it out in the receiver extension, the bolt clears the back of the cartridge by only a mm or two. I can manually get the first round to load but it will not pick up the next round nor will it hold the bolt open if I only load and fire 1 round.
Im thinking the BCG cannot physically travel far enough rearward to operate properly.

below is a shitty photo of my holding the BCG as far back as it will go with a once fired piece of brass in a Larue magazine.
it barely clears it. Seems a bit short to me.
Comparing the BCG stroke of an AR15, the entire BCG travels beyond the back of the magazine and ejection port by a much larger margin than this photo shows. I do understand that they are different platforms and comparing the two at all may be a fool’s errand.

Do I have the wrong receiver extension and/or buffer?

Any other ideas?

I will be running this suppressed 100% of the time. So far, all my testing has been without a can.

 

[wade2big]

You need a carbine length buffer.

 

[USK308]

I recently put together a large frame AR and am having issues with bolt hold open and with the bolt picking up a second round.
Thinking my issue is in the receiver extension and buffer but not having put one of these together before, I’m asking for opinion on the fix.
Parts list:
-Zev matched receiver set
-Criterion 16” barrel, rifle gas
-Superlative adj gas block, set wide open for the moment
-JP FMOS carrier and HP bolt
The following I purchased as a kit supposedly for large frame AR setups IIRC
-Vltor RE-A5 receiver extension (measures 7 15/16 inside length)
-Vltor A5-2 buffer (measures 3.995” in length)
-Vltor spring.

When I manually pull the BCG back with the charging handle as far back as it will go, bottoming it out in the receiver extension, the bolt clears the back of the cartridge by only a mm or two. I can manually get the first round to load but it will not pick up the next round nor will it hold the bolt open if I only load and fire 1 round.
Im thinking the BCG cannot physically travel far enough rearward to operate properly.

below is a shitty photo of my holding the BCG as far back as it will go with a once fired piece of brass in a Larue magazine.
it barely clears it. Seems a bit short to me.
Comparing the BCG stroke of an AR15, the entire BCG travels beyond the back of the magazine and ejection port by a much larger margin than this photo shows. I do understand that they are different platforms and comparing the two at all may be a fool’s errand.

Do I have the wrong receiver extension and/or buffer?

Any other ideas?

I will be running this suppressed 100% of the time. So far, all my testing has been without a can.


View attachment 8249589

What is the weight of your buffer? Where does the gas tube sit inside the upper?

Also, if you have a digital caliper, you should measure the gas port hole size. (When I built my AR10, my gas port measured 0.0835" on an 18" barrel with rifle-length gas system. To cycle properly, the gas port needs to be ~0.093" FOR AN 18" BARREL. Once I drilled the gas port out to 0.0945" ... my rifle cycled like a champ! Note: I also have a Superlative Arms Adjustable Gas Block and once I dialed the gas vs mass in, the AGB got dialed in a 19 click from closed (1 more than the normal 18), to work perfectly for me.

I got A LOT of great help from @98Z5V on the "Building a 308 AR forum."

 

[WhiteDingo]

You can't use an A5 length buffer in the A5 tube with a 308 gun, as the bolt travel is longer. Large frame guns use a special length buffer in a standard carbine tube, or a standard carbine buffer in an A5 length tube.

Swap to a standard AR-15 carbine length buffer of the appropriate weight and you'll be able to cycle the bolt fully.

 

[B y r o n]

Are you able to manually lock the bolt to the rear?

 

[RHutch]

Barely

Are you able to manually lock the bolt to the rear?

 

[Jsp556]

Put a H3 carbine buffer(AR15) in it and an armalite AR10 carbine spring then you should be good.

 

[RHutch]

I will hunt down an H3 carbine buffer and possibly a spring. Thanks all

 

[Len5E1]

It’s pricy but I’m pretty sure you can run the JP SCS in the a5 tube. I run one on my ar10 in a rifle tube and it’s so smooth.

 

[USK308]

It’s pricy but I’m pretty sure you can run the JP SCS in the a5 tube. I run one on my ar10 in a rifle tube and it’s so smooth.

