The way I understand it, the original configuration of the M16 is the ideal condition, and ought to be emulated as closely as possible, when using shorter gas systems. I suppose this can theoretically be done by scaling that original ratio down; if you want a shorter barrel on a shorter gas system, you shrink the amount of barrel beyond the port in a way that keeps that ratio as similar as possible, while also reducing port size due to the higher pressures experienced nearer the chamber. If I am wrong in that understanding, please help me out.
All that being said, if one wants to configure a gun with a silencer, and doesn't care if the gun is able to function properly without it, and wants to use say, a 12 inch barrel, is the weapon better served with a carbine gas with a reduced port, or a mid gas with less dwell time, in regards to reliability and shootability as a fighting carbine. I understand this is a loaded question; I am really just hoping for y'all to help me think through it, and I understand there's not a right answer. I look forward to your informed opinions.
Carbine gas, no question.
A few companies have started to really get behind 12.5 midlength. I have a centurion 12.5 mid, which I've only shot suppressed, and its "operational window" is noticeably tighter. The lower pressure from midlength combined with the short dwell time doesn't leave much ability to push through somewhat dry or fouled conditions. A can that keeps system pressure elevated for a bit longer ("high back pressure") might do better with my specific setup, but it would still be a compromise from the carbine gas length, and it's still extracting at a higher chamber pressure, so it's going to be inherently more prone to extraction and cycling issues.
The upside with 12.5 mid is that it's a tight gun that handles extremely well, and it shoots quite soft and flat. It's my favorite rifle currently, but it's not one I'll keep loaded and handy for use at a moments notice.
Would I be able to play with my buffer weight, open the port a touch, and probably achieve a weapon that will cycle over a greater range of conditions? Yes, but that's a lot of personal effort, time, and ammo to do the testing needed to know it will perform well in those conditions. I'd much rather get a 12.5 carbine gas barrel from the hop, run it wet, and avoid all of the "what ifs" on a gun I need to bet my life on, and deal with the slightly higher gas system pressure and cyclic rate that affects perceived recoil.
Dwell time matters because we're talking about force (gas pressure) working over a period of time, as short and imperceptible as that may be. During this time the gas pressure gives the initial acceleration to the carrier, which is unlocking a still-pressurized bolt. When the system is uncorked by the bullet leaving the barrel your work has to be done as the pressure in the system is going to drop to where it can no longer do much more than puff into a cloud around the gun.
The 20" rifle length is "ideal" because the pressure dynamics of unlocking and extracting the spent casing, and pressure present over the time available, determines things like the mechanical timing of the cam path for the bolt, and the amount of mass and spring pressure needed for the whole thing to run well - it's what the whole thing was designed for. Anything shorter has to run at higher pressure and needs to be rebalanced to get back in line to the original design, or some compromise thereof. But this is why CRANE testing for the Mk18 led to carbine gas 10.3" as the absolute shortest "reliable" configuration, but it's still a configuration that sees far more extraction issues, much higher wear on parts, needs heavier buffers and springs, benefits greatly from improvements like LMT's elongated cam path, Vltor's A5 buffer system, or some of the other enhancements that have all been centered around "fixing" overgassed short AR's.
I was wrong; it seems there is a right answer.
So, another circumstance, to maybe help me further understand: Say I have the ideal 20" gun, and I want to use a conventional silencer full-time. How much barrel could/should I lose, forward of the gas port, in order to maintain the ideal operation characteristics? I understand that a silencer has a different effect on things than any amount of rifled bore does, but is there a balance point?
I can't answer that, and I don't know of anyone who can. Instead, I'm going to ramble on for a while and hope this helps.
I don't know of anyone else that could or would go through the expense and effort of creating a model - computational flow dynamics software, until very very recently, was a software package that started at around $10-20,000 per license, and is the tool one would need to make a predictive model for this, and we haven't even started on how to setup a test rig to record pressures in the system to validate it. That level of engineering on AR gas systems might exist at one or some of the bigger military industrial complex outfits (Sig and Knights come to mind), or certain smaller companies, but it would be proprietary R&D info they'd be pretty well served to not share. Most gun companies, even a lot of the "big" well known ones, lack the engineering resources required (if they have any at all). If a suppressor company is working on that level I wouldn't expect them to care about modeling the system of the gun at all.
