Uppers...300BLK vs 5.56 pistol length reliability?

[danasaki]

From what I have read on several AR websites, the 5.56 caliber SBR or pistol upper seems to run best when in lengths 10.5" or longer. I know there are folks who have 7.5" ish ARs who claim great reliability, and that's fine. I am not here to dispute their claims nor start a pi$$ing contest over whether or not the shortest uppers in 5.56 can an do deliver reliability.

I am after more technical advice as to whether the 300BLK in a short (think 8.5 to 9.0") barrel might have the same issues as the 5.56 caliber uppers in an SBR or more especially, an AR pistol.

I am wanting to build an AR pistol and use both caliber uppers. I have decided on a 5.56 upper, but as for the 300BLK upper I am still looking at the characteristics involving the "shorter" of the short uppers in that caliber. I really like the 8.5 to 9.0" 300BLK uppers and I am hoping I can glean some solid info on using that length barrel in that caliber with regard to overall reliability. Also, whether gas impingement vs piston makes much difference in the short 300BLK upper's reliability, think cycling, ejection, etc..., not just the typical gas vs piston comparison.

I have AR carbines for training so this AR pistol is more of a hobby project and range toy, although one I want to depend upon if needed.

Thanks for any helpful insight. And yes, I did search but could not find the info for which I'm looking.

[bzdog]

There are lots of folks running the Noveske 8.2" and similar. I'm no expert, but I don't see why it would perform differently in pistol form.

My understanding is the 300 BLK works better out of short barrels than 5.56. For example, it looks looks like you only lose 164fps with the Barnes 110gr between a 16" and 9" barrel (2350 vs 2186).

Note Noveske configures theirs with pistol gas and H2 buffer.

-john

[skydivr]

I am in same boat so will enjoy reading the responses of those more informed than I.

[rsilvers]

This whole business about being limited to 10.5 inches I think has a lot to do with people not building rifles correctly. Did they use a extractor spring with enough tension? Did they use a pistol-length gas port? Did they use an H2 buffer? I have a 7 inch 5.56mm that is reliable and I have done 100 round Beta mag dumps with it.

300 BLK "sees" barrels much differently than 5.56mm.

A 7 inch 300 BLK is like a 10.5 inch 5.56mm.

A 6 inch 300 BLK is like a 9 inch 5.56mm.

[danasaki]

I appreciate the quick replies, fellas. For my build project, I will build the lower but only use built uppers from proven companies.

Are the buffer weight and extension and spring the critical pieces to properly mate to the upper for cycling reliability? I presume so, but have not gotten that far into this project yet to experiment hands on yet.

[skydivr]

Keep going Mr. Silvers....pls elaborate further. Additional discussion regarding suppressed/nonsuppressed would be sincerely appreciated. Is a H2 buffer same as a Spikes T2 buffer?

While I'm going to build the lower, I also think a built upper makes the most sense for my level of knowledge.

[rsilvers]

A Spikes T2 buffer is not as heavy as an H2 buffer. I would call it an H1.3. It is not even an H1.5.

It i probably ok, but my personal preference would to just get an H2 buffer.

[danasaki]

...Did they use a pistol-length gas port?...

I presume this would only be an issue with someone building their own upper (perhaps not knowing what to properly use?) and NOT from a company built upper?

[skydivr]

A Spikes T2 buffer is not as heavy as an H2 buffer. I would call it an H1.3. It is not even an H1.5.

It i probably ok, but my personal preference would to just get an H2 buffer.

The ST-T2 is 4.05 OZ and the H2 is 4.3 OZ so it's pretty close, but I'm sure you are correct. I understand it's the weight of the buffer that counts. Other than that, any other differences in lower parts that has much difference between a 5.56 or 300BLK build?? How about CH's in the upper?

[rsilvers]

H2 is 130 grams/4.6 oz. The HK-416 buffer, like the T2, is powdered W, and it is 4.7oz.

The T2 is not very close to the H2 or 416 buffer. They either use less dense W, or less of it.

There are no differences in lower parts.