I have one in my Ar10. Just have to use the spacer for A5 tube!

 

[LRRPF52]

I recently put together a large frame AR and am having issues with bolt hold open and with the bolt picking up a second round.
Thinking my issue is in the receiver extension and buffer but not having put one of these together before, I’m asking for opinion on the fix.
Parts list:
-Zev matched receiver set
-Criterion 16” barrel, rifle gas
-Superlative adj gas block, set wide open for the moment
-JP FMOS carrier and HP bolt
The following I purchased as a kit supposedly for large frame AR setups IIRC
-Vltor RE-A5 receiver extension (measures 7 15/16 inside length)
* Vltor A5-2 buffer (measures 3.995” in length) *
-Vltor spring.

When I manually pull the BCG back with the charging handle as far back as it will go, bottoming it out in the receiver extension, the bolt clears the back of the cartridge by only a mm or two. I can manually get the first round to load but it will not pick up the next round nor will it hold the bolt open if I only load and fire 1 round.
Im thinking the BCG cannot physically travel far enough rearward to operate properly.

below is a shitty photo of my holding the BCG as far back as it will go with a once fired piece of brass in a Larue magazine.
it barely clears it. Seems a bit short to me.
Comparing the BCG stroke of an AR15, the entire BCG travels beyond the back of the magazine and ejection port by a much larger margin than this photo shows. I do understand that they are different platforms and comparing the two at all may be a fool’s errand.

Do I have the wrong receiver extension and/or buffer?

Any other ideas?

I will be running this suppressed 100% of the time. So far, all my testing has been without a can.


View attachment 8249589

That jumped out at me immediately. Your picture is exactly what I would expect to see if you use an AR-15 A5 buffer with an A5 RET on an AR-10 build.

The A5 RET is the best RET you can use for a carbine stock AR-10 build, but it’s meant to be used with an AR-15 carbine-length buffer.

ArmaLite figured this out back in the day when they did the engineering on their first ArmaLite AR-10B carbines. The AR-15 carbine RET was just too short, didn’t allow enough buffer and spring to consistently manufacture a reliable AR-10 carbine, hence the birth of the ArmaLite AR-10 RET, later referred to as the Vltor A5 RET and buffer system. Vltor figured you could put a more substantial back-end on the AR-15 with the AR-10 carbine RET, but with a buffer that was between the length of a carbine and a rifle-length, with more weight in it like a rifle buffer.

If you ever look at the old DPMS carbine buffers for the AP4, they are these tiny little pathetic-looking things that don’t make a lot of sense for an AR-10 action. They’re good for AR-15 PDWs with even shorter RETs with a really stiff spring, but not .308s.

Once you put a heavy AR-15 carbine buffer in it with a stiff rifle spring, you should be good to go shooting suppressed.

16” RLGS will be good for 100% suppressed as well due to the limited plug dwell on that, combined with the bore volume. It should handle smoothly. 16” CLGS .308 suppressed is a PITA unless you run a tiny port or flow-through can. 18” MLGS suppressed can be abusive to the action with the cyclic rate as well.

Please follow-up with the thread once you get it running so others can see why you need an AR-15 carbine buffer length with the A5 RET for AR-10s, and hopefully you will have a reliable blaster with the suppressor.

 

[mettee]

Once you put a heavy AR-15 carbine buffer in it with a stiff rifle spring, you should be good to go shooting suppressed.

Agree 100% here, go as heavy as you can with the buffer, you will need it with most cans unless flow through. I'm running Clints Heavy Buffer in a 7- 5/8 extension with the rifle spring. The rifle spring is another piece worth emphasizing.

 

[RHutch]

H3 and rifle spring was the ticket. Runs great both with and without the can with 6 clicks open on the gas block.
Accurate too which is a plus.
Shot 2 168FGMM at 100 to verify operation and remind myself how that trigger felt, screwed on the can and put three in the black dot. Ran 6 more rapid to verify suppressed operation without a hiccup.
Got to get it outside to check ejection pattern and get it broken in.
Thanks to you all.