Anyone else have trouble with LMT M4 uppers and light loads?

[RyanB]

I am thinking that the gas port on my old rifle (sold it to a friend) is a little small. It runs great with military ammo. With anything lighter, like Wolf, Olympic or Black Hills blue box it will short stroke. The same ammunition, on the same lower, works well through my Colt 6520 upper.

Port size or something else? Does anyone know what port size a 14.5 inch barrel should have? Tally?

[C4IGrant]

I am thinking that the gas port on my old rifle (sold it to a friend) is a little small. It runs great with military ammo. With anything lighter, like Wolf, Olympic or Black Hills blue box it will short stroke. The same ammunition, on the same lower, works well through my Colt 6520 upper.

Port size or something else? Does anyone know what port size a 14.5 inch barrel should have? Tally?

What happens when you load one round in the mag and fire it? Does the bolt lock back?


C4

[RyanB]

No. I am sure it's short stroking, but I am hesitant to blame it on the port until I know if something else could be causing it.

[M4arc]

Could it just need a good cleaning? You said "old rifle" so I assume it's seen a lot of rounds and has been around for a number of years, correct?

[VA_Dinger]

You need a "Vickers Lube".

[RyanB]

Could it just need a good cleaning? You said "old rifle" so I assume it's seen a lot of rounds and has been around for a number of years, correct?

I would be surprised if it had more than 5,000 rounds through it, and it is clean.

[wedgehead30]

I have a 14.5" 1/7 LMT upper on my patrol carbine. I've run just about everything under the sun through it with zero problems. M193, Black Hills blue & red box of various weights, Wolf, various reloads, Federal american eagle, Winchester white box, Radway Green and some Guatemalan surplus. Are just a few of the different rounds in the last month alone. None gave me any problems and all shot with acceptable to outstanding accuracy. As a matter of fact I have yet to have any problems with this upper. It's at least 3 years old and I'd estimate round count at 5,000 +.

Wedge

[VA_Dinger]

I find it hard to believe you have anything wrong with your gas port or even the upper.

I would start with:

Lubrication - Triple the amount most of us would use after a cleaning and you’re getting close to what Yrac & myself named "The Vickers Lube" technique.

[BravoCompanyUSA]

I have LMT 14.5" that have short stroked - when I use sub par ammo. The upper is just fine. LMT uses the milspec size gas ports. All those gas port sizes were designed to run around military ammo (in a variety of enviroments). We ran the same gas port sizes on the BCM line up. And I fully expect to get ocasional calls about uppers finicky with certain ammo.

Many commercial market uppers have their gas ports hogged out (from the factory). And that is probably the smart move (from a business perspective). But the LMT line up was designed first as a offering to Crane and then as available to the civvie market.

Off the top of my head the current milspec size for 14.5" gas ports is right around .0625" give or take a thou... (that should give you cycle rates of 700-920)

[RyanB]

Thanks guys. I will advise my buddy to open up the port or just shoot all military surplus ammunition.