16" intermediate gas, gas port sizes. My SR15 is undergassed, lets talk ideal sizes.

[TMS951]

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.

[phixion]

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.

[opngrnd]

I'd contact Clint at Black River Tactical if all else fails.

[markm]

For a 16 inch mid gas, .076 would be about right.

[TMS951]

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.

[Rascally]

What buffer weight and action spring are you using? Does it work well with a suppressor?

[Todd.K]

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.

[TMS951]

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?

[scottryan]

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?

[TMS951]

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.