Semi Auto carrier for 7.5" Barrel?

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Condor1970
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Semi Auto carrier for 7.5" Barrel?

Post by Condor1970 » Tue Jul 28, 2020 5:28 pm

Just curious what your thoughts are on this. Almost done building my AR pistol. I have a 7.5" barrel, and can't help but think a semi-auto BCG (vs a full auto) might be better for a 7.5" barrel, due to being a little lighter. Thoughts?

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BJK
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Re: Semi Auto carrier for 7.5" Barrel?

Post by BJK » Wed Jul 29, 2020 9:06 am

I think if you go too light you can correct that with a heavier buffer. I don't have huge experience with this, I don't do many builds and only one 300BLK build. I don't think it would be wrong to do as you intend, but my answers are always never that short. So here goes...

Mine uses a 10.3" barrel, but has the same pistol port location as yours. I used a JP Ent' full mass BCG. A standard weight buffer and standard buffer spring. I'm getting a correct ejection pattern @ about 4 O'clock.

More than that I don't have. Below are links so that you can compare what you have in mind to what I have working in mine if that will help.

My consideration was that the firearm must work in all temp's it's likely to encounter and I didn't want to go too far astray from the AR/M16 as it was designed. The basic design works pretty good without reinventing the wheel. Now if I had a specialized reason to change things up, like competition or some such I might have considered changing innards, but I just wanted it to function reliably without, again, special considerations.

But unless I miss my guess I'd be willing to bet a few million ARs have been built using semi-auto BCGs and they work fine. I think you might run into unforeseen issues if you go with too light a BCG, then you might need an adjustable gas block and other stuff not required on a more standard build.

https://www.ballisticadvantage.com/10-3 ... arrel.html
https://jprifles.com/1.4.7_FMOS.php
https://www.wingtactical.com/firearm-pa ... er-spring/

I had problems initially with cycling, but not due to anything other than a rifle buffer spring in the handgun. I cut off 3 coils and it worked fine. Now I have a flat wire carbine buffer spring in it and it works fine as long as I give it enough gas pressure. One issue that might be a problem for some... That flat wire spring appears to be "twangy" and it's been many decades since I notice buffer spring noise in the design. It doesn't bother me, it's just something I noticed but could bother others. Yeah, I know what the text at the link states. But I know what my ear picked up. Maybe I noticed it because it sounded differently. I don't know and didn't analyze it.

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dellet
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Re: Semi Auto carrier for 7.5" Barrel?

Post by dellet » Wed Jul 29, 2020 9:08 am

Depends on how the rest is set up.

The lighter carrier will have a bit less recoil and movement taking the sights off target, and be more likely to be over gassed

The heavier carrier will feed more reliably in poor conditions, but have trouble cycling loads on the edge.

All this changes is if the gas system set up wrong for the carrier choice.
300 Blackout, not just for sub-sonics.

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