Answer Me this. Carbine length gas system: different barrel lengths.

[rickyrick]

So the tribal knowledge says that 16" barrel and carbine length gas system is over gassed. Needs a heavier buffer everyone says.

14.5" barrel and such with a carbine gas system is more optimal. This might suggest that a standard carbine weight buffer is good to go. Makes sense by virtue of the reasons given for the downsides of a 16" barrel and carbine length gas.

So when looking into 10.5" everything else the same; logic would dictate an even lighter buffer would be required as the bullet is in the barrel an even shorter amount of time after the bullet passes the gas port.... Why does everything I find recommend a heavier buffer?

 

[Sharkbite]

Most 10.5" barrels have a PISTOL length gas system and are just as over gassed (maybe more) as a 16" carbine gassed gun.

The cronic problem of over gassed AR's in 556 is the reason for adjustable gas blocks.

Putting in a heavy buffer does not change the over gassed issue. It does mask the symptoms and reduce the effects. An adj gas block will allow you to fine tune the amount of gas actually getting into the system.

 

[MarkCO]

Adjustable gas blocks introduce another failure point and dirft off of the set after a while.

I much prefer to alter the other parts of the system...load, buffer mass, carrier mass, spring rate. The adjustable gas is a good band-aid, but better still is a barrel with a reduced size gas port, or, better yet, an optimally sized one.

 

[rickyrick]

I'm thinking of 10.5 and the carbine gas system, so am I correct in my thinking that it may be slightly under gassed?

 

[MarkCO]

It will not be under-gassed if the port size is right, but the dwell time may be too short. That can be mitigated with reducing buffer mass or using a faster powder, or lighter carrier or reduced power buffer spring. However, that might lead to tight extraction, so a Colt gold spring and black buffer under the extractor might help as well.

It is a system and if you keep the parts of the system in the proper balance, it works. I have built 100% reliable systems with pistol to rifle gas shooting loads from 35 grains at 2000 fps to 350 grains at 2400 fps all on the AR15 platform. All about balance.

 

[rickyrick]

Ok I think you explained what was in my head.

I shot my 10.5 build for the first time and it short stroked a few times. (Failed to strip a few rounds and occasional failure to hold open on last round.

So I decided to keep running chalking it up to new gun needing a little breaking in.

My thinking was, I planned on playing with buffer weights if it didn't straighten out.

About the last four mags after a couple hours of shooting it all straightened out. The gun loaded every round and BHO worked... Had great fun.

Was still considering lightening the buffer.

Has m16 carrier and bolt.

When I got home I found that the ejector roll pin had worked out and bent (obviously had been getting caught on barrel extension lugs). This may have caused the short stroke symptoms.

I've fixed the bolt, but now I still wonder if I need a lighter buffer as well.

 

[MarkCO]

Pop out the roll pin on your buffer, remove one weight, test. If you like it, you can replace the length with a piece of gas line cut to length.

 

[rickyrick]

Excellent
Thanks

 

[marine6680]

The closer the gas port to the chamber, the higher the gas pressures (port preasure) going into the system. The farther away from the chamber, the lower the pressures.

The smaller the gas port hole, the less gas can be funneled to the rear. The larger the port, the more gas can get rearward.

The closer the distance of the gas port and the muzzle, the less gas can funnel rearward before the bullet exits the barrel, and less time the pressure has to impart force. The farther apart the gas port and muzzle, the more gas can funnel rearward and the pressure has longer to impart force. (dwell time)


Its all a balancing act... Higher port pressures need less dwell time and port size/amount of gas, to impart sufficient force to the system for reliable cycling. The opposite for lower port pressures.


I have read one civilian trainer for the military recommend an H3 and extra power buffer spring for a carbine length gas system on a 16in barrel when running 5.56 spec ammo. That was his personal setup and it was claimed to run reliably for thousands of rounds.


Since manufacturers tend to make their gas ports on the larger side, in order for the rifle to run reliably with many type of ammo (like cheap steel cased) and when the rifle is in various states of care/cleanliness... Most rifles are very over gassed... So heavier Buffers are better.

Short barrels and short gas systems amplify the problem... This is why heavier Buffers are recommended.

 

[Fishbed77]

Most 10.5" barrels have a PISTOL length gas system and are just as over gassed (maybe more) as a 16" carbine gassed gun.

In my experience, a carbine gas system works perfectly with a 10.5"barrel. I use an H2 buffer and get perfect reliability.

FYI - the Mk.18 CQBR also uses a 10.5" barrel and carbine gas system.

