Bolt inconsistently locking back on 10.3" carbine

[26 Inf]

I load on an XL650 and I have a 62 gr BTHP load that is sub MOA at this charge weight. It's a PITA to adjust the powder measure, but I'm more than willing to bump up the charge weight on the 55 gr load if that makes this gun happy.

I load on a 550 - same powder measure - I bought one of these and found it helpful in keeping track of things - https://dawsonprecision.com/arredond...owder-measure/

Probably costs a nickel to make but $4.95 isn't much, I just threw it onto a sight order.

Anyways, tomorrow would you be will to PM me your recipe? 'Preciate it.

[MistWolf]

Not that I'm questioning what you say, buy could you tell me where I can find that info - I'm a copy and paste kind of guy. Thanks.

Colt uses a gas port of .0625" and an H buffer for the 16" 6920.

Colt uses a gas port of .0625" and an H2 buffer for the 14.5" M4.

[MWAG19919]

I load on a 550 - same powder measure - I bought one of these and found it helpful in keeping track of things - https://dawsonprecision.com/arredond...owder-measure/

Probably costs a nickel to make but $4.95 isn't much, I just threw it onto a sight order.

Anyways, tomorrow would you be will to PM me your recipe? 'Preciate it.

I've always wondered about the powder measure upgrades. I got so tired of adjusting the measure in the wrong direction that I wrote +/- arrows in sharpie, but it's still very much a guess & check operation. And I'm so anal that I weigh 10 charges electronically and won't stop adjusting until I get 250.0 gr with 10 throws for a 25.0 gr load.

Btw, your inbox is full.

[26 Inf]

I've always wondered about the powder measure upgrades. I got so tired of adjusting the measure in the wrong direction that I wrote +/- arrows in sharpie, but it's still very much a guess & check operation. And I'm so anal that I weigh 10 charges electronically and won't stop adjusting until I get 250.0 gr with 10 throws for a 25.0 gr load.

Btw, your inbox is full.

I think just having a dial with markings on it helps. I check mine just about the same way, and recheck the drop every second primer tube.

In terms of repeatability, this one would seem to be pretty accurate and repeatable: http://www.uniquetek.com/product/T1231 It is $129.00 installed on a new bar.

This is probably the next best thing: https://www.doublealpha.biz/click-ad...le-powder-knob

[Iraqgunz]

NiB is garbage. In addition NiB changes the dimensions of the components.

I'm genuinely curious why you say that. The toolcraft was merely a spare I had lying around until this build happened, but I would expect a BCG with a lower friction coefficient to lock back before a phosphate coated one.

Not being argumentative, just curious.

[Iraqgunz]

It comes from first hand examination of the carbines as well as measuring ports in barrels.

Not that I'm questioning what you say, buy could you tell me where I can find that info - I'm a copy and paste kind of guy. Thanks.

[26 Inf]

Thanks!

[100]

Mechanical devices fail, and sometimes mechanical devices are sold that dont meet spec.

[Pappabear]

Is this a gun that would take a A5 buffer system? If so, why not go there, it fixes so many ills, just why not.

Sorry if it's in middle of thread I skipped past.

PB

[MWAG19919]

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.