The typical method that's been used since the military got serious about adopting a gun shorter than the 20" rifle has been to guess and check. Mission dictates overall size, sets barrel length, collapsible stock, equipment and designs available at the time, bureaucracy, all that other stuff to design the gun. Final port selection starts with an educated guess and is refined in testing. Shoot the crap out of a lot of guns, to the point of total failure (because ports erode), and record lots and lots of data. Make a final selection based on weighted criteria, where "the gun works" is high on the list and "operator comfort" is quite low on the list. (and as an aside; this is why I bow to the knowledge of someone like fritz on here- bullets through barrels counts for a lot more than "I think it should work" and the ponderings and posturings of engineers with insufficient data)
"Modern" mil standard gas ports.
Mk18, 10.3" - .070 (carbine length)
M4, 14.5" - .0625 (carbine length)
Mk12 18" - .0995 (rifle length)
M16 20" - .093 (rifle length)
Keep in mind these are for a specific recipe - specific ammo, spring/buffer, vendors with consistent gas efficiency, unsuppressed, etc. - but they're all meant to deliver consistent function in relatively the same operational window. So we have some data on the subject of achieving the same performance by only changing dwell time, and we know what the tradeoffs are. Unfortunately info for more civilian applications outside of mil testing and anecdotal accounts is hard to come by, and knowing what to do with that information isn't readily apparent from a "This gun is just legos, parts are parts" mindset toward building ARs.
And to go back to that "ideal" - the 20" rifle gas port has had a change in specification a few times over the 60-odd years it's served. But so has the ammo, so has the manufacturing, so have a lot of things. So your baseline "ideal" is not necessarily as much of a well-defined target as you might be thinking. And tying things together - the military has been tuning these guns over those 60 years, and their "ideal" for what the entire fighting force needs in a gun somewhere between new and rattled out, using their maintenance schedule and issued lubricant, might not be the ideal for you and your application/mission.
And then you want to add suppressors into the mix. So what is a suppressor actually doing from a gas dynamics perspective, and why do we care so much about "backpressure"? When the bullet exits the barrel it uncorks the pressurized system. The suppressor is an expansion chamber for that gas, causing pressure to drop by increasing volume. The pressure is still well above atmospheric pressure, but it is significantly below the pressure present during unlocking. Whether or not this gas can contribute toward "dwell time" depends greatly on the design of all the parts present, how much gas is still in the system, where it's at, and what pressure it's at. Gas will find the path of least resistance to equalize the pressure. If the bullet is still in the can and doing some amount of sealing by creating an orifice with the bore of the suppressor during extraction, not only are you adding to the force and stress that goes into unlocking and extracting, the gas will want to flow out of the chamber, and through the gas port and tube into the action. Delay unlocking, even by a small amount like with a buffer weight, and you've given yourself enough time to eject more gas out of the bore and cut down the amount flowing into the gun and ultimately the shooter.
So back to your question - how much barrel would you chop to make adding a suppressor full time have "no affect" on gas dynamics? Well, knowing that I can't answer that but wanting to know more; I would ask what tradeoff you'd give up to keep the rest as balanced as possible. And what are you really trying to achieve, and how does the addition of the suppressor without removing any barrel length change that? And to ground the theoretical; if I could give you an answer you'd need to be able to quantify how your gun/suppressor/ammo differs from the model I used to know whether the information is correct.
So what do you think?
I think I need to read that at least two more times, before I can respond in any meaningful capacity. Thank you for your thorough replies, Rustpot.
From your first reply:
How is the mid, with or without the silencer, extracting at a higher chamber pressure than the carbine?
My curiosity, boiled down, is: when we're considering a full-time suppressed AR15, why is the discussion always more about gas port size and recoil components, than it is about gas system length and dwell time? It seems to me that the ratio of those two factors could be very meaningful, when combined with the proper gas port, of course. Perhaps it is because adjustable gas blocks, springs, and buffers are easier pieces to fiddle with, when compared to modifying barrel proportions.
For most of the last couple years, my priorities were pleasant recoil, minimal gas in the face, and minimal filth in the gun, when using suppressed rifles. I was willing to make compromises in reliability, in the name of shootability and cleanliness. My priorities have changed; I find myself caring less about recoil impulse and cleanliness. I think the gas in the face will remain mitigated with a properly configured rifle anyway, but even it is not so much of a priority anymore. I also want to avoid any adjustable pieces, and vented carriers. I use a silencer all the time, so I'd rather tune with a BRT tube than an adjustable block. The vented carrier isn't a deal breaker, but I'd like to minimize the noise it may contribute.
I have two rifles I think may be good candidates for this approach: One is a 16" with rifle gas, and the other is a 14.5" with mid gas. Based on your assessment of the 12" hypothetical, it seems the 16" rifle setup may lack the dwell time needed for healthy reliability, with the rifle port pressure. Where it all confuses me though, is that gun currently has a vented carrier and an H3 buffer with carbine spring, with a presumably .093" port, and has never had a hiccup. I don't have thousands of rounds through it, of course, but it's never given me a problem, even with that counter-intuitive short dwell time, combined with lower port pressure. I'll consider your replies more, and respond more, later. If you have any additional notes, based on my comments in this post, I look forward to hearing them. Thanks again.
