Any one know what size the gas port needs to be in an 18" barrel with full length gas system?  

avrgr,

depends on many factors and gas port sizes vary from maker to maker.

Size is determined by the distance from the gas port to the chamber and the distance from the gas port to the end of the barrel.

Gas port (GP) get larger the further it is from the chamber. GP gets larger the closer it gets to the muzzle. If the GP is close to the chamber it doesn't need to be very large. There is no point in making the gas port larger than 1/8" as the limiting factor is the ID of the corresponding hole in the front sight base.

No doubt you wanted numbers. Is this a custom (undrilled) job or are you checking an existing GP size?

This is a custom job that has been drilled into a Krieger Barrel.  The size it was drilled to is .104, I’ve been having some reliability problems and was checking if I needed to go larger or change my reloads.

.104 is the size used by OAI on their 20" barrels. It's on the large end of spectrum for 20"ers. There's really not much larger you can go before you reach .125" (1/8").

I'd leave drilling the port bigger as a last resort. It's a simple check, load one round into the mag, lock and load, fire the round. Bolt should lock back.

I'd try some known good ammo first, mil spec loadings.

Then start looking into the gun.

What kind of problems are you having? What kind of gas manifold (where the gas tube ties to the barrel) are you using?

I had some cycling problems i.e. failures to feed.  I have a Young Manf.  Combo gas block/ front sight.

Failures to feed means the round doesn't make it past the feed lips on the mag. Is this what you're seeing?

If you load one round into the rifle, hold the rifle very loosely, and fire it, does the bolt lock back normally?

If the gas block is one of the screw on types it may be misaligned with the gas port.

Check for short stroke (2nd paragraph) first before making any changes.

I shot a bunch of green tip thru my gun with no problems. My reloads is what is giving me the problem.  I'm loading 69gr Noslers with 25.3 gr Winchester 748.  That is a fast burning powder.  I'm Going to try a slower burning powder next i.e. VV N540.  The gas block is aligned properly and was checked with a bore scope.  My gas rings are new. My gas Key is tight and staked on. I have a Young Manf. Lightweight bolt carrier and a Les Baer bolt. As far as the failures to feed I probably used the wrong term.  The gun was short stroking and not picking up the new round.  It did the about 10 times out of a hundred so I’m thinking I have a marginal gas pressure.  If I hold the gun loosely it will not cycle. So bottom line I need more pressure from somewhere.... either enlarging the gas port or higher-pressure rounds.  The army's new SPR rifle is 18" I would like to know what their gas port size is.  I'm going to go to the range tomorrow and try out my new loads.

What is the diameter of your barrel under the gas manifold?

If you have one available, try a GI bolt carrier. The LW carrier may not have the necessary inertia on the battery stroke.

The barrel is .750 under the gas block.

Just making sure it wasn't a bull barrel. Normally the larger the diameter the larger the GP. You're running at the high end of the scale regarding GP size right now. Opening it up may allow the rifle to operate with your handloads but it'll overfunction with everything else.

Finesse the loads and try a different carrier.

Thanks for all the advice.
Tony