A5 vs Sprinco Blue

[CaptainDooley]

I've got a BCM 9" .300 with a standard lower setup and an H3 buffer. Runs pretty good overall after my single range visit. I didn't think to track ejection angle, so I'm not sure where it's landing. Using new USGI mags it would consistently not lock open after the last round about 50% of the time with both Freedom munitions 147gr. and Barnes factory 110gr. Ive got a Sprinco blue spring on the way to see if that helps, but I was wondering if that was the best way to go or if I should think about the A5 system. I don't really want to make the rifle any longer if I don't have to, but at the end of the day it's only .75" longer…

What do you guys think? If I do go A5, what buffer weight should I look at?

[tom12.7]

I've tended towards the A5 for better operation over different springs for the carbine action for 5.56 and 300 Blackout. You still need to tailor your buffer mass for your combination.
The carbine action H3 may be too much without a can for that combination with that barrel. Another choice could be to reduce the buffer to better fit your application.

[CaptainDooley]

I was wondering if the buffer was too heavy (H3 is the BCM recommendation). My plan was to take an H2, H and the Sprinco blue spring next range trip and see if any of those solved the problem.

[tom12.7]

I'm not so certain that BCM recommends the H3 for supers without a can. Did you test this with a can?

[CaptainDooley]

No can. Actually, I've never fired a gun with a can - which I need to remedy.

This is from the BCM product page: "These barrels were designed to run with a carbine buffer with subsonic ammo and a H3 buffer with high velocity ammo. Fine tuning may be required in limited instances based on ammo and buffer choices that work best for the desired application."

http://www.bravocompanyusa.com/BCM-9-300-BLACKOUT-KeyMod-KMR-A8-p/bcm-urg-300blk-9-kmr-a8.htm

Like I said, I've got a the spring to go up, and buffers to go down. My question really was "if the spring fixes it, would it be smarter to look at the A5 or just go with the spring?"

[tom12.7]

My choice would be of the A5 variety with a buffer mass that suits operation. The time increase in bolt over travel to the magazine helps the 300 Blackout more with the black tip more than 5.56. I have little data on the 147 loads that did not have the design intent originally for 300 Blackout.

[CaptainDooley]

Cool - thanks for the help. Do the A5 buffers use the same size weights as the carbine buffer? If they do, I've got enough parts to make all the different weights with just buying an H0

[tom12.7]

Same dimensions for most, except for the weight with the spring cut for the A5. Not all buffer masses are the same, some use more or less density in the tungsten, but you can take advantage of those differing densities to make a better choice. If you want to make it easy, buy a A5H0 and a A5H4, you could make any offering inbetween them, A5H0-A5H4.

[daniel87]

Aac for their 300 9inch calls for a standard car spring and an h2 buffer.

Try thd h2 first.

The stronger spring would be a temp bandaid.

Sent from my SM-G900P using Tapatalk

[tom12.7]

The AAC 9" upper with the SDN-6 with the H3 and red HK spring does tend to overspeed some magazines. The action work can be better applied with more time in duration in some areas more than some others.

[Clint]

An extra strength blue spring will be even more difficult to compress and lock back, so skip it for now.

Swap the H3 for an H2 with the standard spring and try that.

ETA: That's assuming the issue is too little gas and dhort stroking, rather than way too much gas and out running the magazine follower's ability to lift the bolt catch.