16" or 16.5" Barrel with a Rifle Length Gas System: Why Not?

[jkrispies]

I've been PM'ing and emailing lots of folks about the viability of using a 16" or 16.5" barrel with a rifle-length gas system in a direct impingement gun, and I consistently get one of two answers:

1) I know it works great because I have (or my buddy has) one that gets shot all the time with no issues!

or

2) You can't do that because everybody knows there's reliability issues with dwell time!

So, here's my question: Can anybody please provide either first hand or published or personally derived emprical evidence that proves a dwell time issue truly exists with this type of barrel setup? Or, conversely, who can attest/prove that no issue exists?

I'm totally open to this "gun urban legend" being proven/disproven either way. I'm even good with something along the lines of "it can only be reliable with a low mass operating system" or "you need to limit its use to heavy bullets," etc., etc., but what I really want is better proof than "everybody knows" or "I heard if from a guy who heard it from a guy."

Thanks,

J

Edited January 10, 2015 by jkrispies

[outerlimits]

Two words...mid length.

[MarkCO]

Sure it can be done, but the other parts of the system may have to be adjusted to compensate. It is a SYSTEM with an operational window. If a part takes you outside the window, another part change can move you back inside the window. I have been told I can not run 50s at 2300...Done. I have been told I can not run 320 grainers at 2400 in a .450BM...also done.

Once you leave the vaunted TDP behind, it is amazing what can be some with the AR15... Thankfully JP has shown there are many good options outside the TDP. Sadly, some leave the TDP without understanding the system and materials well enough to make beneficial changes.

[jkrispies]

Mark, thanks for the thoughtful answer. I know that you've done a ton of experimentation with the AR system. Do you have any personal experience with this layout? If you were to take a guess (no garauntees written or implied, lol), do you think necessary adjustments could successfully be made with something as simple as an adjustable gas block and buffer weight experiments?

(On a side note, I'm looking at pistol barrels, and from eyeballing them, it looks to me like these barrels would have the same or less dwell time as a 16" rifle length... but I don't hear complaints about them.)

Edited January 10, 2015 by jkrispies

[CMV]

I'd like to do some testing in this area myself. Voodoo's 16.5" ULW barrel with rifle gas intrigues me as a starting point for a very light 3 gun rifle...

[Vlad]

(On a side note, I'm looking at pistol barrels, and from eyeballing them, it looks to me like these barrels would have the same or less dwell time as a 16" rifle length... but I don't hear complaints about them.)

On a pistol barrel dwell time is probably not as important because of the much higher gas port pressure.

[MarkCO]

It would depend on the load (burn rate and pressure) and gas port size. The are no adjustable gas systems on the market that are truly long term adjustable, so if gas volume volume is what you want to adjust, and orifice system might be helpful.

I'd start with a slow powder heavy bullet load, make sure the gas system does not leak and use a bolt with a high lubricity coating, new gas rings and a great lube. Since Dwell is what you will be chasing, a slower moving bullet gives you that, so don't think it is all about velocity.

And, yes Vlad is correct on the pistol port having much higher port pressure.

Edited January 10, 2015 by MarkCO

[Mark Gale]

@jkrispies - What do you hope to gain with this barrel over a proven configuration? If this is a "because it's there" issue, I'm OK with it. I just don't think your going to find any upside to it.

Edited January 10, 2015 by Mark Gale

[jkrispies]

Eurekia! MicroMOA just provided me with this link on their testing, which helps a lot: http://micromoa.com/?page_id=1190

Mark Gale, I'm trying to gain a few things having to do mainly with balancing the gun, but also recoil impulse. I'm not very concerned with barrel length for the sake of barrel length-- velocity difference between a 16" and 18" is small, and frankly it's easily "correctable" by me knowing my holds.

(NOTE: Before I go on at this point, I'll add that I haven't ruled out an 18" barrel at all-- I'm just asking questions about this system as I search for quantifiable data that has so far been elusive.)

What I want, in order of importance:

1. I want a rifle lengthed gas system for the benefit recoil impulse

2. I want a barrel that is lightweight due to how I want the gun to balance. (I'm still playing with weights to figure this out more precisely, but 1.5-2 pounds is looking like a good number to me)

3. I want a barrel that won't lose MOA with heat, so I prefer a medium thickness barrel (READ: thicker than a "pencil" barrel) though barrel thickness, per se, isn't a deal breaker if I'm confident enough in end-user reports that a particular barrel make holds up to heat

So far, the best candidate I've found that meet all three criteria is the MicroMOA 16" in rifle length. After that, the next closest is probably the 18" Adams Arms and 18" Faxon barrels, but both of those are pencil barrels which makes a tad me nervous.

I'll also add that I've gotten nothing but good reports from users of the 16.5" Adams Arms barrel, and I'm not opposed to that barrel at all... except being short AND a pencil barrel it appears a bit too light for my balance point. That being the case, I think the greater benefit for me--should I go with Adams Arms--is to opt for their 18"... not just because it is closer to my weight point but because it would make all this 16.5" questioning moot.

