It looks like I'm going to be building an upper for myself soon. I'm looking to build a light weight upper for a calling gun so I'll probably go with a 16" light weight barrel. I notice I can get 16" barrels with either carbine or mid-length gas port holes pre-drilled. My question is is there an advantage to one over the other? If so what are they?

Mid-length gas blocks allow for a lower pressure gas impulse to work the action. With the lower pressure gas impulse, you get a slower and less violent rearward movement of the bolt carrier with smoother feeding and ejection.

The advantage of the carbine length is being able to run your bayonet! Doesn't everybody run a bayonet?

The downside is your bolt will crack probably twice as fast, but you'll have a bayonet to fall back on!

Ummm, your bayonet would be a bit stubby on a carbine gas system...

Like said above, it less pressure with a longer sight radious thrown in.

Downside is it's a fairly new system that while sort of proven has a bazillion rounds to go to even get close to the time proven carbine gas system in all weather conditions and ammunition.

I have a carbine, mid and intermediate, setup with the right gas port sizes and spring/buffers, I can't tell you one will outlast the other or is any easier to shoot.

That said, given a choice, I'll go midlength when I can but when I grab my Colt light weight, I know it's gonna work.

Yes, but that is why it is there....really.

Stubby? Yes, but the hoards in Haiti were sufficiently intimidated by "stubby" a time or two.

And Colt M4's routinely break/crack bolts...really. This ain't hearsay. This was extremely rare with the old 20in rifles but fairly common today.

M4's are also famous for the dreaded "bolt over base" malfunction that will ruin your day. I believe the high port pressure causes this.

Go with mid-length. If you're running A2 style sights it gives you a longer sight radius in addition to the already listed benefits outlined by MichiganScott.

The carbine gas system has been proven on 14.5" barrels aka the M4. The 16" barrel combined with the midlength gas system is ideal.

One last benefit, the carbine gas length looks really goofy with a 16" barrel when it comes to proportions. The mid-length looks a lot better.

So that's performance, reliability, and looks all improved. Could you ask for more?

my colt carbine seems to throw brass further than my Noveske midlength which piles it up neatly in the same area. I still prefer the carbine because I know its going to work until it breaks really bad. I would not want a carbine gas system on a second tier gun like a smith, or armalite, bushmaster, or dpms because you know up front they cut corners on their components.

Ummm, the USGI standrad issue M4 has a 14.5" barrel, the govt has never used a 16" barrel....really.

I wouldn't say "routinely" but yes they do break, however the service life is well above anything we need to worry about here, though I do carry a spare Colt bolt. Also you're talking 14.5" barrels not 16"s. Yes the extra 1.5" makes for more pressure on the bolt but after firing thousands upon thousands of rounds in a 16" carbine, I'm not skeered.

If the "bolt over base" is bolt bounce, that is why we have H and H2 buffers with loose weights inside, run a soldi buffer like the 9mm buffer and see what happens.

TWR, a 16in and a 14.5 M4 have the same gas port pressure, if the guns are other wise configured the same. The longer barrel has a slightly lower pressure at the crown because it has a longer dwell time.

A 16in barrel also pushes M855's effectiveness out about another 50-75 yd. By that I mean it'll break at the cannelure like M193. Mid-length 16in is the way to go.

Having carried an M4 on a few continents, I never knew how long the damn barrel was, thanks.

You are correct that often a buffer change can fix all manner of problems.

Bolt-over base is when a piece of brass is extracted but doesn't clear the ejection port and gets lodged ABOVE the bolt and under the charging handle. If you pull the charging handle, it jams it harder.

A Leatherman MUT is the fastest way to clear this, without a proper tool, just beat the butt against the ground for inertia to unwedge the bolt.