View Full Version : 16" intermediate gas, gas port sizes. My SR15 is undergassed, lets talk ideal sizes.
I have an early SR15 barrel/bcg I'm using.
Its chronically under gassed. Clean its ok, but as it gets dirty in something like a class it slows down and starts fail.
The easy solution is to send it to KAC and have them update it to the newer size. Well, not so easy. I have modified the upper. The gas block has been switched for a pinned Vltor and it uses a Geisselle MK16 rail. KAC doesn't want to see it. They also don't want to tell me what gas port size to get it drilled to. I find this super annoying. The Vltor gas block is a tight fit and does not leak. I am running a sand cut KAC carrier.
So lets talk numbers. What is ideal, what size does KAC currently use? I measured this port at .078"
The gun will reliably cycle 77gr. MK262 clone. M193 will fail to lock back most of the time. I also use speer gold dot soft points in 62gr. and 64gr. and Federal fusion in 62gr., these too are underpowered in this gun.
What i'd like to run:
5.56 55gr.
.223 Speer and federal fusion 62gr.
What does not need to:
All steel cased ammo
.223 55gr ammo like federal or PMC
Where I am can get as low as -30 out, I try no to go out in that weather. I would like it to cycle at -15 F though, this is a more common deep winter day time temp.
It will ware a can some time. Its a sure fire socom 5.56 RC2.
So how much bigger do I need to go? will .08 do it? Is .0078 already big enough?, it appears to be a common 16" mid size. Has any one measured a new SR15?
ETA: I thought about it and re did the math the gas port is already .078 or 5/64. Not the .068 I wrote earlier. I remembered wrong when posting.
I'm not sure where I'm pulling this number from, but when I hear 16" intermediate, I recall 0.078" as the gas port size.
I'd contact Clint at Black River Tactical if all else fails.
For a 16 inch mid gas, .076 would be about right.
I have an early SR15 barrel/bcg I'm using.
Its chronically under gassed. Clean its ok, but as it gets dirty in something like a class it slows down and starts fail.
The easy solution is to send it to KAC and have them update it to the newer size. Well, not so easy. I have modified the upper. The gas block has been switched for a pinned Vltor and it uses a Geisselle MK16 rail. KAC doesn't want to see it. They also don't want to tell me what gas port size to get it drilled to. I find this super annoying. The Vltor gas block is a tight fit and does not leak. I am running a sand cut KAC carrier.
So lets talk numbers. What is ideal, what size does KAC currently use? I measured this port at .068"
The gun will reliably cycle 77gr. MK262 clone. M193 will fail to lock back most of the time. I also use speer gold dot soft points in 62gr. and 64gr. and Federal fusion in 62gr., these too are underpowered in this gun.
What i'd like to run:
5.56 55gr.
.223 Speer and federal fusion 62gr.
What does not need to:
All steel cased ammo
.223 55gr ammo like federal or PMC
Where I am can get as low as -30 out, I try no to go out in that weather. I would like it to cycle at -15 F though, this is a more common deep winter day time temp.
It will ware a can some time. Its a sure fire socom 5.56 RC2.
So how much bigger do I need to go? will .07 do it? It would seem like .076 is the biggest I'd want?, it appears to be a common 16" mid size. Has any one measured a new SR15?
Why did you bother making all of these modifications in the first place?
When you say MK262 clone, what does that mean? How did it work with actual MK262 Black Hills? I find it hard to believe real M193 did not lock back in this upper's original configuration.
Why didn't you sell the legacy KAC upper and buy a MOD 2 upper?
Why did you bother making all of these modifications in the first place?
When you say MK262 clone, what does that mean? How did it work with actual MK262 Black Hills? I find it hard to believe real M193 did not lock back in this upper's original configuration.
Why didn't you sell the legacy KAC upper and buy a MOD 2 upper?
I took the upper in trade long ago. I ran decent on heavier ammo, and clean. I sent it out for a different gas block and pinning when I had a DD RIS II installed, at the range it had functioned well. I used the KAC URX II rail to build an 11.5" upper.
Once I started running this upper in classes I noticed problems with it when dirty. I decided to run a light weight carrier and my problem was solved. I no longer want to run that carrier, it had its own reliability issues.
Now its easy to take apart. I swapped the DD rail for the MK16. At this point I want not he softest shooting gun, but a more reliable one. I have left it alone so far because you can't make a gas port smaller so I thought it could have a future value.
