300 Blackout AR Pistol Cycling issue

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re mcx, I prefer the 11.5" barrel of the mcx to the 10'5" of the brn-180S, but I don't prefer the $2000 cost or the weight. I noticed they have a 9" 300BLK version which is cool.
View attachment 726011
the sig rattler is a 5.5” 300blk. they’re fun to shoot. also the sig virtus is a 9” they’re fun to shoot. i just dont like the price tag.
 
Get a recoil spring specifically for the 300bo and it should solve your problem. I shoot the 208gr subsonics and had short stroke issues at first, but the lighter recoil spring solved the problemo.

Edited to add: You can go with opening up the GP and adding an adjustable gasblock, but it will screw you over when switching between loads and suppressor on/off

 
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Get a recoil spring specifically for the 300bo and it should solve your problem. I shoot the 208gr subsonics and had short stroke issues at first, but the lighter recoil spring solved the problemo.

Edited to add: You can go with opening up the GP and adding an adjustable gasblock, but it will screw you over when switching between loads and suppressor on/off

Did you try a lighter buffer instead of a lighter spring? If not, was it just a "let's try this" option, or did you have a reason to try that before trying a lighter buffer? My reading suggested that a buffer had more influence over the rearward motion of the BCG than a spring, I could potentially see both being a factor when it comes to whether the BCG comes back far enough or not... Obviously it worked for your setup, but just curious on your thought process to get there.
 
Did you try a lighter buffer instead of a lighter spring? If not, was it just a "let's try this" option, or did you have a reason to try that before trying a lighter buffer? My reading suggested that a buffer had more influence over the rearward motion of the BCG than a spring, I could potentially see both being a factor when it comes to whether the BCG comes back far enough or not... Obviously it worked for your setup, but just curious on your thought process to get there.
I was using a carbine spring and a Spikes T1 buffer (lightest I had at 3oz)
I've been using ARs for a long time and considered the shortstroke had to do with either undergassing, or too much spring resistance. I gave the bbl mfg the benefit of the doubt and went with the 300bo recoil spring and it solved the problem with the heavier 208gr Hornaday Subsonics.

Edited to add: I may have gone over to arfkom to look for spring recommendations, but can't recall for sure.
 
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I was using a carbine spring and a Spikes T1 buffer (lightest I had at 3oz)
I've been using ARs for a long time and considered the shortstroke had to do with either undergassing, or too much spring resistance. I gave the bbl mfg the benefit of the doubt and went with the 300bo recoil spring and it solved the problem with the heavier 208gr Hornaday Subsonics
Do you know the length or how many coils are on your 300BO spring?
 
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I was using a carbine spring and a Spikes T1 buffer (lightest I had at 3oz)
I've been using ARs for a long time and considered the shortstroke had to do with either undergassing, or too much spring resistance. I gave the bbl mfg the benefit of the doubt and went with the 300bo recoil spring and it solved the problem with the heavier 208gr Hornaday Subsonics.

Edited to add: I may have gone over to arfkom to look for spring recommendations, but can't recall for sure.
After moving to the lighter spring, how was it shooting the lighter (e.g. 125gr, 150gr) supersonic loads? Do you need to swap springs back and forth?
 
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I’m running the .300 upper on my Rattler (I have the 5.56 as well but the gun is better with the .300) and I run it with my AAC can. Zero cycling issues from 110g to 240g. I have a couple other .300s but haven’t used them near as much as this one.
What are you running for buffer weight, spring type, gas system length, etc?
 
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Just another story here, but my buddy was having trouble with his as well, and we did everything... swapping all kinds of different parts. Buffers, buffer springs, bolts, and BCGs. Turned out his fancy, self lubricating, adjustable gas BCG was the problem. Thing ran great with a cheap BCG.
 
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What are you running for buffer weight, spring type, gas system length, etc?
The Rattler platform doesn’t use the traditional buffer, buffer tube, etc. Mine are straight-from-SIG complete uppers. I bought the Rattler 5.56 as a pistol, Form 1’ed it, and added the .300 upper.

