16" Midlength AR Problems
- Thread starter Noah
- Start date
- Status
Another anomaly I never understood.
Bushmaster introduced the Dissipator concept and got it right by using the Carbine gas length and mounting another false gasblock/sight tower in front of the functioning shaved top gas block.
Other people tried copying the design, on a budget apparently, that used the one rifle length gas system on a 16" barrel to cover both and with far less than success.
I put around 10,000 rounds through my early Bushmaster Dissipator with no major hiccups before rebarrel.
That rifle ran very well and convinced me there might be some worthy merit to the M16 type rifle after all.
Actually, it still does, kinda, I still have the Lower assembly.
Sam1911
Moderator Emeritus
Most likely because of the NFA 16" restriction on barrel length. If you're going to have to have a 16" barrel, might as well have a gas system that's not so short and harsh as the carbine gas system, and doesn't require adding another "fake" gas block up forward to hold the front sight up.
Middys work really well, when the parts are matched to each other well (just like rifles or carbines) and are generally more reliable (or at least more flexible in ammo) than carbines. Not quite as gentle as rifle-length, but more so than carbine gas systems to a degree that is significant.
I don't think I'd say Bushmaster "introduced the ... concept." Colt did in 1965 with their 605 carbine.
Bushmaster came out with the Dissipator NAME and the fake forward gas block/FSB idea, and yes it works just fine.
Middys work really well, when the parts are matched to each other well (just like rifles or carbines) and are generally more reliable (or at least more flexible in ammo) than carbines. Not quite as gentle as rifle-length, but more so than carbine gas systems to a degree that is significant.
I don't think I'd say Bushmaster "introduced the ... concept." Colt did in 1965 with their 605 carbine. Bushmaster came out with the Dissipator NAME and the fake forward gas block/FSB idea, and yes it works just fine.
MachIVshooter
Member
Proper dwell time and aesthetics. The ability to mount a conventional bayonet is a fringe benefit that few people care about.
It took the rest of the industry over a decade to catch on, but Armalite's middy system has become more popular than carbine gas for 16-18" rifles, and is even frequently found on 14.5"/14.7" (the latter of which does not make sense to me).
MachIVshooter
Member
Probably could have got it to run everything by either increasing port size or using a back pressure increasing muzzle device like the Noveske KX3/KX5. That said, running a carbine or middy gas system under the hand guards with an A2 FSB further forward acting only as a sight is optimal.
If all else fails see if a KX3 or similar device will give the back pressure needed to force it to run. Gas ports aren't to finicky in my experience. A wooden dowels down the bore and a good drill gun have served me well in the past with AK and AR.
My AR troubleshooting Algorithm:
Gas Port -> Gas Block -> Gas Tube -> Gas Key -> BCG Weight -> Buffer Weight -> Buffer Spring -> Check for burrs or like on moving parts -> Polish BCG contact surfaces -> Add muzzle device for pressure like KX3 or Suppressor -> Reem for .223 Wylde chamber -> Handload dangerously hot ammo to force to run or sell it
My AR troubleshooting Algorithm:
Gas Port -> Gas Block -> Gas Tube -> Gas Key -> BCG Weight -> Buffer Weight -> Buffer Spring -> Check for burrs or like on moving parts -> Polish BCG contact surfaces -> Add muzzle device for pressure like KX3 or Suppressor -> Reem for .223 Wylde chamber -> Handload dangerously hot ammo to force to run or sell it
kimberkid
Member
- Joined
- Feb 5, 2010
- Messages
- 1,653
The reason "full spec 556 ammo" Wolf Gold and works is because it uses a faster burning powder and builds more pressure quicker than Silver Bear 62's and the like.
The problem with a 16" barrel and a Midlength gas system is that there isn't enough barrel between the gas port and the end of the barrel ... in other words, your gas pulse is too short and all pressure in the barrel is relieved when the bullet leaves the barrel.
I don't think an adjustable gas block is going to do you a bit of good unless you open up the gas port because you still aren't getting any more gas into the gas-tube.
The right way to fix it would be to switch to a carbine barrel which moves the gas block back further and gives you more barrel between the port and end of the barrel ... this gives you a longer gas pulse before the bullet leaves the barrel and relieves all pressure.
A KX3 may help as it does provide a bit more back pressure but I wouldn't count on it.
The problem with a 16" barrel and a Midlength gas system is that there isn't enough barrel between the gas port and the end of the barrel ... in other words, your gas pulse is too short and all pressure in the barrel is relieved when the bullet leaves the barrel.
I don't think an adjustable gas block is going to do you a bit of good unless you open up the gas port because you still aren't getting any more gas into the gas-tube.
The right way to fix it would be to switch to a carbine barrel which moves the gas block back further and gives you more barrel between the port and end of the barrel ... this gives you a longer gas pulse before the bullet leaves the barrel and relieves all pressure.
A KX3 may help as it does provide a bit more back pressure but I wouldn't count on it.
MachIVshooter
Member
A 16" middy has the correct dwell time; same port-to-muzzle distance as a carbine length 14.5 and a rifle length 20". That was the entire purpose behind the midlength gas system, and with a correct port size, they run just fine. A 16" gun with a carbine gas system will tolerate an undersize port or weak ammo better because they are over gassed to begin with; downside is, they are more violent-most notably producing a sharper recoil impulse.
wally
Member
Total BS, I've shot many thousands of rounds of the stuff and while its maybe ~2 moa accuracy wise and perhaps "dirtier" than some, its functional.
