Can vs no can - effects on pressure/cycling?

BHuij

Active Member
I posted this on Reddit, but it seemed like the kind of question better suited to the folks in this community.

I’m doing my homework on a 10.5” 300 BLK AR-15 pistol build. Honestly, I don’t really see the point in having a 300 BLK firearm unless you’re shooting subs with a can (personal opinion).

I’d like to be able to work up a really good subsonic cast bullet load (reasonably accurate, 100% reliable cycling) while I wait for my tax stamp to come through. Assuming I found a load that worked well, would it translate seamlessly to shooting quiet? Or would adding a suppressor change chamber pressure or anything else significantly?
 

fiver

Well-Known Member
suppressors will raise barrel pressure, well back pressure anyway.
think about them like you would a cars exhaust system, a good system will help the vehicle actually run more efficiently and have more power.

a can, can help with cycling, but a pistol length gas system will pretty much be using the gas long before the can can do it's job.
if however you have gas volume available but no pressure, the can can help by holding the pressure a little longer letting the gas actually force the bolt back.
 

Ian

Notorious member
A can increases both bullet velocity and dwell time, so you may have to back down on the powder charge a little once you put a can on it.
 

fiver

Well-Known Member
that's what I said...LOL
I just liked typing in can can and wanted to see how many times I could work it in in one sentence.
 

waco

Springfield, Oregon
What powders suitable or the 300 generate the most gas volume? Anyone try Cfe Blk yet?
 

Ian

Notorious member
that's what I said...LOL
I just liked typing in can can and wanted to see how many times I could work it in in one sentence.

Heh heh...that explains it. I wondered why you pulled a me and wrote two pages for a one-sentence answer.
 

BHuij

Active Member
Sounds like if I’m having any trouble getting cycling with my subsonic load, the can might push me across the finish line.

That said I’m not too worried about failure to cycle with heavy bullets and a short gas system.

I’m just trying to decide between getting the Lee 155gr .312 mold and making a mouse fart subsonic load to keep my CPR down, or doing it “right” with the Lee 230gr .309 to get closer to full power.
 
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Ian

Notorious member
The 155 won't cycle and stay subsonic. Get the Lee 230 and powder coat them. If you don't plan to powder coat, get something else.
 

fiver

Well-Known Member
depending on the twist rate,,, a 1-7 yeah.
1-8 I'd go with the NOE 200.

depending on your throat of course.
I have seen 3 different ideas about the 300 throat in 3 different guns so far.
the 2 bolt guns I have right now will not interchange ammo.

my money is on AA-1680 giving the most gas volume.
I liked 4227 it has a broad window and reliably operates even carbine length tubes with 200 gr bullets at slow nuff speeds.
 

Ian

Notorious member
1680 is really good for supers in the 125-160 range, but absolutely sucks with subsonic heavies. After 10-15 rounds the FCGs on my and L1A1Rocker's short Blackout ARs both looked like a full teaspoon of ground pepper had been dumped in them, and his fancy custom speed hammer started failing to reset. Mine worked fine but probably wouldn't have after a couple more magazines. The other problem we had was velocity being all over the place; one would crack and the next barely cycle.
 

fiver

Well-Known Member
that's the problem with wanting slow and gas volume, it's real easy to get one or the other.
not so easy finding the balance.

when I want slow and heavy in the bolt guns I go to a fast pistol powder and barely put any in the case.
when I want fast and light I go slower on the powder speed and push them to the muzzle.
 

BHuij

Active Member
So far I'm leaning towards 4227 since it's such a short gas system and the bullets are heavy. Guess the Lee it is. I have such a hard time justifying spending on 1 non-Lee mold what would get me 3+ Lee molds. I get that it's necessary for HV applications, but for shooting quiet 900 FPS slugs from a "just for fun" 300 BLK, I think I can get acceptable accuracy without splurging on a really nice mold with perfect throat fit.
 

Ian

Notorious member
It'll shoot. Some 50-yard sight-in groups to prove it. "Pct. 30" is my 10.5", and had a 2-moa red-dot sight. Now it wears a thermal scope. Putting on a thicker coat and sizing to .3095" instead of .3090" took care of the yawing from the 10.5" long 1:8 barrel. Darth Maul is a 1:7" X 16.5" barrel.20181104_211145.jpg
 

fiver

Well-Known Member
10 grs is right there with 4227 also.

my 1-7 is right at the too slow tipping point with 3.5grs of 700-X with a rifle primer.
a pistol primer will allow them to become unstable.
the 8 twist rifle needs almost another half grain to shoot it's best at it's slowest.
 

BHuij

Active Member
Since I haven't built the AR yet, sounds like you guys are making a pretty strong case for the 1:7 barrel :D
 

BHuij

Active Member
Actually, I'm finding a lot more barreled uppers in my price range that are 1:8 twist. Do I lose anything by going to the Lee 200gr .309 round nose mold instead of the 230gr .309 5R? I'm sure the ballistic coefficient is significantly lower on the 200gr, but for plinking within 100 yards I don't think that matters.
 

Ian

Notorious member
For a pistol-length gas system...no. For a carbine-length gas system, 220gr is minimum with a .120" gas port.

I'm a big fan of CMMG barrels.
 

BHuij

Active Member
Yeah definitely sticking with the pistol length since it's a very short barrel. I read somewhere that 300 BLK only needs 10" of barrel to get a full burn. I'm assuming that refers to rounds specifically within certain military specs (IIRC the cartridge was designed as an AR-platform replacement for suppressed MP5s, aiming for superior terminal ballistics).

I like the 10.5" barrel option, and don't really think I need to go any longer. That said, my understanding is that a longer barrel = less pressure at the muzzle when the projectile exits = quieter suppressed performance. Is this going to be noticeably quieter at 11.5" or noticeably louder at 8" for example?
 

fiver

Well-Known Member
some but I doubt 10 decibels is that big of a deal.
I was putting some Hornady 200gr eld's together for a buddy with a medium length tube 10grs of 4227 was the first load we tried and it was 100% successful.
I never got a chance to try backing the load down or even running it over the chrono.
he was ecstatic with the load and that's where he stayed.