What they’ve done with their design/build is pretty amazing.
 
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Haven't tried the lube yet, but plan to in the couple days, but with that suggestion set aside for now, here's the end of the email I plan to send back to the Mfg... Anyone feel like I missed something or was really off-base?

=== QUOTE ===

Further options that were suggested...

-A lighter, 300BO specific buffer spring (this one was suggested by a couple people as working well from 125 grain to 240 grain in an AR pistol with a standard 3 Oz buffer, like we have in the uppers you supplied). This might be worth trying first...

-A lighter buffer was another suggestion, but might result in more felt recoil and having the bolt unlock sooner (thus more gas in the face), so might not be as a good of a solution as a 300BO buffer spring

-Enlarged gas port (drilling out progressively 1/64" larger than stock until the 208 and 220 208 grain cartridges cycles consistently). Possibly an okay solution, but isn't reversible and might require an adjustable gas block if hole gets too big, otherwise it might get over-gassed such that the supersonics may not run well.

There are definitely a lot of people out there with uppers that run all weights of 300BO successfully, so it seems we just need to tweak the right variables to get things properly functioning... The options above were discussed quite a bit, and it would seem that if the lighter spring cycled all bullet weights successfully, it would probably be the best option with the least downside/risk.

=== /QUOTE ===
 
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I've not tried any lubricant at all just yet, other that anything that might have come on it from the manufacturer. The BCG is nitride, feels nice and smooth (not fouled up yet, as it's probably only had about 40 rounds through it so far), but it definitely doesn't feel greasy, oily, or otherwise wet. I'd think it should work relatively dry when fairly new, but I guess a little lube would be an easy thing to try. It would be nice to know that it's not needed to function.
Lube first, IMHO it is unrealistic ( no offense ) to expect reliable performance from a 300BK with out lube.
Allow the upper to break in, with lube. Fire the most reliable ammo used so far for break in.

Trying to see if it runs without lube is something for after break in.

The 300BK with the large diversity of bullet weights / powder charge weights / gas system length ( especially a carbine length ) can be finicky.

There are a lot of add on parts available for the 300BK uppers... but, I'd suggest pursuing those after a reasonable break in period.
 
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Lube first, IMHO it is unrealistic ( no offense ) to expect reliable performance from a 300BK with out lube.
Allow the upper to break in, with lube. Fire the most reliable ammo used so far for break in.

Trying to see if it runs without lube is something for after break in.

The 300BK with the large diversity of bullet weights / powder charge weights / gas system length ( especially a carbine length ) can be finicky.

There are a lot of add on parts available for the 300BK uppers... but, I'd suggest pursuing those after a reasonable break in period.
[/quote]
None Taken, and thanks for the input. That does make sense. :) But, for now, something else is already in the works, though this may come back into play. More on that in my general update below. :)
 
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General Status Update on my issues...

After sharing my post above with my vendor and discussing some options, they had their supplier finally get some 10.5" 300BO nitride barrels in stock, which is what we ordered originally, but with pistol-length gas systems (recall that through processed we had switched from 10.5" carbine-length to 7.5" pistol length), they're going to test those with some 220 grain bullets, and if they cycle correctdly, they'll send me those, which, in the ened, is closer to what I originally wanted. (I really didn't want to go shorter than 8.5", but decided to try it, but I'll be happy with the original 10.5", if they function consistency.)

Will update more after I receive/test them. Thanks again for all the advice so far. Given that the option we're trying now is more along the lines of my original order, if I have any issues, I'll probably try more of the things suggested to find the optimal config, but hopefully it just works. :)
 
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Quick update...
Got the 10.5" barrel with pistol-length gas system back. But busy, so not a lot of testing yet, but I ran 3 rounds of each:
125gr
150gr
208gr (SS)
220gr (SS)
And all ran without issue so far. That's a good start anyway. Now I just need to finish other projects so I can get to some more in-depth testing. :)
 

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