View Full Version : SBRs and their purpose
10.5, 11.5, 12.5....
what was the original intended purpose for each?
what is the current general useage of each?
what are the benefits of each?
and lastly, what are gas port sizes for each?
thanks, gentlemen.
MisterWilson
09-27-09, 00:56
Tag for gas port info...
http://img200.imageshack.us/img200/831/img0104xu.jpg
These were the weapons of the Navy EOD guy's attached to my platoon back in 2006 when we were evacuating Lebanese American Citizens out of Lebanon to Cyprus. When I was in Iraq, I met a few SEALs and they were carrying 10.5" M4's with KAC Cans. Other than those two occasions I have not see a whole lot of usage with SBRs, during my time in the Marine Corps.
http://img200.imageshack.us/img200/831/img0104xu.jpg
These were the weapons of the Navy EOD guy's attached to my platoon back in 2006 when we were evacuating Lebanese American Citizens out of Lebanon to Cyprus. When I was in Iraq, I met a few SEALs and they were carrying 10.5" M4's with KAC Cans. Other than those two occasions I have not see a whole lot of usage with SBRs, during my time in the Marine Corps.
Third from the left - Aimpoint/mount on the weapon backwards????
whiterabbit05
09-27-09, 13:06
^haha
Looks that way, doh!
The Navy EOD guys that were attached to us were a bunch of Drama Queens. Apparently getting people to safety wasn't sexy enough...You should of seen their pallets of gear.
Voodoochild
09-27-09, 17:04
Ak44 whether or not you liked the EOD guys or not. Under no circumstances are you to make derogatory remarks about the men and women in service. Continue down that path and you may have a short lived time here.
Nice pic Ak44,
Between PCC's and PCI's I cant even think of a good reason why it would ever be on backwards, except typical ****offery like flipping off someone elses safety.
Who knows, I never even noticed to look at the picture very hard until someone said something haha. That was the first time I saw a Redi-Mag.
FLipping the aimpoint is funy as hell. That is agood ball bust on your buddy. I state that assuming that this was a break in a training cycle.
We, in LE, are ruthless on one another. Ballbreaking has it's time and place. I think it builds the team better/faster. I work in a unit of 6 guys, if a stranger walked in you would think we hated each other. But, if ANYONE comes in to mess wth one of us it is 6 on one.
Good stuff,
By the way I went with a 10.5 LMT because it is easier in and around a car, not tomntion going inside structures.
Jack
bump for actual responses.
:)
decodeddiesel
10-01-09, 12:04
http://www.dtic.mil/ndia/2003smallarms/john.ppt
This is a power point presented by NSWC Crane outlining the MK18 MOD0 basic configuration and modifications. It talks about gas port sizes and the 10.3" barrel length. I think this data is a direct correlation to the LMT 10.5" uppers as well.
thanks for the post. i actually have that PPT on file, but it was good reviewing it again. good reminder of how short a life you get from a relatively expensive and time-consuming endeavor.
---------------------------------
gas tube replacement at 6400 rounds? i assume this is from carbon build up? anyone know how it was established that the tubes needed replacing? i think the few times ive ever replaced a gas tube was simple PM in conjunction with other work, since it's cheap PM.
i'm also surprised there werent more extractor and/or spring replacements, or bolt breakage. too bad they only tested 4 weapons- i'm sure the MRBS would have gone down as the number of tested weapons went up. doesn't seem very conclusive.
so if the gas port for the CQB, traditionally, has been .062 for a 10.5, should i assume that gas port size for 11.5 and 12.5 is .062" as well? is this simply out of laziness? definately seems you could optimize these weapons by finding the proper port size for each according to dwell time. i bet some of you mentats could do the calculation in your heads, and without any more information. what port sizes are current manufacturers using for their SBR barrels?
decodeddiesel
10-01-09, 17:40
Actually in the document they mention the gas port being opened from .062" (standard size for a 14.5" Colt M4 barrel) to .070".
I do not know what port sizes the 11.5 use, however I can remember reading on this forum some time ago about people chopping LMT and Colt barrels to 12.5" (right ahead of the M203 notch) and not having to open the gas port at all for reliable functioning.
Honestly I have no idea as to why those weapons went through the gas tubes so quickly. I would like to know the nature of the failures as well. Excessive fouling would be my guess as well.
Gas tubes will get soft and sag with very heavy firing.
Actually in the document they mention the gas port being opened from .062" (standard size for a 14.5" Colt M4 barrel) to .070".
but that's just for these particular upgraded weapons (and whoever else chooses to do it). i imagine most SBRs are still gonna be drilled at .062. it certainly makes sense to open the port. in fact, as i write this, i can remember reading a thread in here where it was theorized that a larger port may serve two important functions- more gas for harder cycling, as well as decreasing port erosion by reducing the gas bottleneck/reducing the pressure of the gas as it flames through. dunno who wrote it, or how accurate that is.. also seems to me as thought it might do the exact opposite- by opening surface area of the port you might be allowing for even faster erosion, as you're letting it eat more material away with each shot. that's gonna depend on the physics and causes of port erosion, which i probably don't understand. perhaps some searching for "port erosion" is in order.
