*SKIP DOWN TO THE "##" TO GET TO THE POINT*
Ok so i have decided on my next ar build will ultimately be a suppressed, piston "sbr" (a.k.a. ar15 pistol with a sig brace).
I opted for 300 blk and piston for its obvious advantages in a suppressed weapon.
The piston that I was looking at is the Adams Arms carbine length. i chose the carbine length because: 1. The piston conversion is $200 cheaper than the pistol model.
2. Carbine length barrels in 300 BLK are easier to find in stock.
3. Carbine lengths are milder in recoil than pistols
4. If all else fails -i scrap the build or for some crazy reason i don't get my suppressor- I can slap it on my carbine length government profile colt barrel. I have built 2 ar15s but this is my first pistol, my first 300 blk, and my first piston. Cuz if you're gunna be a bear, be a grizzly.
##Do i need to widen the gas port?
If so, to what size?

thanks in advance guys.

Widening your gas port is going to be dependent on the MFG. There is no consistency there. One of the benefits of the piston system is most have the ability to adjust gas. So until you go out and hammer on it at the range, you really won't know about gas issues.

How long are you going to make your barrel?

if you look around, no major manufacturer sells a pistol length piston 300blk... there's a reason why...

the physics of the round and the requirements of the piston do not match to make a reliable weapon.. it may work some of the time but not all the time with all rounds... primarily in suppressed subsonic rounds, they do not cycle...

the adam arms piston was designed for 556 not 300blk...

the shortest I know that works is the ARAK in 12.5 barrel..

you can always find out the hard way, I looked into this not that long ago and did a ton of research online (other forums) and they all came to the same conclusion... there was just not enough pressure and the curve was too fast, too short to move a piston reliably..

opening the gas port markedly will likely make it cycle for subsonic, but then break it if you go supersonic..

one more thing, if you cut down a carbine length barrel, you end up with a single shot weapon...

there's a reason why pistol length gas systems exist

no offense, but anybody that thinks this is a good idea, needs to spend a few more hours understanding and researching the AR piston gas system and the physics behind it..

the length of barrel that is left between the gas port and when the bullet leaves is what determines your gas pressure (as well as the hole size)... cutting that length you lose a lot of pressure..

http://www.at3tactical.com/blogs/news/9 ... as-systems

do remember the chart on this link above is for 556.. for 300blk, it is totally different and is actually less forgiving..

@xdm9cwa: the PWS MK109 has a 9.75" barrel. Regardless, I don't have that kind of money. I think I'll take your advice and attempt this build when I understand it a little more. In the meantime....

@massivedesign: i was thinking 10-11" in. DI now. I was interested in that Spartan lower group buy, but the G.I. bill doesn't kick in until the 1st.

I'm not sure how pws made it work, still doubt it would feed all ammo reliably.. the physics just isn't there

Actually you should do the build but just scrap the piston idea.. not necessary for a 300blk pistol.. get a 10.5" barrel with a pistol gas tube and you're good to go.. get an adjustable gas block if needed..

Don't make it more complicated than needed