View Full Version : gas port size for 16 1/2" barrel?

June 12, 2002, 09:24
Working on my Imbel carbine, barrel length is 16 1/2" with a Entreprise "zero climb" brake attached. Need to know a good starting point for the gas port size from anyone who has built one with this barrel length. TIA

June 12, 2002, 10:17
I usually start with .110". That almost always works, unless the piston is worn, the gas tube is worn, or you have a small gas leak. Just make sure you open it up in small increments so that the drill bitt doesn't break off. Usually the addition of a muzzle brake adds a little back pressure and the port won't have to be opened up as much as one without the MB.

Seth Livzz
June 12, 2002, 10:18
I believe .110 is the standard if there is any.

June 12, 2002, 10:25
thanks guys, so what size drill bit do I need?

June 12, 2002, 15:34
Most of the gas ports start around .90" or .92", if I recall correctly. That would be about a 3/32" drill bit. .110" is about a 7/64" drill bit. Make sure you use a very hard drill bit also.

I would try building the rifle and testing it. It may run on the standard gas port. If not, all you have to do is take the front sight out to open the port up. Make sure you place a cleaning rod in the bore so that when the bit goes through, it won't nick the rifling on the opposite side of the bore :)

June 12, 2002, 16:10
thanks, 187. I will try it out before I drill it, just want to know what I will probably be needing.

June 12, 2002, 20:22
rob1, fire the rifle before you drill. My Entreprise Incher Aussie 18" had to go to .125 :eek: and it functions with totally reliability on 5 (no muzzle thingey). I used the step method and I went what gunplumber and others consider the max. I am glad I could stop there. Go slow and in small steps by all means.

ratas calientes
June 13, 2002, 09:35
My first build was a 16.25-inch carbine to which I added the DSA short brake. After a lot of reading, I just bit the bullet ;) and drilled it out to 0.120 in two steps: (1) first to 0.106 (drill #37) then (2) 0.120 (drill #31). It runs great on 5-6.

I set the barrel up vertically in a vise and used a hand drill so I could have better "feel." Motor oil makes a good cutting fluid for drilling steel. I used the oil that I use to lube my swampcooler bearings. I rinsed out the chips with WD-40 using the straw "thingie."