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.



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