First AR Build and Some Basic Questions

[Joelpat]

Hi Guys. I've built several bolt guns and a 10/22, now I'm jumping into ARs. I have a Sig MCX, so I'm not completely new to the design, but obviously a proprietary piston gun isn't the same thing. I have no doubt I could build a 5.56AR pretty easily. But I don't do easy very well.
I've got two New Frontier 80% lowers. One for a PCC and one milspec. I'm not in any hurry, which is good, because I'm going about this the hard way.

I want a 357sig PCC. Not easy, but not impossible. And I also would really like to build a piston design. I know... My theory is that the higher chamber pressure of the 357 should be make a DI or piston setup possible. I have a 357 pistol, so it saves me having to load or buy another caliber. If I want to go subsonic, I can make up light loads and mimic a 9mm sub.

I have access to traditional and CNC milling equipment if I need it, so I'm gonna give it a try. Setting aside the piston stuff for now, I'm trying to get a better handle on the caliber issue first.

So, a few questions:
1. What's the difference between a pistol caliber BCG and a milspec AR BCG (assuming both are DI)? Assuming you are using a cartridge that fits the dimensions of the upper, why would you need a "Pistol Caliber" carrier? I know the carriers have to be milled for magazine compatibility.

2. I've read that a 9mm bolt can be opened up for a .40/357 (.424). This make sense to me, am I right?

3. I haven't found a source for a barrel yet. Even if I find a .357sig barrel, it probably won't be set up for DI. I guess I could rechamber a 9mm barrel?

Thanks for the help.

 

[LoonWulf]

You probably wanna talk to @MachIVshooter about this project. Hes done some work along these lines.

 

[bbqreloader]

1. What's the difference between a pistol caliber BCG and a milspec AR BCG (assuming both are DI)? Assuming you are using a cartridge that fits the dimensions of the upper, why would you need a "Pistol Caliber" carrier? I know the carriers have to be milled for magazine compatibility.

So I'll give a quick idea from when I built my 9mm AR, before some real pros chime in. The pistol BCG has no gas port. It is also heavier to compensate since it is a blowback design, also needed is a heavier buffer, instead of venting off the gas anywhere. There is also no bolt assembly. Besides the firing pin being able to be removed, it's roughly one unit, at least mine is.

 

[RecoilRob]

Even with a carbine length gas system, I'm doubting whether the Sig will generate enough port pressure to run the gun reliably. It IS a pistol round after all...and the case volume very limited so you're pretty much stuck using fast pistol powders in it. Even using light bullets how much 2400 or 296 could you load....and even they won't have much pressure left by the time they reach the port. This is just a gut thinking with no experience in 357 Sig to back it up. Will be interesting to read about your experiences if you proceed on this quest. Best of luck!