Range Report: 300 Blackout SBR

[Netpackrat]

You might be surprised to learn just how far down the o-ring rabbit hole goes.

http://www.ar15.com/archive/topic.html? ... 8&t=336520

Not sure I ever did get to the bottom of it, but near the end of that thread a guy claiming to have developed the use of it in the first place, said that the correct part is not an o-ring at all, but rather a "d" ring that Brownell's sells for $10 each. And that after they developed the fix, the .mil contractor screwed them and went with the cheaper generic o-ring that they'd supposedly found doesn't work as well. And the people who sell the plain o-rings are not saying what they are, so they can keep selling them for a few bucks each rather than a few cents, to the willfully ignorant tacticool crowd. Seems like everybody else is using a #006 viton o-ring, which currently sells for $4.48 per 100 at McMaster-Carr.

[Denis]

You might be surprised to learn ... that the correct part is not an o-ring at all, but rather a "d" ring that Brownell's sells for $10 each.

Firm, rubber, D-shaped ring surrounds the extractor spring to multiply extractor spring tension by 400%. Easy installation prevents virtually all extraction problems associated with AR-15 and M16 firearms.

I know nothing about AR-pattern rifles, but wouldn't the fact that the tension of the original spring needs to be quadrupled to ensure reliability tend to suggest there is something wrong with the specs of the original spring?

[Netpackrat]

I know nothing about AR-pattern rifles, but wouldn't the fact that the tension of the original spring needs to be quadrupled to ensure reliability tend to suggest there is something wrong with the specs of the original spring?

You won't get much argument out of me, but I don't actually like the AR. I think it's an overall crappy design with a few really good points, and nothing in my ownership so far has dissuaded me from that view. The only reason I own one is because it is the easy button for shooting 300 Blackout with a semi. Note well, that I didn't buy a shitty generic bolt carrier group... I bought what is widely regarded as one of the best available. And I still had to resort to what strikes me as being a ghetto fix, albeit an institutionally adopted one.

That said, from what I read elsewhere, the o-ring solution originally came about for use with carbine and SBR length barrels and shorter gas systems. Apparently they cycle more violently, so the extractor needs some help. Per what I was reading last night, the extractor spring on its own is generally adequate with the rifle length barrels and gas system that the rifle was originally designed for.

And 300 BLK seems to be a whole 'nother ball of wax. It may be designed to allow conversion of a 5.56 AR by changing only the barrel, but there's no getting around the fact that the system is designed to run 5.56. In general it works pretty good, but it's more prone to issues. Given the purported purpose of the o-ring in dealing with more violent cycling, the fact that mine wasn't extracting right unsuppressed doesn't seem to make a lot of sense... Since the added back pressure with the can ADDS to the violence of cycling, one would think that the problem would manifest itself suppressed, rather than unsuppressed.

I'm not going to spend much more time trying to understand that; I am happy that my rifle seems to be working OK now. Despite the things I don't like about it, it is accurate and a lot of fun to shoot. It's a good thing it's fun to shoot because cleaning the SOB sucks ass compared to what I am used to.

Edit to add; totaled up all of my saved brass tonight, and it looks like I have 530 rounds through it so far. I am down to my last 3 boxes of subsonic ammo. However much of that I have in my range bag, tends to be the amount of it that I shoot on a given outing. I see an XL650 in my future.

[rightisright]

You might be surprised to learn just how far down the o-ring rabbit hole goes.

Oy. I found the other McMaster SKU while doing a search for BCM o-ring. I didn't realize there was such debate about lowly o-rings... but, then again, it is Arfcom.

[Netpackrat]

These groups were all fired with the relatively inexpensive UMC 120 grain ammo that is supposed to most closely match the BDC reticle on my scope. Since the last time I benched it and checked for zero, I had removed and installed the scope countless times, and there didn't seem to be any significant shift (that was a cold October day, so I am not sure how close I got it in the first place). Go Bobro.

Fired a couple groups unsuppressed (not shown). Despite the troubles I have had with this can (SDN-6) on other rifles, the 300 SBR seems to like it a lot. I didn't note enough POI shift to worry about with the can removed, although the groups got bigger without it. It locks up as tight on the 51T mount as it did the first time I tried it... Wish I could say the same about it and the 51t mounts on the other rifles that will take it.

Oh... This was the first time I fired it on the second (Aero) lower I SBRed. Not sure yet what I am going to do with the SBR Palmetto lower. I moved the G2S trigger and the Hogue stock/grip over to the Aero lower, so the only difference I could feel was the lack of rattle between the upper and lower.

[D5CAV]

I don't have your patience.

I picked up a .300 BLK upper because some guys in one of my classes were raving about it. They were talking about turning an ARF into AK ballistic performance.

I put it on one of my ARF lowers and discovered I had a straight-pull AR.

I figured I needed to play with the spring or mess with the gas port. Maybe the o-ring would work for me as well.

Anyway, I wasn't particularly impressed with the cartridge performance either. I decided if I felt a burning desire to launch .30 projectiles, I'd do it with a 7.62x39 or 7.62x51 launcher.

Project got dropped after 2 trips to the range.

[Netpackrat]

So far it's been worth the initial hassle, although the constant cleaning due to the rifle shitting where it eats gets a little old after a while.

[rightisright]

So far it's been worth the initial hassle, although the constant cleaning due to the rifle shitting where it eats gets a little old after a while.

Do you have an ultrasonic cleaner? They work great for the BCG.

[Netpackrat]

I bought a Hornady ultrasonic unit on sale a few months ago... I haven't been all that impressed by how well it works, so far.

[HTRN]

Try a different cleaning agent. Also, they take quite a while to work..