ZCI Aluminum M4 Outer Barrel for M4 / M16 AEG Rifles (Length: 14.5")

"They measured it end to end not to where the receiver meets just be aware
that doesn't mean that they do stick out of it
good product

"Just buy some locktite since it's a two part barrel.

"Gun: Matrix/S&T (Rebranded S&T) M4 Stubby in Dark Earth

The outer barrel fits fine in the gun, however there are a few minor details to be aware of if you own the same gun (and just in general).

1. The stock inner barrel for the listed gun is approximately half an inch longer than the end of the threads on this barrel. This is not an issue for me since I intend to always use a tracer unit and it seems to not mind the slight overage.

2. The stock barrel/amplifier setup (the amplifier is glued on and would require more work than the cost of the barrel for me to remove) has an internal spring pressing the inner barrel/hop assembly against the nozzle. There is a pocket in the stock barrel and the spring fits in there a bit deeper than it does in the ZCI barrel... so I just removed it entirely. The tiny spring that usually provides pressure seems to be doing the job well enough to keep the gun accurate out to ranges that are not justified by the stubby barrel. TLDR - you will be losing the larger inner barrel spring but it shouldn't matter much.

Overall the barrel is well made and feels quality. Fit and finish are good and aside from the minor issues that, honestly, are more my own lack of using a ruler (i could have just measured all this stuff precisely but... eh).

I should have purchased 6" or more. The Acetech Lighter BT fits on the end of this and the knurled ring that holds the thread adapter in place sits just ahead of the start of the forward grip tube section thing (which has a name that I am too lazy to remember or look up... you know... the place where all your rails are where you put your pointless flashlight I use to locate your position).

Buy one. Use a ruler. Don't be me.

"Came with the screw-in alignment pin. NO endcap washer or o-ring under endcap (transfer those parts from your existing outer barrel). This is probably a slight step up from a basic outer barrel. There are no existing gas block divots, if having a gas block is important to you you will need to add those yourself.
It is two-part but I could not easily separate them so I left as-is. If they were to loosen then as another noted some thread locker would help.
Getting useful dimension info to help pick your correct part is tough. Especially when altering to another length because dimensions vary, are missing, or not obvious from where to where being measured. These ZCI outer barrels measure from end-end (uninstalled).
Generally if you subtract 1/2" from the end-end length that will tell you how far out from the receiver edge the barrel crown will reach to. Then the threaded section is another 1/2" to account for (subtract) in case you are mounting a tracer.

I measured the 10.5" version as 10.51" end-end; so the thread section will "just" clear a 9.5" length rail when using a tracer/suppressor. Rail lengths tend to vary so in some cases "just" enough and others "just" shy. It has the very end OD reduced from 19mm to 17mm. My transferred 19mm endcap washer fits a bit loose over it but not really a big deal.

"Not a one piece design. Barrel unthreads near the receiver end. Suggest thread locker if you plan to use a mock suppressor or tracer unit that will be removed often. Also my former 5.5" outer barrel measured 5.5 from the flange to the tip. This measures 10.5" total length. Expected it to extend past my 10" handguard but it does not.