M193 will run reliability for about 360 rnds in warm weather then start having failures to feed from short stroking. In the cold it won't always lock back on a clean gun.
ETA: my mk262 clones are IMI and CBC 77gr. 5.56 ammo. Not black hills.
Failure2Stop
01-08-19, 15:56
Can you better define exactly what barrel you have?
Pics will help.
On the progress through the legacy, Mod 0, Mod 1, and Mod 2 guns there have been pretty significant changes to the system.
So I thought about it more and felt like .068 was not possible when compared to .076 of a middy. I redid the math, it was a 5/64 still bit that fit the gas port perfect. .078 not .068
So no more puzzling why in many configurations has this upper run like its under gassed if it’s not? Gas block is tight. Different carriers have been used.
The bolt, gas tube and barrel are the constants here. I’m going to look at the gas tube and maybe replace it. A recent thread where it turned out to be a worn gas tube has my interest.
What buffer weight and action spring are you using? Does it work well with a suppressor?
You can't just send the stripped barrel in to get the port fixed?
You should not be looking at mid length for a port size. It is actually close in size to the Vltor intermediate gas system.
The gun uses the sr15 marked carbine buffer and colt carbine spring and I've tried a white sprinco spring.
The bulb on the end of the gas tube is non existent. When I run my nail over it its like it was never there, just a smooth polish all the way to the end. I am going to replace the gas tube and see what happens. At 57$ on operation parts its definitely F-U expensive, but thats the pleasure of KAC ownership I guess...
I'm still interested if the .078" it measures at is considered too small for the gas system length?
The gun uses the sr15 marked carbine buffer and colt carbine spring and I've tried a white sprinco spring.
The bulb on the end of the gas tube is no existent. When I run my nail over it its like it was never there, just a smooth polish all the way to the end. I am going to replace the gas tube and see what happens. At 57$ on operation parts its definitely F-U expensive, but thats the pleasure of KAC ownership I guess...
I'm still interested if the .078" it measures at is considered too small for the gas system length?
I've heard of that happening with gas tubes before.
pointblank4445
01-09-19, 12:48
The gun uses the sr15 marked carbine buffer and colt carbine spring and I've tried a white sprinco spring.
The bulb on the end of the gas tube is no existent. When I run my nail over it its like it was never there, just a smooth polish all the way to the end. I am going to replace the gas tube and see what happens. At 57$ on operation parts its definitely F-U expensive, but thats the pleasure of KAC ownership I guess...
I'm still interested if the .078" it measures at is considered too small for the gas system length?
ETA: disregard, this is in regard to KAC's intermediate spec.
I've heard of that happening with gas tubes before.
There was a recent thread where it ended up being the issue. Had I not recently read that thread I never would have guessed. I've been chasing it for years, I realize its probably slowly been getting worse which makes sense.
The other thing that now clicks is when I went and swapped rails on it the other day I noticed how dirty the gas tube was from where it goes in the BCG and then forward about 6-7". At that time I thought maybe the tube had ruptured and that was my issue, I cleaned it up and found no splits in it. I now realize it was so dirty because gas was shooting forward out of the gas key due to the loose seal from the gas tube.
I don't know the round count on this upper. I have a 3k-7k rounds through it, I really don't keep track. When I took it in trade it was very clean and looked good, but round count is unknown to me. The only thing I think to attribute the ware to is one place I take training classes is all fine sand. A lot gets in the action in Vtac classes I took there, actually sand goes everywhere and in everything. I eventually stopped taking classes there because of the sand.
Due to the ware on the gas tube I am going to install a KAC bolt repair kit I have at the same time to freshen the whole thing up.
For future reference, can you measure the diameter of the bulb and report back?
The bulb on the end of the gas tube is non existent. When I run my nail over it its like it was never there, just a smooth polish all the way to the end. I am going to replace the gas tube and see what happens. At 57$ on operation parts its definitely F-U expensive, but thats the pleasure of KAC ownership I guess...
For future reference, can you measure the diameter of the bulb and report back?
Yes. Once I recieve the gas tube I’ll take the rail and gas tube off, measure both gas tube ends and photograph them side by side. I’ll also photograph any barrel markings while it’s apart as well.
I'm wondering if the aftermarket gas block is misaligned. I would have someone with a bore scope check it. It is how I check for port alignment.