It is likely relevant that I am, so far, using carbine recoil assemblies. I have considered the A5, as it seems quite proven, at this point, but the varying offerings from Sprinco have me wanting to stick to carbine.This message has been edited. Last edited by: KSGM,
The easy answer is an adjustable gas block.
Determine the barrel length and suppressor configuration that is optimal for your uses.
Most non custom AR barrels are ported to run unsuppressed. Most of them are also ported for a margin of reliability with crappy ammo. Higher end barrels usually are ported for high quality ammo and may be undergassed with low powered steel case ammo like Tula or Wolf.
Adding a suppressor to a properly ported barrel will make it overgassed.
Throttle down the gas with an adjustable gas block and you can tune a rifle to run optimally with a suppressor. You can also choose a happy medium where it will run reliably with or without the suppressor, at the expense of a little more recoil and gas to the face when suppressed.
Buffer weight and spring rate also play a role in this equation.
My advice would be for you to choose the barrel and suppressor setup that meets your needs. Then tune the gun with via buffer weight and adjustable gas.
I have fired tens of thousands of rounds through AR15s equipped with both Syrac (out of business) and JP adjustable gas blocks. I have never had a failure of a gas block, despite them being often pointed to as somehow adding a weak point to the rifle.
Over time, the gas will eventually cause the gas block to seize and become non adjustable. The solution for this is a few days of putting a few drops of kroil onto the adjustment screw. Eventually the kroil will free the adjustment screw.
I have never been unable to free a frozen adjustment screw with applications of kroil and a little patience.
There are also adjustable bolt carriers on the market. I have never used one so I cannot comment on their efficacy.
I can say that tuning the gas on an AR that is shot suppressed can dramatically reduce, almost going so far as to state eliminate, gas to the face when shooting a suppressed AR.
If you are new to suppressed ARs, you will be shocked how much dirtier they get. They function fine when dirty so long as they were lubed properly initially. But you will need to increase your cleaning frequency somewhat, depending on your current habits.
I do not white glove clean. Wipe off the goo form the carrier, and out of the upper. Relube and roll on. I rarely to never use a cleaning solvent on anything other than the bore.
Relative to a 20" a 12.5 barrel will be extracting at higher pressure, is what I think I meant. Choosing carbine over midlength, or choosing a 16" rifle gas setup isn't because these things won't work. The latter has been fielded in combat, even.
I've written a response to this a few times and deleted it. Email coming.
I was familiar with JP, but not their adjustable blocks. Those look quite nice indeed; much more robust than other offerings. I wish they made that .308-dedicated one in a 5.56. I'll consider JP, in the future; and I don't disagree with your assessment of the practicality of an adj block; they just don't often work out, in the style of rifle I have been preferring over the last few years.
So, my 16", rifle gas, dedicated-suppressed rifle ran into some trouble the other night. I really just hadn't shot it enough yet, to reveal some bugs. It had about 120rds through it, in relatively short order, and began to act a bit sluggish, to include a pitiful performance in chambering on a reload, when held at high port. I have since removed the vented carrier, and swapped to a stronger recoil spring, while retaining the H3 buffer, and still with a (presumably) .093" gas port. Ran some diagnostic rounds through it, to include a bolt catch reload, and it is clearly much more reliable, but has the recoil impulse to go with it. I'll likely try a slightly reduced-port BRT tube, on top of the recent changes, in an effort to find a happy medium. It'll have to wait though, as this gun is destined to use a different (still pending approval) silencer, which may have different performance characteristics. I also need to ensure proper function across the different types of ammo I intend to use.
I'll instigate a bit of a thread drift, based on my previous comment. When attempting to remedy the gas surplus of adding a silencer, why is an adjustable block always preferred, while things like buffers and springs are seen as band-aid solutions? It seems to me that the gas boost, when balanced by a heavier buffer and stronger spring, could result in an ultimately more reliable gun, albeit with some arguably undesirable recoil and wear characteristics. Using the above example of a full mag bolt catch reload with a fouled gun held toward the sky: a gun whose silencer was balanced by an adjustable block, while retaining, we'll say, a carbine buffer and spring, could choke; while one with the gas port unchanged, and a spring and buffer adjustment, would power through.
I apologize I haven't sent that email. I haven't found the time to write something a bit longer that won't be hard to follow or sound like complete nonsense without a longer conversation about related topics.
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