Putting together a 18”Spr.   With generic upper, aro Spr 18” barrel, low profile gas block, rifle length gas tube. It’s short stroking sometime it will elect and not pick up the next round and sometimes it will not even elect and just tries to re-chamber the spent round. I’ve tried it on a different lower and its the same. Different gas tube and block, same thing. Different bolt same. That leaves the port size, I think. I haven’t measured the port size yet , but was wondering what size it should be, and what size drill bits I should be looking at or I’m I going in wrong direction.  

Thanks if anyone has ideas.

Before drilling out holes.

Have u tried different magazines?  Buffers or springs?  What buffer system carbine or rifle? Have u inspected the bolt? What ammo ?

How about an adjustible gas block before ya start mutilating it

Order a pin gauge set to see what the gas port diameter is.

You probably need to be around .090 to .100 and check the carrier and key to ensure they're both mated properly.

What buffer weight are you using and what kind of ammo?

I assume rifle gas?  I had an 18" with rifle gas that had the same issues.

Have you tried an empty buffer?  Pop the pin and dump out the weights.  If it will run on an empty buffer, run it.  If you need to add a little mass there are aluminum weights available to mix with steel to tune.

OOPS, went back and saw you tried a different BCG.

If the gas port is too small, that won't help.

This. lighten it up and tune down your buffer

I've had to drill the gas port out on two 18" rifle length gas barrels from different manufacturers to get them to run.  My thought is that these barrels tend to be used in "SPR" clones that will be primarily run suppressed.  🤷

Adjustable gas blocks can’t add gas.

That said, we need to know specs for everything else.  I’d be trying a lighter weight buffer before drilling out the gas port.

Start with confirming the GP diameter.

Thank you for all the suggestions I will try and respond to all the questions. Tried 3 different mags, 2 different types of ammo Magtech 55 and 62. Two different bolts, two lowers one that has a rifle stock and buffer one that is a carbine stock and buffer, two different gas block/tube combos, with and with out muzzle device. So at one point it was just the upper and barrel that wasn’t changed out alway with the same result

That’s what I was thinking…G/P size.  Can one effectively measure that with numbered drill bits?

Drill shanks will get you relatively close.

Thank you I think I'll try that.    Maybe buy some from .090-.1 ?

Look at wire gauge bits. Ace Hardware carries Irwin brand. $3.50-$4 each. Example sizing: #38 is .1015" and a #39 is .0995".

Sweet.   I have to drive right by an Ace tomorrow

If this is an Aero/BA barrel, it is no surprise.  I have several and they are definitely undergassed. I wound up using an Aero spring and 3.0 oz. buffer.  I couldn't get them to run with a Sprinco white spring. Generally I fing that on an 18" rifle length barrel I need a .097 port to get them to run well.

Adjustable gas blocks CANNOT increase gas flow, they can only restrict it.  No adjustable gas block ever made increases the amount of gas flow if the barrel gas port is too small.  It kills me that every time someone has an under gassed problem someone always says, adjustable gas block.  It boggles the mind.

1.  Check gas block position.  No blowing through it BS, take measurements if a dimple is not present.
2.  Check gas tube installation.  Tube not upside down or pin in front of tube instead of through it.
3.  BCG gas key screws not broken.
4. Gas rings in good condition.
5.  Ammo known to function properly in other rifles with same specs.
6.  As others said, measure gas port in barrel and compare to port size in other functional barrels.
7.  Last resort, enlarge gas port .002-.004 at a time until it functions with known ammo that works.

I work with wildcats a lot and have drilled out a lot of gas ports to function.  It’s not something to be afraid of because, if you DO go too large, THEN you can go with an adjustable gas block.  I’ve yet to need to do so myself, but there is always tomorrow.

If you're not running a can, you should be.

UPDATE.   Gas port was .0955  went up to .0980 works now

Awesome.  When I built and 18" for a coworker I had to open the gas port, which gave it the most consistent ejection I've ever had in an AR.