Sam1911
Moderator Emeritus
As MachVI said, the reason to have a Midlength system is so that the gas system works correctly with the 16" federal minimum length.
The original 20" barrel has a gas system that leaves about 6-1/2" of barrel in front of the port. The 14.5" M$ barrel's "carbine" gas system leaves about 6-1/2" of barrel in front of the port. A "middy" gas system on a 16" barrel leaves about 6-1/2" of barrel in front of the port. That's how it should be.
Get too much barrel out in front of the port (say with a "carbine" gas system on a 16+" barrel) and you've got too big a push on the bolt carrier. Cut the barrel too short in front of the gas block (like the old Colt 605s) and the gun may not cycle well. You can use non-standard (big or small) port sizes to try and compensate for this, but it's better to get the lengths right to begin with. And remember that the pressure is way higher closer to the chamber. So rifles are a bit more gentle (softer recoil) than middys which are a bit more gentle than carbines.
Zerodefect
member
IMO: The 14.5" carbine is the way to go to get a legal 16" rifle. And 14.5" middys are great for race guns. Softer cycling and ammo sensitive.
A Colt 6920 cut down to 14.5 with a PWS556 comp pinned on is a sweet thing.
A Colt 6920 cut down to 14.5 with a PWS556 comp pinned on is a sweet thing.
Noah, your rifle isn't undergassed. The problem is with underpowered ammo. If you tune the rifle to run reliably on underpowered ammo, carrier speeds will be a bit high when using full power ammo, giving harsher than necessary recoil, causing bolt bounce and increasing the chance of extraction and ejection problems. Tune the rifle to meet your needs, just be aware of the trade-offs. TANSTAAFL
carbine85
Member
So why do other mid lengths like mine work with any ammo your feed it? I'm also running the H buffer and FA carrier.
Gordon
Member
16" (or 18") is correct for mid length gas as noted. Unless your gas block is perfectly located (preferably pinned or at least index dimpled for the set screws as my Noveske 16" Rogue hunter is) there can be problems. The diameter of the gas hole is of course really important and unless it was checked and reamed after it was drilled there can be problems on inexpensive barrels. I would bet the hole dia. or the gas block alignment is the problem.
dprice3844444
member
http://www.ar15.com/forums/t_3_4/550213_Gas_port_size_and_distance.html
noah.check gas port hole diameter.will above thread help?
http://www.ar15.com/forums/t_3_4/625909_Barrel_gas_port_size_question.html
Colt specs the USGI M4 14.5" gas port diameter at 0.063", and the USGI M16 20" at 0.093" The ideal gas port diameter for a 16" mid-length gas system should be 0.078".
4th reply down.
noah.check gas port hole diameter.will above thread help?
http://www.ar15.com/forums/t_3_4/625909_Barrel_gas_port_size_question.html
Colt specs the USGI M4 14.5" gas port diameter at 0.063", and the USGI M16 20" at 0.093" The ideal gas port diameter for a 16" mid-length gas system should be 0.078".
4th reply down.
Last edited:
Because your rifle is tuned to run on lower pressure ammo, or perhaps you haven't tried low pressure ammo yet. I have a 16" middy that I have to change to a lighter buffer to shoot low pressure steel cased ammo reliably. However, recoil is harsher if I shoot full power ammo with the carbine set up that way.
Keep in mind some rifles are more tolerant of changes in ammo pressure than others. Some simply need to be shot more to be broken in. If you have a rifle that runs 100% reliably on full pressure ammo but chokes on low pressure ammo, there's nothing wrong with it. But if you make changes so it runs reliably on low pressure ammo, you're making trade-offs that can affect how reliably the rifle will run on full pressure ammo. If that's what's needed to meet your mission requirements, go for it
MachIVshooter
Member
I disagree. A middy that is built right should run everything from Tula to hot handloads, just as all of mine do-including 40 gr. loads that clock 3,600 FPS from the 16" tubes. Of the four 16" guns in the other room, three have midlength gas, and are not the least bit ammo sensitive, nor do they exhibit excessive carrier speeds.
Not everyone wants a permanently attached muzzle device, or to spend $200 on an SBR stamp.
Personally, I haven't found a 14.5" to really be any handier than a 16". I have a registered SBR, but have no intention of bothering with a 14.5" upper. 12.5" and shorter make a noticeable difference, though. I have a 12.5" BCM that is a great general purpose configuration, and my 7.5" upper with a KX3 is a very handy PDW-sized weapon.
Zerodefect
member
You can't complain about reliability if you're shooting Tula or Wolf. Deal with it, or switch to quality ammo like Federal.
Exibar
Member
I also say it's a gas problem... BUT, it very well could be a bad ammo problem... really bad ammo... so I dont' think it's the Ammo, as even the "cheap stuff" should cycle the firearm properly...
check for a clog in the gas tube, check the alignment of the gas block, it might be blocking the port just a hair... Check for blockages in the gas block itself, there might be a bit of swarf in there that is blocking a bit...
Mike B
check for a clog in the gas tube, check the alignment of the gas block, it might be blocking the port just a hair... Check for blockages in the gas block itself, there might be a bit of swarf in there that is blocking a bit...
Mike B
- Status