Gas tubes will get soft and sag with very heavy firing.
makes sense- i imagine tubes get extremely hot during sustained firing. does most of the droop occure rapidly while the weapon is hottest, or is it an overall progressive thing?
would NRW care to divulge the sizes of their SBR gas ports?
decodeddiesel
10-01-09, 19:53
Gas tubes will get soft and sag with very heavy firing.
Well might as well ask it straight from the source, Todd what gas port size do you use with your 12.5" and 10.5" barrels?
decodeddiesel
10-01-09, 20:03
but that's just for these particular upgraded weapons (and whoever else chooses to do it). i imagine most SBRs are still gonna be drilled at .062. it certainly makes sense to open the port. in fact, as i write this, i can remember reading a thread in here where it was theorized that a larger port may serve two important functions- more gas for harder cycling, as well as decreasing port erosion by reducing the gas bottleneck/reducing the pressure of the gas as it flames through. dunno who wrote it, or how accurate that is.. also seems to me as thought it might do the exact opposite- by opening surface area of the port you might be allowing for even faster erosion, as you're letting it eat more material away with each shot. that's gonna depend on the physics and causes of port erosion, which i probably don't understand. perhaps some searching for "port erosion" is in order.
I do remember reading somewhere (and for the life of me I can't find it :() that the TDP for the MK18 call for a .070" gas port. Now that would apply to the Colt CQBR but perhaps not to the LMT 10.5 as it was a COTS item and not regulated by a TDP.
As far as the benefits of widening the gas port, absolutely agree with increasing the gas flow in system, however I am not sure of the erosion benefits. That would be a great question for some of the mechanical engineers here. I will qualify my remarks with the statement that I have not personally finished my degree in mechanical engineering, but I have had some materials classes which would apply here. I think the rate of erosion would be inversely dependent of the diameter of the gas port. Therefor the larger the port is the the less the erosion occurs. Still though the cumulative effects of the erosion would be greater on a larger gas port as there's less material to erode away before the barrel is condemned. Then again I could be totally off base.
RogerinTPA
10-01-09, 20:07
I was under the impression that the design was originally for SOF folks for CQB and later utilizing them while performing PSDs.
Firecop203
10-01-09, 20:32
This is the information I was able to Google about gas port sizes.
10.5 .093
11.5 .081
14.5 .063
This is the information I was able to Google about gas port sizes.
10.5 .093
11.5 .081
14.5 .063
da ****... that must be commercial/SAAMI size or something. where'd you pull it from?
Thomas M-4
10-01-09, 21:09
I thought the gas ports were bigger on a SBR because of the decreased dwell time.
Less than 4 1/2'' in front of the gas port you need more gas to reliably cycle the carbine.
I thought the gas ports were bigger on a SBR because of the decreased dwell time.
Less than 4 1/2'' in front of the gas port you need more gas to reliably cycle the carbine.
you thought right
decodeddiesel
10-02-09, 09:59
This is the information I was able to Google about gas port sizes.
10.5 .093
11.5 .081
14.5 .063
Holy crap! Those are much larger than I would have thought. I know that varies from manufacturer to manufacturer though so I wonder if these are Bushmaster and the like's "will run anything I feed it" or LMT "if it ain't 5.56 don't bother". Yes I know most LMTs will run .223 just fine with the right buffer, but the system was designed specifically for 5.56 pressure ammo.
I thought the gas ports were bigger on a SBR because of the decreased dwell time.
Less than 4 1/2'' in front of the gas port you need more gas to reliably cycle the carbine.
Correct. This is part of the puzzle as to why SBRs run so harsh. In order to cycle the action in the shorter time window presented by the lower dwell time you must really up the pressures and therefor speed of the BCG.
I called Colt and asked them what the port size was on an 11.5 Commando and I was told they can’t tell me because it is part of the TDP.
Third from the left - Aimpoint/mount on the weapon backwards????
Bruce Willis's rifle.
Bruce Willis's rifle.
http://www.filmweb.no/bilder/multimedia/archive/00101/Bruce_Willis_i_Tear_101959o.jpg
they're probably inert... how was he to know?
Robb Jensen
10-12-09, 07:52
FWIW factory LMT 10.5" barrels have gas ports of .071"
For 11.5"-12.5" I like .067" for 5.56mm NATO pressure ammo.
For .223 Rem SAAMI pressure I like them a little bigger. 10.5s" to .073" and 11.5"-12.5" to .071"
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