 

[rickyrick]

While I think the ejector pin coming halfway out was the root cause of the short stroking symptoms; I went ahead and opened up the buffer.... Wasn't the usual 3 steel weights, it was one steel bar inside.

I went ahead and ordered a buffer and action spring from a reputable source. I'll tweak the weights later if needed.

 

[Mobuck]

I must be from another planet. I feel the adjustable gas block is a far better solution for the "oversized gas port syndrome" than heavier buffers or buffer springs. Heavier buffers/springs may resolve some ejection problems but add the possibility of "over drivng" the BCG forward due to the increased weight/momentum of the overall reciprocating assembly. This batters several areas which are important to the life of the system and could contribute to the rare but possible slam-fire.

 

[tirod]

High speed video of a carbine gassed rifle shows light buffers allow the bolt to rebound and unlock. Increasing buffer weight is the recommended answer per Crane on the MK18.

Nobody wants a primer to ignite just about the time the bolt is unlocking from having too little mass.

Rebound is often caused by high cyclic speeds, which are caused by a lot of gas. Crane was working with a tight window of timing but a wide variety of bolt speeds - getting a gun to run right at the armorer level was a lot easier by recommending a buffer weight change.

 

[rickyrick]

Thanks to those that contributed.

I think the budget buffer and action spring was the problem.
The spring was a full inch longer than a standard carbine spring and the buffer was a pretty cheap copy at best. I have a new buffer and carbine spring in the gun and an h3 buffer on the way. I plan to use the weights from the h3 to adjust if needed. I plan on dividing them up between the 16" rifles if the pistol runs fine

 

[Dragline45]

This chart may be helpful.

Just for reference, an optimal dwell time is 0.200 ms

Properly sized gas ports can make all the difference. For example, notice that the 14.5 mid length has a dwell time of only 0.139, which is why when buying an off the shelf upper or rifle in a 14.5" midlength it's a good idea to get one from a reputable manufacturer like BCM or Daniel Defense who have the 14.5" midlengths down to a science. Alot of cheaper manufacturers who put out 14.5" midlengths, or some who build them themselves, you will see will have a tough time cycling weaker ammo, and you will often have to bump down to a standard carbine buffer to get it to run. On the other hand alot of these cheaper manufacturers knowing this will purposely overgas their rifles to get them to run all kinds of ammo, pretty much negating benefits of a midlength gas system. Though this trend goes across the board for all barrel lengths and gas systems with low tier AR's, most will be overgassed on purpose just so they run reliably with a wide variety of ammo.

Even though a 14.5" midlength is not an optimal dwell time, my BCM 14.5" midlength will eat cheap .223 wolf steel cased with an H buffer because it is built and gassed properly. On the other hand my buddy who built a couple 14.5" midlengths has one that will only run full power 5.56 unless he drops down to a lighter carbine buffer.

There is a reason for each gas system they have recommended barrel lengths which is usually 16" for midlength, 14.5" for carbine, and 20" for rifle, these three by no suprise have some of the most optimal dwell times on the chart.

 

[rickyrick]

Thanks,

That data tells me that a lighter buffer would be required for a 10.5 barrel and carbine gas, all other things being equal. But, as I read through forums, the consensus is heavy buffers.

The barrel that I'm working with is a 10.5 carbine gas from aero precision
I emailed them about gas port size but haven't heard back.

I'm gonna try again with a reputable spring and carbine buffer. The one I used came with the reciever extension, but was not good quality. I liked the tube, but cut the corner on the spring and buffer because it came free with the tube. Lesson learned.

 

[Dragline45]

Yeah if you were experiencing short stroking I would def try dropping down to a lighter buffer. Seems like after your initial break in the rifle started to run better, so I would suspect a lighter buffer will solve any short stroking issues you had. Also, not sure what ammo you were running, but if you haven't yet try running some full power 5.56 like M193 through it. If all else fails you can open the gas port a bit, but I am betting a lighter buffer will be enough.

 

[rickyrick]

Yup,
New carbine buffer and spring from a good vendor made this thing run like a champ. Burned thru a good chunk of my ammo stash. These AR pistols are a hoot to shoot.

Mix of bulk 5.56 and .223 not one failure.

I replaced the buffer and spring, double checked gas port alignment...

Wish I'd built one of these years ago.

 

[Sharkbite]

The next step is to do a form 1 and SBR the little guy. A stock makes it even COOLER

 

[rickyrick]

I bet you are right...
I put more ammo thru it in one setting than I ever have with my other firearms