[jkrispies]

It would depend on the load (burn rate and pressure) and gas port size. The are no adjustable gas systems on the market that are truly long term adjustable, so if gas volume volume is what you want to adjust, and orifice system might be helpful.

I'd start with a slow powder heavy bullet load, make sure the gas system does not leak and use a bolt with a high lubricity coating, new gas rings and a great lube. Since Dwell is what you will be chasing, a slower moving bullet gives you that, so don't think it is all about velocity.

And, yes Vlad is correct on the pistol port having much higher port pressure.

Mark, I actually prefer to shoot heavier bullets (77's, generally) so the bullet recommendation works perfectly for me. I also previously thought this would help with the dwell issue, though it's more coincidental on my end since I already happen to prefer the heavies.

Also, though nothing is settled at all yet--still researching and probably will continue for awhile--I like the looks of the Adams Arms low mass bolt carrier group, and expect to match an AA made bolt to the carrier.

Vlad, thanks for that info!

[MarkCO]

If you plan on shooting full pop ammo, you might just get a Stretch16 and call it good. Those pencil barrels don't pass muster past about 300 yards with the rate of fire typical in 3Gun.

[jkrispies]

Here's a somewhat separate (and complicated) question... What minimum distance is generally considered needed between port and crown for reliable operation across a wide range of ammunition? We'll say 55gr to 77gr? Could it be that the 16.5" Adams Arms barrel is in the "safe zone" whereas a 16" is not? Would a 17" with a rifle length be safely reliable???

[dauntedfuture]

Years ago CMMG used to make a 16" with a rifle length gas system. When I asked them about it I was told that in order to make the system run reliably with a standard operating system you would need full power ammo to make things run. Voodoo has apparently figured something out and make and sell a 16.5 with a rifle length gas system and I don't think there are reliability issues with the system. I believe there are some comments and concerns about how such a thin profile barrel will hold up under a sustained rate of fire in regards to maintaining accuracy over a 30-50 rd COF. Experience tells me that thin barrels walk when they get hot but that's another topic.

[jkrispies]

I believe there are some comments and concerns about how such a thin profile barrel will hold up under a sustained rate of fire in regards to maintaining accuracy over a 30-50 rd COF. Experience tells me that thin barrels walk when they get hot but that's another topic.

Thanks, dauntedfuture! Another topic, true, but an important point!

[Mark Gale]

What minimum distance is generally considered needed between port and crown for reliable operation across a wide range of ammunition?

The size of the gas port is the other half of this equation. That being said, barrels with different size gas ports are going to have different sweet spots.

[MiniUzi]

jkrispies Viable yes. Absolutely.

I'm on the Micro MOA shooting team and I've been running the 16" rifle length gas since July with the Govnah gas block. Mark CO is 100% correct it's a system and it needs to be healthy. The barrels Mico Moa uses has single edge polygonal rifling which lends itself to less bullet deformation and a tighter seal around the bullet which results in less loss of gas around the projectile giving a bit more pressure.

Some awesome rifling coupled with the Govnah gas block, which I think could be considered a long term adjustable gas block, seems to be a solid solution thus far. Due to the design of the gas block plate system we know exactly what each setting for the gas block is since the holes restricting the gas are drilled. We have four 16" rifle length guns in team members hands running a variety of adjusted port sizes for optimal gas ranging from .082 up to .096. depending on the gun configuration. The info on some of the specs is detailed here .

The main issue that seems to be pop up in these discussions is dwell time. How much gas do you get into the gas system and how long do you get it for. What does it take to push that bolt carrier back and cycle the action 100% of the time. We haven't seen issues so far that would point to any dwell time related issues. We have thousands of rounds and dozens of matches in the books with zero issues.

We did have two of our guys decide to see how far they could game their loads and they went single shot right off the bat which was to be expected. They pushed the gas plates over and went to the full gas setting and finished the course of fire (and match) without issue. We have never seen a single issue with this system running full gas. Not one.

We would like to do an objective test on a 16" rifle length barrel this year and put this argument to bed. The only remaining question we need to answer is "will the system degrade, and if so , at what point ?". So far those involved in the project say it's not going to degrade at all and although I believe them I'd prefer an outside opinion. If anyone has suggestions as to a number of rounds and method of testing to achieve this we're listening.

On a side note, if you see any of our folks running around with a Micro MOA logo on their shirt at a match, ask them for trigger time on the gun, try it for yourself. It's in their contract to hand over their blaster and share it with others. If any of you are going to the ATL versa max challenge we'll have a couple there with team members at the end of the month.

-Todd

[jkrispies]

Todd, thanks for the reply! I do like that Guvnah concept! Great info and I like how your team is seeking some outside input.

[StealthyBlagga]

As others have indicated, your are dealing with a SYSTEM, with many interdependent components. When you choose to lock in on a barrel with a very short distance between gas port and muzzle, you need to compensate by adjusting things like bullet weight, port pressure, port size, bolt/buffer mass and buffer spring rate. Certainly this can be made to work, as many have proved.

