Q's related to low-mass bolt carriers

[bluejeans]

i'm in the planning phase for a lightweight big bore for my work and i have two quesitons for those who are savvy one low mass BCG's.

1. chambering heavy-grain bullets. i was reading an old post where a buy with a .358 win. AR said that his lightweight carrier wouldn't reliably chamber bullets over 220 gr (memory.. can't find the post now..) makes sense; the lighter reciprocating mass of the carrier is mostf easily stopped or slowed by a heavier shell and fails to fully close. has anyone else experienced this. what bullet weight was the max? (ar-15 or 308 sized) could this be overcome by a heavier spring and more gas?

2. (.. and considder this seperately from the issues in q#1) we all know that when using a lighter carrier we need an adjustable gas block to tune/reduce the gas.. so would i be able to run a pistol length barrel with a mid length gas system? (or similar) and be closer to the gas requirements for cycling the lighter carrier? i ask because i see very little selection for big bore pistol barrels for the ar-15 andeven less for the 308 sized action.. but with plentiful carbine pencil barrels, and owning two lathes, i could easily cut down a 16 or 18" barrel to 12" and rethread/crown.. and i could choose between the carbine, rifle, or mid length gas port. (portss @ 7", 12", and 9" repectively)

anyone played with this?
thanks in advance.

 

[Varminterror]

1) If a cartridge is failing to chamber, it's FAR more likely a poor ammo sizing job than a bullet weight issue. I'm not buying the whole "the bullet weighed too much for the carrier to push it" BS. More gas won't fix a battery issue, other than relying upon bolt bounce, which again, is just silliness. Proper spring rate, clean feeding, and proper ammo size for the chamber, it'll close. Small base, sufficiently bump the shoulder according to the actual chamber - not just bump based on the fired cases, and make sure you're not choking the throat or jamming the lands, and make sure you're not crashing rounds on the ramp or extension with poor feeding. Guys come up with the silliest of excuses for why their rifles don't run - more likely, he was either seating those 220+ bullets too long, or they were too blunt and crashing the ramp or extension and stalling the BCG, not some magical inertial ceiling for picking up rounds from the mag.

2) I buy my barrels custom ordered for projects like this, so I can pick whatever gas length I want. You're far more restricted in trying to do this with cheap, off of the shelf stuff. I tend to prefer to an AGB on a shorter gas system (standard length for the barrel length), rather than trying to "dissipate" with a longer than standard-for-barrel-length system. The AGB will let a short system act like a long system, but there's no way to force a long system to act short.

I think a lot of mil-spec mafia like to blather on about how lightweight reciprocating systems don't work, because it sounds good online. They're really not that challenging to make run - just give them a stiff enough spring to not bounce at the front, and light enough gas so they don't try to blast out the back, they'll run fine.

 

[IndianaBoy]

My experience with low mass carriers in the AR-15 are confined to tuned match rifles shooting 223/5.56 from 18 and 20 inch barrels. I consider an adjustable gas block to be mandatory to extract the benefit from using low mass carriers. That has grown to me adding adjustable gas blocks to rifles that are using full mass carriers as well, especially those that are shot suppressed as the suppressor changes the amount of gas going into the system significantly.

It is important to understand that low mass carriers were developed to reduce reciprocating mass, not to reduce the overall weight of the rifle. You might be ahead to use a standard carrier and use a slightly slimmer barrel profile.


Are you building an AR-10 or an AR-15?

What cartridge are you using?

What barrel length do you want?

 

[IndianaBoy]

If you are cutting a barrel down to 12", you will probably want a carbine length gas system. I have a carbine length 11.5 in 223 that functions perfectly with and without a suppressor.

Cartridge selection will play a role of course. 300 blackout is often using a pistol length gas systems on barrel lengths from 8 to 12 inches. But that is often to ensure function with subsonic ammunition.

 

[Varminterror]

It is important to understand that they were developed to reduce reciprocating mass, not to reduce the overall weight of the rifle.

This is accurate, but, in fairness, doing one job can also do the other.

 

[bluejeans]

Are you building an AR-10 or an AR-15?

What cartridge are you using?

What barrel length do you want?

ar-10/15 undetermined. i have several lowers for each one ready to finish. i'm looking into light carriers as the finished weight is largely what will determine if i can afford the bulk of the larger frame.

358 win if i go big... if that's too heavy then on the ar-15 i'd be looking at the 358 yeti and the 450 bushmaster.

barrel length probably 12" but could be anywhere from 10.5-14.5" if i can avoid any sharp drops in velocity -depending on the final caliber. but 12" is what i'm aiming for.

 

[d2wing]

One of benefits of heavier BCG or buffer is to slow down the bolt speed. An AGB does the same thing. Another issue is that depending on the cartridge used, you need dwell time to provide enough gas to operate the action. Dwell time is the time from when the bullet clears the gas port to when it leaves the barrel. The shorter the dwell time, the hotter the load or more gas you need. It takes someone smarter than me to figure out how an AR pistol works.
A heavier bullet moves slower and I would think provide more pressure and dwell time.