.300 BLK 10” minor function issues

[757_Magnum]

I would hope it to work perfectly when new and dry… but when a mish mash of various manufacturer’s pieces are put together and run with a short barrel….

Might need parts to break in.

I’ll bet after 300 rounds there will be shiny spots on places where there weren’t before and it’ll cycle with less resistance.

From my understanding, a 9 to 11 inch 300 BLK with pistol length gas tube has about the same pressure curve as a 16" carbine or mid-length 5.56. It's not the same as a 10.5 inch in the latter caliber, where you start pushing the envelope for reliability.

[JCN]

From my understanding, a 9 to 11 inch 300 BLK with pistol length gas tube has about the same pressure curve as a 16" carbine or mid-length 5.56. It's not the same as a 10.5 inch in the latter caliber, where you start pushing the envelope for reliability.

It’s interesting for MCX that the gas port is in a different place for their 4.5, 9 and 16 inch barrel 300 blackouts.

I figure his build will be just fine after parts wear in a little.

But I personally wouldn’t put pieces of parts together in a home brew build and expect it to be self defense vetted without an extended break in.

[JCN]

The first thing I would suggest is to try standard 5.56 mags. As far as I know, the 300 BLK specific mags are mainly to accommodate the ogive on certain subsonic rounds. I've had great luck with standard Lancers on my two 300 BLK builds. I haven't tried my gen 3 Magpuls just because I'm reserving those for my 5.56 rifles.

I bought some of the rubber band label things to go on some of the Magpuls to identify them quickly.


https://www.amazon.com/Blackout-Maga.../dp/B084M73CYF

[LockedBreech]

Very much appreciate all the input. I’ll table using it as a home defense gun for now and continue using my off the shelf DDM4 for that purpose. I’ll update the thread when I’ve had no less than 500 rounds through it.


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[JCN]

Very much appreciate all the input. I’ll table using it as a home defense gun for now and continue using my off the shelf DDM4 for that purpose. I’ll update the thread when I’ve had no less than 500 rounds through it.


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Do you have a can yet for it? Suppression also can help cycle.

[LockedBreech]

Do you have a can yet for it? Suppression also can help cycle.

I don’t, and hasn’t really intended to, but that does make an awful lot of sense for a home defense gun.


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[JCN]

I don’t, and hasn’t really intended to, but that does make an awful lot of sense for a home defense gun.

My version. Sig MCX rattler with the 4.5” barrel swapped for a 9” plus can (wrapped to not burn myself).



Shooting supersonic.

[LockedBreech]

My version. Sig MCX rattler with the 4.5” barrel swapped for a 9” plus can (wrapped to not burn myself).



Shooting supersonic.

That is pretty freaking sweet.

[HCM]

This conversation has already made me realize I don’t know much about long guns. I had no idea about mating the bolt and I don’t have a pistol lower to compare recoil spring length too (unless it’s just a normal carbine spring in which case I have two). 90%+ of my AR rounds have been fired through my Daniel Defense DDM4V1 that I got off the rack in 2011. It never needed any particular break in.

I will hold off on the pricier ammo until I can grab a large chunk of bulk basic ammo to run and break it in. Sorry if I jumped the gun error reporting, I’ll report back and bump the thread if 500+ rounds and more lube doesn’t fix the issue.


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Are there any signed of birds, or unusual warmarks indicating something being out of spec?

What weight buffer and what buffer spring are you using ?

The interaction between the gas coming back the weight of the buffer and the spring or what run your cycle of operation.

One thing to check is to load a single round and see if the bolt locks back when fired. If it does that means the gas coming back is adequate.

You may need to use a lighter buffer or a stronger buffer spring.

Given current price and availability of 300 blackout ammo trying a slightly stronger spring from Sprinco might be cheaper than breaking the gun in with half a case of ammo.

Adding a suppressor may also change that dynamic. Adding a can will increase the back pressure making the gun cycle faster. Unless you keep a set of active hearing protection with your home defense gun a suppressor makes a lot of sense for an HD long gun.

[SecondsCount]

It sounds to me like the headspace is on the tight side which could be the ammo or the bolt/chamber. You would need a go-guage to check the bolt, or a set of case guage tools to measure the ammo.

If the chamber is close to tolerance, shooting it will wear on the lugs and open the headspace a few thousandths. That's probably all you will need.