The challenge you will face is that the shorter dwell time will mean you need gas to flow to the bolt carrier faster to compensate for the shorter-than-normal impulse time. This typically means hotter ammo and/or larger gas port. If the port pressure changes (e.g. different ammo, different environmental conditions etc.), the system may cease to be reliable - in technical terms, you have a "narrow operating window". In the world of military weapons, this dilemma is typically circumvented by increasing gas volume (i.e. bigger port) and by increasing the reciprocating mass... unfortunately, both of these will increase the felt recoil impulse, which is contrary to your stated intent.

In a timed sport where there are no reshoots for malfunctions, I submit that a safer and more proven configuration like a midlength or intermediate gas system would be wiser in the long run. If configured with low-mass reciprocating parts and adjustable gas, I am skeptical you would even notice the difference in handling compared with your dream configuration. If you persist in pursuing your proposed setup, please post your experiences here so we can all learn from the story of your exciting journey.

Edited January 11, 2015 by StealthyBlagga

[EaZeNuTZ33]

Here is the YouTube video from Surefire in Vegas this year. I used a 16.5" rifle gas Voodoo UltraLite barrel. I didn't have any issues shooting paper, a rifle spinner, then long range before shooting more paper and missing way too many times on an offhand plate rack after running.

I've shot 55gr without issue several times, but shot mostly 69gr SMK handloads at major matches and never had issues.

As to the SYSTEM......Syrac GenII gas block, RCA lightweight carrier, Taccom ULW and tune able buffer system, and Rolling Thunder comp. I have no idea how this would work with standard parts in/on the gun and a variety of ammos.....but if buying a rifle gassed carbine length barrel for competition, why wouldn't you buy the supporting parts?

[MiniUzi]

Some more food for thought in regards to the healthy system concept.

In our testing we found a few interesting things that popped up.

#1 Trigger / Hammer system in gun plays a much greater role in how much gas you need to introduce to make the system reliable. We found that the angle on the hammer of some trigger systems actually created friction on the bolt while traveling rearward . This of course caused for more gas to be introduced into the system to make it reliable. Hiperfire triggers have an interesting angle to the top of their hammer which we found allowed us to reduce the gas needed compared to other triggers.

#2 Magazines: Some of us go prone and get our magazine or a pair of coupled magazines on the ground. It's not uncommon to use a clamped pair of magazines as a monopod of sorts on long range shots. Most magazines have a little vertical movement to them.Weight on the gun while prone and resting on magazines can also increase the friction on the bolt while traveling rearward causing more gas to be needed to ensure reliability.

-Todd

[Headworked]

Todd,

Will try to find one of the MicroMoa team members at Atlanta VersaMax to test out your rig

[KPIC24]

Bumped my load a little and have had no issues wih 16.5 voodoo, was a little under gased to start oh and increased the gas port a little. Running 75 gr and 55 gr. Mark I run a 16" ultra lite voodoo all off last year at tarheel and regionals and have had no issues but you get it hot enough and it will wonder for sure. Miniuzi number 2 is the reason I changed port size and it went away after that and bumped load a little

[Therealkoop]

Ive been interested in a setup like this for a while.

Since it sounds like you have to run a larger gas port/opening (as compared to 18" rifle gas), how does this affect the recoil impulse in relation to the 18" bbls?

[jkrispies]

Ive been interested in a setup like this for a while.

Since it sounds like you have to run a larger gas port/opening (as compared to 18" rifle gas), how does this affect the recoil impulse in relation to the 18" bbls?

Great question!

[MiniUzi]

Ive been interested in a setup like this for a while.

Since it sounds like you have to run a larger gas port/opening (as compared to 18" rifle gas), how does this affect the recoil impulse in relation to the 18" bbls?

Good question.

My 18" was a stag 3G and my optimal gas was a .089

My 16" rifle gas Micro moa is .096 gas for optimal gas.

They aren't quite apples to apples in terms of configuration but I do not "feel" any difference in the impulse. Felt recoil is subjective. With that being said I'm working on another project that was spawned by my need to have actual data instead of "yo bro this shoots so flat" type range logic on evaluating the effectiveness of component changes.

This year we have the ability to actually measure the g forces, axis shift etc... accelerator and gyroscope paired with some blue tooth and an android app in a package a little bigger than a quarter. This is a prototype but it works and works well.

I hope in a few months we can have solid data on a number of components and know exactly what they do to the system from a data standpoint. Saying "it feels better" versus saying "this is 20% reduction in force on the shoulder between setting A and setting B" is where I want to be. Data is good thing.

Perhaps I'm not sensitive enough to feel the difference in the 18 to 16" variations but there is more gas with the configuration I choose to run. My setup is not "optimal gamer" . I'm running regular full auto carrier , rifle stock( luth AR) and regular buffer.

A freakishly low gas option so far that we have found is being run by team member JP Thompson. He has .082 gas setting, hyperfire 24c trigger,light carrier, JP Captured spring, Govnah gas block on the 16" rifle and I think a SJC titan break (another important factor for system pressure) . His barrel port is .101 , optimal gas is .082. You can feel the difference between full gas and his optimal gas for sure.

-Todd