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9/22/2017 12:11:25 AM
Posted: 8/5/2005 8:44:15 PM EDT
I am wanting a Dissipator style upper and wondered if I had a 20" standard barrel cut down would I need to enlarge the gas port to insure reliability?

Mike
Link Posted: 8/5/2005 9:08:25 PM EDT
probably, check with mongo. He did it several times.
Link Posted: 8/5/2005 9:40:27 PM EDT
I bought a DPMS dissy, which has a full length gas system. I had to open my port to 1/8" for it to work with all types of ammo: XM193 to Wolf. Others have had to do the same thing, but with various port sizes.

Even with all the problems I've had with it, I still prefer the 16"-18" barrel lengths, with full sight radius, the best.

You may want to get a 20" barrel that is light under the handguards to save weight.

Let us know how it works for you.
Link Posted: 8/6/2005 3:04:55 AM EDT
I cut down a 20 inch to 14 1/2 for my RR m16. It took a lot of work. When cut down the 20 inch to 16 inch you will have to move the gas hole. I spent a lot of time making sure I put the hole in the right place. When going down to 14 1/2 I had to make the gas hole larger then what was in the 20 inch. It runs 100%
Link Posted: 8/6/2005 4:05:03 AM EDT
I have cut a couple of 20" down to 16". I enlarged the gas port to a #42 drill size; works great!
Link Posted: 8/6/2005 4:33:34 AM EDT
You can do 17-18 inches without modification. 16 inches requires a .110" gas port and possibly a coil cut off the buffer spring. I have an 18", BTW, that runs fine without mods.
Link Posted: 8/6/2005 1:13:03 PM EDT
When I cut down my 20 inch to 14 1/2 I only opened up the gas port. I never had to cut anything of the spring
Link Posted: 8/6/2005 2:01:47 PM EDT
[Last Edit: 8/6/2005 2:06:14 PM EDT by mongo001]
I've done four so far from 20" to 16". The gas ports on mine had to be opened up to 0.100" +/-. I worked them up in 0.002" increments until they ran on a wide range of ammo, from underpowered Wolf to semi-hot handloads. I ran this on an unmodified carbine buffer system (no mods to the spring) using an M16 bolt carrier.

I have two for sale right now. Email/IM me if interested.

Here's one of them.

Link Posted: 8/6/2005 2:04:00 PM EDT

Originally Posted By usagold:
When I cut down my 20 inch to 14 1/2 I only opened up the gas port. I never had to cut anything of the spring



20" to 14.5"?? New gas block area?? Pics please.
Link Posted: 8/6/2005 6:26:45 PM EDT




You can see the old gas hole location in the photo.
Link Posted: 8/6/2005 6:50:57 PM EDT
At the time I did not have the cash for a CAR upper. The above upper started life as a 20 inch SP1 upper. Cut barrel down, turned barrel down a little,located location for new front sight pins, drilled new gas port. It took me some time but the upper runs 100%. I am happy with it.

The 16 inch 9mm barrel I cut down to 11 1/2 was a hole lot faster to do. Cut barrel down and re mount flash hidder
Link Posted: 8/6/2005 10:15:28 PM EDT
Thanks for all the replies.

Mike
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