As far as the gas tube goes, brownells sells a gauge made by Mark Brown to measure the flared portion of the gas tube while it's still in the upper. It will find wear on the half that's not visible. It's a good gauge.
Can you better define exactly what barrel you have?
Pics will help.
On the progress through the legacy, Mod 0, Mod 1, and Mod 2 guns there have been pretty significant changes to the system.
Wasn't there something too, from KAC, about not running M193 spec ammo? I'll see if I can find it.
I think it is time to part out this upper, sell off the questionable parts and buy new.
elephantrider
01-12-19, 04:54
Wasn't there something too, from KAC, about not running M193 spec ammo? I'll see if I can find it.
IIRC the original SR-15 Mod 0's were ported to run 5.56 pressure ammo exclusively. Of course, buyers complained that Brand X,Y, and Z .223 ammo wouldn't run, and eventually KAC opened up the gas ports. Not sure if they ever got to the point where they offered different porting upon request like Daniel Defense eventually did for their Mk18s, but the current SR-15 gas block, barrel, and gas tube configuration is totally different than the original.
Can you better define exactly what barrel you have?
Pics will help.
On the progress through the legacy, Mod 0, Mod 1, and Mod 2 guns there have been pretty significant changes to the system.
It started life as Mod.0. The barrel has no external markings forward of the barrel nut. I don't think any marks are under here either. The only way it can be identified as KAC is by its gas system length and barrel extension.
For future reference, can you measure the diameter of the bulb and report back?
.174 on the new one. Behind the bulb is .165. The old one was .165 all the way with no visible evidence of a bulb ever being there.
I'm wondering if the aftermarket gas block is misaligned. I would have someone with a bore scope check it. It is how I check for port alignment.
As far as the gas tube goes, brownells sells a gauge made by Mark Brown to measure the flared portion of the gas tube while it's still in the upper. It will find wear on the half that's not visible. It's a good gauge.
Gas block is perfectly aligned. I verified this. The upper has been tried with a number of carriers. A JP lomas, LMT enhanced, the KAC that came with it, and now it has a KAC sand cutter carrier. One of these carriers must have had too small a gas key interior diameter and worn the tube. I say this because the bulb was evenly worn off all around the tube. No signs of pressure from one side.
I am going to buy that tool based on this experience.
Wasn't there something too, from KAC, about not running M193 spec ammo? I'll see if I can find it.
I read similar, basically that it was tuned for heavier 62gr. green tip and there for does not always lock back with m193.
So the gas tube is in, the old one had no bulb at all on it and had a slightly different bend in it. I'll post pictures in follow up. I won't be able to test fire for a week or two I don't think. But clearly there is a big difference in gas tubes. .175 for the new vs. old worn all the way to .164. I expect a big difference. If it is still a little weak for my taste I'll have the gas port opened to .08 or .082. I have a Noveske 18" intermediate barrel I'd like to measure the port on for comparison, its in a Vltor VIS so its kind of a PIA to get to. Pic to follow
please click on pics for larger versions.
Here are the two ends for comparison.
55540
In these two pictures you can see where the bend in a different place, and how the old gas tube pointed down some.
55542
55543
So a bit of an update here.
I installed the gas tube and used my bolt repair kit. There was visible ware on the cam pin and extractor. The new extractor would not fit in my bolt. I contacted KAC and they said they have to special order one and its going to take a long time and they are going to send me another one. Currently waiting on that. In the mean time I used a knife sharpening stone and removed the high points on the extractor where I could see it was sticking.
I managed to do a little very quick test firing. I only had time for 10 rounds. I fired 5 mags, with two rounds in each. # mags with federal fusion 62gr. .223 and two mags with xm193. All 5 mags locked back. The gun was well lubricated with ALG gojuice and very clean. It was 50 degrees out.
I look forward to trying to run this rifle hard come spring and see what happens. At this point I will leave the gas port alone. I am going to try to did up some 55gr. 223 and see what it does on that. Theoretically by design it should not run well, I'm interested in how bad it will be.
You should run the weakest ammo, at the coldest temperature, holding the rifle loosely off your shoulder.
SouthwestAviator
02-07-19, 10:22
I think it is time to part out this upper, sell off the questionable parts and buy new.
This.^^
And yes, KAC has directly said they do not recommend XM193, saying that the quality control is not good particularly with LC, leading to blown primers and the like (not because of bullet weight or pressure or anything). Personally I don't really buy that, never seen a problem with LC XM193 in any standard carbine.
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