Bolt inconsistently locking back on 10.3" carbine

[Iraqgunz]

MIL MK18's use a USGI spring and H2 buffer.

Crossed the 500 round mark yesterday and thought I’d provide an update.

Yesterday was my first time shooting this pistol in the cold (approx 28 degrees). I’m glad I did because it gave me some useful data. I fired 50 rounds of my 5.56 55 gr reloads and 50 rounds of my .223 62 BTHP reloads. Up until yesterday the .223 load had been flawless in the MK18, but after a few hours in the cold I started to have failures to lock back. The 55 gr 5.56 loads continued to lock back, but ejection was very weak (about 5 feet at 3:00). What is interesting is that the 62 gr load was fine at first, but started acting up after a few hours. I found it kind of fascinating to see how the cold affected function.

Going forward I will swap out the blue sprinco for a standard carbine power spring. I know Blue + H2 is often recommended, but I think the newer DD 10.3” barrels with the 0.070” ports are undergassed for that combo. I believe DD includes a standard spring and a regular H buffer with their complete MK18.

That .223 62 gr load is very accurate in our 16” rifles, and I have 750 rounds left, so I’ll address the overpowered spring and see how that goes. I know some will say it’s a 5.56 gun so feed it 5.56 ammo, but honestly I want absolute reliability with any ammo in any temp. Besides, the fact that the 5.56 rounds juuuuuust barely ejected has me thinking the gun is oversprung.

[1168]

MIL MK18's use a USGI spring and H2 buffer.

And if I remember correctly, they also have the same gas port size that MWAG says his barrel has. So that spring and buffer are probably the best bet.

[MWAG19919]

MIL MK18's use a USGI spring and H2 buffer.

Yeah that’s what I’ll go with for now. If that doesn’t work I have an H1 to try.

As much as I want to do sprinco green and A5, I plan to put a Law Tactical folder on this, and I’m not sure I’ll like a LOP that long.

[thegreyman]

I think this is a Reloading and Ammunition thread; thread began with function problems resulting from reloads not from factory ammunition.

[MWAG19919]

I think this is a Reloading and Ammunition thread; thread began with function problems resulting from reloads not from factory ammunition.

I see where you’re coming from, but I wasn’t seeking reloading advice as much as I was seeking advice about my build. The ammo, while not necessarily full power 5.56, was not a problem in 3 other ARs. Although I could likely solve the issue by adjusting my reloads (or by simply purchasing factory 5.56 ammo), I’m more interested in advice about how to set up the gun optimally for my needs.

[Iraqgunz]

Yep that's the one.

And if I remember correctly, they also have the same gas port size that MWAG says his barrel has. So that spring and buffer are probably the best bet.

[1168]

Remember when everyone was complaining about .080 ports on DD and Noveske barrels? I bet someone would trade barrels with you on EE. I don’t think you are going to optimize the current weapon to your ammo; it should work the other way around (I thought that was the point of handloading?).

10.3” barrels have a pretty narrow range of ammo they will tolerate because of having so little barrel after the port. Port size sets ammo type, and the Mil Mk18 was not built with light ammo in mind. I would bet if you put a regular spring and H2 in it, it’ll eat M855 without issue. If you must shoot lighter ammo, you might find a 10.x” with a slightly larger gas port more suitable. If you plan on shooting a range of ammo types, you might find that an 11.5” barrel is more forgiving. There’s a reason that length is popular on M4c, and among LE, whose issued ammo can be subject to change. Soldiers, Marines, Sailors, etc, don’t have to worry about that. Ammo only seems to get hotter with succesive generations.

10.3” barrels like to be suppressed...

I see where you’re coming from, but I wasn’t seeking reloading advice as much as I was seeking advice about my build. The ammo, while not necessarily full power 5.56, was not a problem in 3 other ARs. Although I could likely solve the issue by adjusting my reloads (or by simply purchasing factory 5.56 ammo), I’m more interested in advice about how to set up the gun optimally for my needs.

[MWAG19919]

Remember when everyone was complaining about .080 ports on DD and Noveske barrels? I bet someone would trade barrels with you on EE. I don’t think you are going to optimize the current weapon to your ammo; it should work the other way around (I thought that was the point of handloading?).

10.3” barrels have a pretty narrow range of ammo they will tolerate because of having so little barrel after the port. Port size sets ammo type, and the Mil Mk18 was not built with light ammo in mind. I would bet if you put a regular spring and H2 in it, it’ll eat M855 without issue. If you must shoot lighter ammo, you might find a 10.x” with a slightly larger gas port more suitable. If you plan on shooting a range of ammo types, you might find that an 11.5” barrel is more forgiving. There’s a reason that length is popular on M4c, and among LE, whose issued ammo can be subject to change. Soldiers, Marines, Sailors, etc, don’t have to worry about that. Ammo only seems to get hotter with succesive generations.

10.3” barrels like to be suppressed...

I’m sure one of the overgassed barrels would work, but I’ll bump up the powder charge long before I do a barrel swap. It’s been reliable with the .223 stuff at the indoor range for about 200 rounds, but in the cold it was barely ejecting 5.56, which lead me to believe the extra power spring isn’t ideal.

[thegreyman]

You want to set your MK 18 for light reloads?