"Intermediate" Gas Tube Length


#1

My 300 HAM’R is my first build out of the milspec box. First problem was the gas tube. I never heard of “intermediate” and assumed it was the same as “mid-length”. I understand a lot of people made that mistake. So I get an Intermediate tube. Now the bolt won’t close. It seems Intermediate isn’t standardized and a Noveske tube is 1/2" longer than Wilson Combat’s. Why they arbitrarily chose to shorten it seems to be rooted in some desire to require you to buy all your parts from them. Definitely not in the spirit of interchangeability. I install the Wilson tube. Gun goes bang, bolt doesn’t move. Now I have to wonder if they positioned the gas ports on the barrel and their proprietary gas blocks to be a set that you have to buy together (again, from Wilson Combat). Is there a standard dimension from gas block stop on the barrel to the gas port and from the front of the gas block to the block’s port? I’m going a bit berserk here guys, can you help?


#2

@Dreamsinger Sorry you’re having to go though all this. Intermediate gas system lengths aren’t standardized so this issue tends to come up often.

When you say the bolt doesn’t move, you mean it didn’t move at all? Or just not very much?

You are correct that the first thing to check is the alignment of the gas block, gas tube and the gas port. You can be fairly certain this is the issue if your BCG did not move at all when you fired it. But another way to check without completely disassembling the upper is to insert an EMPTY case or dummy round into the chamber to plug it. Then blow compressed air into muzzle end of the barrel. If air comes out the gas tube then you can rule out a total blockage. Remember, safety first. There is no reason to even have the lower attached to the upper when running this test. You can also remove the BCG. And for those who don’t have an air compressor, use your imagination.

You can check out this video for two examples of how to ensure proper alignment (also you don’t have to buy the gas block alignment pins, you can just use a toothpick or uncooked spaghetti noodle):

You can also check that the gas tube opening is not obstructed by removing the gas block from the barrel with the gas tube still attached and blow in to make sure there is unrestricted airflow. If you want, you can also remove the set screw below the port in the gas block to look through and make sure both the port on the gas block and the gas tube are aligned properly.

I know tracking down these issues can be frustrating but you’re on the right track. Worst case scenario you just have remove the gas system and re-install everything one step at a time, ensuring proper alignment and air flow at each step.


#3

I went through it then checked for flow. My radial alignment was correct but the roll pin appears to misalign the tube port to the block port by about 30%ish. In addition the barrel port and the block port aren’t quite concentric which constricts flow a little more. I’m thinking the cure is to remove the tube and elongate the hole then attempt to elongate the hole in the block to fully catch the barrel port. Does that make sense?


#4

You could try modifying the tube and block but if the block is misaligning both the gas tube and not lining up with the gas port on the barrel then the block itself seems like the culprit and it might be easier (and better) to just replace it. What brand / type of gas block are you using?

If your gas port is ~ .33" from the shoulder when measured from the shoulder to the outside edge of the gas port (as shown below) then it is almost certainly an out of spec gas block.


#5

Mine measures close to .33" with a front handguard cup, ironic since no one makes a drop-in handguard for intermediate gas length AR’s. You have to use a floating handguard. That will change. Black guns wood.com is making me one. They’ll have both Noveske length and Wilson. The gas block is problematic in that I need 2 Pic rails; one for the front sight and another on the bottom for a bipod. Since the handguard is vertically split I can’t put a short rail on the bottom of it.


#6

My gunsmith called today with good news. After a little micro machining, the port in the gas tube aligns with the gas block port and the barrel port aligns with the gas block inlet port. It locks back on the last round and ejects and feeds reliably. I’m picking it up tomorrow morning and going straight to the range to boresight it and get my 200 yd zero set.


#7

Some miscommunication on my part kept the gun at the shop. Because this will have wood furniture it requires the front retainer cup. Rather than milling the face of the gas block to compensate for the retainer’s thickness he just removed it. Back to the bench. The idea for this gun has been kicking around in some form for years. I’ve always liked the idea of the Cooper scout rifle and its all purpose nature but but being a right hander who has to shoot left, bolt guns don’t work well for me. Besides, Cooper himself said that technology would eventually make a semiauto scout a possibility. The 300 HAMR seemed to make a lot of sense but my ignorance of the lack of standardization of “intermediate” gas systems has been a royal PITA.